Regulation of H₂O₂ Stress-responsive Genes through a Novel Transcription Factor in the Protozoan Pathogen Entamoeba histolytica

Background: How gene expression is regulated in response to oxidative stress is unknown in Entamoeba histolytica.

Results: Motif AAACCTCAATGAAGA, enriched in promoters of H₂O₂-responsive genes, specifically binds amoebic protein EHI_108720.

Conclusion: EHI_108720 is a transcription factor that mediates up-regulation of gene expression in response to elevated H₂O₂ levels.

Significance: Determining the molecular basis of H₂O₂ stress response is critical to understanding parasite virulence.

Abstract

Outcome of infection depends upon complex interactions between the invading pathogen and the host. As part of the host's innate immune response, the release of reactive oxygen and nitrogen species by phagocytes represents a major obstacle to the establishment of infection. The ability of the human parasite Entamoeba histolytica to survive reactive oxygen and nitrogen species is central to its pathogenic potential and contributes to disease outcome. In order to define the transcriptional network associated with oxidative stress, we utilized the MEME and MAST programs to analyze the promoter regions of 57 amoebic genes that had increased expression specifically in response to H₂O₂ exposure. We functionally characterized an H₂O₂-regulatory motif (HRM) (¹AAACCTCAATGAAGA¹⁵), which was enriched in these promoters and specifically bound amoebic nuclear protein(s). Assays with promoter-luciferase fusions established the importance of key residues and that the HRM motif directly impacted the ability of H₂O₂-responsive promoters to drive gene expression. DNA affinity chromatography and mass spectrometry identified EHI_108720 as an HRM DNA-binding protein. Overexpression and down-regulation of EHI_108720 demonstrated the specificity of EHI_108720 protein binding to the HRM, and overexpression increased basal expression from an H₂O₂-responsive wild-type promoter but not from its mutant counterpart. Thus, EHI_108720, or HRM-binding protein, represents a new stress-responsive transcription factor in E. histolytica that controls a transcriptional regulatory network associated with oxidative stress. Overexpression of EHI_108720 increased parasite virulence. Insight into how E. histolytica responds to oxidative stress increases our understanding of how this important human pathogen establishes invasive disease.

Introduction

Hydrogen peroxide elicits a robust transcriptional response in multiple organisms (1–7) and, along with other reactive oxygen species and nitrogen species (ROS² and RNS), targets multiple cellular components (8). In response, microbes have developed a wide range of defense mechanisms to either directly tackle ROS and RNS or repair the damage that they cause (9–11). Understanding how this response is coordinated at a transcriptional level provides important information regarding the ability of microorganisms to survive in hostile environments. In Escherichia coli and yeast, the transcription factors OxyR and YAP1, respectively, have been identified as the principal players in coordinating the transcriptional response to hydrogen peroxide (12, 13). These transcription factors are directly impacted by elevated hydrogen peroxide levels and display altered DNA binding specificity (OxyR) (14, 15) or elevated protein levels in the nucleus (YAP-1) (16), resulting in up-regulation of multiple stress response genes (reviewed in Ref. 17).

Entamoeba histolytica, a protozoan parasite, is an important human pathogen that must survive changing oxygen tensions and ROS in order to establish infection. Ninety percent of individuals infected with E. histolytica remain asymptomatic, whereas 10% develop a potentially lethal, invasive disease (18). The basis of this variable disease presentation is not fully understood but is probably due in part to the virulence potential of different parasite strains. Both virulent and non-virulent strains of E. histolytica have been identified (19), and comparative analyses of the proteome and transcriptome have identified multiple virulence determinants (20–22). One particularly striking difference is the increased expression in the virulent strain of the surface molecule peroxiredoxin, which degrades hydrogen peroxide (23). It has been demonstrated that virulent E. histolytica strains survive exposure to oxidative stress better than avirulent strains, in part due to the presence of peroxiredoxin (21).

On the transcriptome level, microarray studies demonstrated that exposure to sublethal quantities of H₂O₂ or dipropylenetriamine-NONOate (a nitric oxide releaser) results in greater changes in transcript levels in a virulent than in a non-virulent amoebic strain (2). The percentage of genes regulated by these compounds in the pathogenic strains is larger and the magnitude of changes observed in individual genes is higher than that observed in the non-pathogenic strains (2). The majority of the known factors, including peroxiredoxin, that protect against ROS and RNS are more highly expressed in the virulent strains, but because they are already expressed at robust levels, they do not significantly alter their expression levels in response to stress. This suggests that the virulent strains of E. histolytica utilize transcriptional networks in response to ROS or RNS to regulate the expression of either novel protective factors or factors required in other aspects of increased virulence.

Transcriptional regulation remains a poorly understood aspect of E. histolytica biology, and only a few transcription factors and their corresponding DNA binding motifs have been characterized (reviewed in Ref. 24). Of those transcription factors that have been well characterized, most were originally selected due to sequence similarity to known factors, such as EhMyb10 or the EhTBP (25, 26). However, some unique transcription factors have been successfully identified in E. histolytica; the best characterized of these is URE3-BP, an EF-hand domain-containing protein that is regulated by changes in calcium levels (27, 28).

In this study, we identify a new E. histolytica transcription factor that plays a role in coordinately regulating gene expression in response to hydrogen peroxide exposure. We employed a bioinformatics approach to identify an H₂O₂-responsive motif (HRM) that was enriched within promoters of genes up-regulated following exposure to stress. Our functional studies demonstrated that this motif specifically binds to an amoebic nuclear protein(s), and mutation of this motif resulted in altered gene expression. We utilized a combined DNA affinity chromatography and mass spectrometry approach to identify the HRM-binding protein (HRM-BP), EHI_108720. This protein specifically interacts with the HRM, and manipulation of HRM-BP expression levels altered basal expression and stress responsiveness of an H₂O₂-responsive promoter. These data represent the first steps in elucidating the transcriptional network responsible for coordinating changes in gene expression following H₂O₂ exposure in the important human pathogen E. histolytica.

EXPERIMENTAL PROCEDURES

Microarray Data

Microarray data using custom-generated arrays from Affymetrix identified 184 genes that are up-regulated in the E. histolytica strain HM-1:IMSS in response to a 1-h exposure to 1 mm H₂O₂ (2). For the purposes of this study, this set of genes was reduced by the removal of genes that are up-regulated in response to NO (2) or heat shock (29). Reannotation of the genome (30, 31) resulted in a further reduction and resulted in 57 promoters being used for analysis in this study. The gene promoters used for this study are listed in supplemental Table 1.

Databases

The E. histolytica genome was downloaded from Pathema along with the location of all predicted open reading frames (ORFs) (download date, January 8, 2009). This enabled the retrieval of −300 to −1 nucleotide regions relative to the predicted translation start site for each ORF. For each predicted promoter region, sequence was retrieved irrespective of the location of surrounding predicted ORFs.

DNA Motif Identification

The MEME and MAST programs were used as described previously (29, 32). In brief, the MEME and MAST programs were obtained from the University of California San Diego (33, 34). MEME was performed with the command line: -dna -mod zoops -minw 6 -maxw 10 -minsites 5 -nmotfs 20 or -minw 10 -maxw 14. This set of commands identifies 20 motifs that must have zero or one occurrence in each promoter and occur a minimum of five times within the total number of promoters. The motifs found would have a length between 6 and 10 nucleotides or between 10 and 14 nucleotides. The MAST program was utilized to determine the total number of occurrences of each motif in the promoter sequence databases. The command line arguments used for this purpose were as follows: -ev 500 -remcorr. This allows sequences with an e value of less than 500 and examines each pair of motifs and removes motifs that have a correlation coefficient of greater than 0.7. The hypergeometric distribution was used to determine the significance of enrichment for each motif identified. Motifs with a p value of less than 0.01 were determined to be significantly enriched within the promoters of the 57 H₂O₂-up-regulated genes (supplemental Table 2). Sequence logos were generated using WebLogo (35).

Isolation of Cytoplasmic and Nuclear Enriched Fractions

E. histolytica HM-1:IMSS were grown axenically in TYI-S-33 medium at 36.5 °C as described previously (36). Nuclear extraction was performed using previously published methods (29, 37) with the following modifications. Amoeba were harvested and resuspended in Buffer A (10 mm HEPES, pH 7.9, 1.5 mm MgCl₂, 10 mm KCl and 0.6% IGEPAL) with protease inhibitors 1 μm leupeptin, 1 μm E-64-d, and 1× HALT protease inhibitor mixture (Pierce) and incubated on ice for 20 min. Samples were briefly vortexed and centrifuged for 10 min, at 1000 × g at 4 °C. The supernatant was removed and stored at −80 °C and represents the cytoplasmic fraction. The nuclear pellet was then washed in 500 μl of Buffer A (without IGEPAL), followed by centrifugation for 10 min at 150 × g and 4 °C. The supernatant was removed and the nuclei were resuspended in 150 μl of buffer C (20 mm HEPES, pH 7.9, 420 mm NaCl, 1 mm EDTA, and 1 mm EGTA) supplemented with the same protease inhibitor mixture. Samples were incubated on ice for 30 min prior to centrifugation (20 min, 4 °C, 18,000 × g). The supernatant was removed, and aliquots were frozen on dry ice prior to being stored at −80 °C. Protein distribution was assessed by Western blot analysis. Cytoplasmic and nuclear fractions (30 μg of total protein/sample) were separated by 12.5% SDS-PAGE, and blots were probed with antibodies to histone H3 (ab1791, Abcam), actin (691001, MP Biomedicals), and Myc tag (9B11, Cell Signaling).

EMSA Analysis

Complementary overlapping probes were designed with the motif of interest centrally located, annealed, and labeled using [α-³²P]dATP and Klenow fragment (Invitrogen) as described previously (29). Binding reactions took place in a total volume of 20 μl and included EMSA binding buffer (10 mm Tris-HCl, pH 7.9, 50 mm NaCl, 1 mm EDTA, 3% glycerol, 0.05% milk powder, and 0.05 mg of bromphenol blue), 5 μg of nuclear extract, 2 μg of poly(dI-dC), and 50 fmol of labeled probe. Supershift assays were performed by the addition of 1 μg of either a specific α-Myc antibody (sc-40X, Santa Cruz Biotechnology, Inc.) or control α-β-actin antibody (sc-47778X, Santa Cruz Biotechnology, Inc.), and competition assays were performed with cold oligonucleotide probes provided in 100-fold excess. The supershift and competition assays required the addition of antibody or competitor to the binding reaction prior to the addition of the labeled probe (30-min room temperature incubation for antibody and 10 min for competitor). Upon the addition of the labeled probe, the final reaction mix was incubated for 30 min at room temperature, and samples were loaded onto a non-denaturing polyacrylamide gel and run for 3 h. Upon completion, the gel was fixed, dried, and exposed to a phosphor screen. Gels were imaged using software from Molecular Dynamics. The oligonucleotides used in EMSA (mutations underlined) were as follows: consensus probe, 5′-acgtcagtAAACCTCAATGAAGAcagt (sense) and 5′-acgtactgTCTTCATTGAGGTTTactg (antisense); mutant 1, 5′-acgtcagtAAACCTCAGATAAAGcagt (sense) and 5′-acgtactgCTTTATCTGAGGTTTactg (antisense); mutant 2, 5′-acgtcagtCAATCCAAATGAAGAcagt (sense) and 5′-acgtactgTCTTCATTTGGATTGactg (antisense); mutant 3, 5′-acgtcagtAAACCTAACTGAAGAcagt (sense) and 5′-acgtactgTCTTCAGTTAGGTTTactg (antisense); mutant 4, 5′-acgtcagtAAACCTCAATGAAGAcagt (sense) and 5′-acgtactgTCTTCATTGAGTGTTactg (antisense); mutant 5, 5′-acgtcagtAAACCTCAAGTAAGAcagt (sense) and 5′-acgtactgTCTTACTTGAGGTTTactg (antisense).

The EMSA probes used for promoter analysis were as follows: EHI_029340, 5′-acgtcagtAAACCTCAATGTTGAcagt (sense) and 5′-acgtactgTCAACATTGAGGTTTactg (antisense); EHI_134960, 5′-acgtcagtAAACCTCAATAAACTcagt (sense) and 5′-acgtactgAGTTTATTGAGGTTTactg (antisense); EHI_176810, 5′-acgtcagtAAACCTCATTGTTCAcagt (sense) and 5′-acgtactgTGAACAATGAGGTTTactg (antisense).

Reporter Gene Analysis

Promoter regions of genes EHI_029340, EHI_134960, and EHI_176810 were amplified from E. histolytica HM-1:IMSS genomic DNA using the following primers: EHI_029340, 5′-ACAAAGCTTTCATGTTGAGAATTAAGTGGTTCA-3′ (forward) and 5′- ACACCTAGGTGGTGTTGTGTTTGACATGAAT-3′ (reverse); EHI_134960, 5′-ACAAAGCTTTTCTTAATTAGATTCATGGTTATTC-3′ (forward) and 5′- ACACCTAGGTTTATCACTTTTTTTCATAATGAA-3′ (reverse); EHI_176810, 5′- ACAAAGCTTTTGAGTTATATATTCAAGAAAAGAA-3′ (forward) and 5′- ACACCTAGGAAGTGCATATCTTATCATTTTATG-3′ (reverse). Approximately 500 nucleotides were selected for each promoter region. All reverse primers incorporated the first 15 nucleotides downstream of the start codon for the given gene, which has previously been demonstrated to be important for optimal gene expression in E. histolytica (38). Promoters were cloned into the pKT-Luc vector using HindIII and AvrII restriction sites underlined in the primers. Site-directed mutagenesis was performed to insert the desired mutations into the DNA motif using the QuikChange site-directed mutagenesis protocol (Stratagene). All vectors were sequenced prior to being utilized in functional analyses.

Transient transfection of E. histolytica HM-1:IMSS parasites was achieved as follows. Amoebae were plated into a 35-mm tissue culture dish 24 h prior to transfection. Medium was removed, and cells were washed with 4 ml of M199 and then covered with 1.8 ml of M199 plus 15% fetal bovine serum. Transfection was achieved using 10 μg of experimental plasmid DNA and 1 μg of a control Renilla luciferase plasmid, incubated at room temperature for 10 min with 30 μl of SuperFect (Qiagen) in M199 medium to a total volume of 200 μl. This mixture was added dropwise to the amoeba, and dishes were incubated at 36.5 °C for 3 h. Subsequently, the dishes were iced for 10 min to release the parasites, and the amoeba were transferred to a standard 16-ml culture tube and resuspended in E. histolytica culture medium. Following a 16 h incubation, transfected amoebae were chilled on ice for 5 min, and the cells were pelleted at 1000 × g for 5 min. Luciferase assays were accomplished using the Dual-Luciferase reporter assay system (Promega) following the manufacturer's protocol. Pelleted cells were resuspended in 100 μl of cell lysis buffer (plus 1 μm leupeptin, 1 μm E-64-d, and 1× HALT protease inhibitor mixture (Pierce)) and incubated on ice for 10 min. Samples were vortexed briefly prior to centrifugation (30 s, maximum speed), and supernatants were transferred to fresh tubes. Of each sample, 10 μl was assessed for expression of both firefly and Renilla luciferase, and firefly luciferase results were normalized using the Renilla luciferase readings. Stress was applied by the addition of 1 mm H₂O₂ to parasites for 1 h prior to isolation of whole cell extracts, which were then processed as described above.

Enrichment and Identification of EHI_108720

Protein EHI_108720 was isolated using a DNA affinity chromatography technique described previously (39, 40). The following double-stranded wild type and mutant biotinylated probes were designed: WT P1, 5′-biotinylated ACGTCAGTACACAACAAACCTCAATGAAGACAGTACGT-3′; mutant P1, 5′-biotinylated ACGTCAGTACACAACAGACCATAACTAAAGCAGTACGT-3′. Biotinylated probes were fixed to streptavidin-Sepharose beads (Cell Signaling) and incubated with 300 μg of crude nuclear extract (precleared with non-biotinylated mutant probe and poly(dI-dC)) for 30 min at room temperature in a 1.5-ml tube. Beads were washed twice with 100 mm KCl buffer (20 mm HEPES-KOH (pH 7.9), 100 mm KCl, 1 mm DTT, 1 mm EDTA, 0.01% Nonidet P-40, and 15% glycerol). The first 100 mm KCl wash was kept and is the equivalent of the flow-through sample obtained when performing the DNA affinity chromatography protocol exclusively on columns. Beads were resuspended with 1 ml of 100 mm KCl buffer and loaded onto a Mobicol minicolumn (Boca Scientific). The columns were washed with 200 mm KCl buffer (6 ml), and bound proteins were eluted with 500 mm KCl buffer (0.8 ml). All samples were concentrated using an YM-10 Microcon column (Millipore) (to ∼30 μl), and 5 μl was used for EMSA analysis, with the remainder of the eluted samples from the WT and mutant columns being sent for mass spectrometry.

Direct Isolation of the Protein(s) of Interest

Three EMSA binding reactions were performed as described above. Two reactions contained ³²P-labeled HRM consensus oligonucleotide probe, and the third contained unlabeled probe. Samples were run on a polyacrylamide gel with the cold sample being flanked by the two radiolabeled samples. The gel was exposed to film overnight and then aligned on top of the film, allowing the location of the three samples to be determined. A small section of gel was excised that corresponded to the location of the cold probe-protein complex and was analyzed by mass spectrometry (MS).

Mass Spectrometry

Samples were submitted to the MS facility at Stanford University and processed in the following manner.

Gel Slice Sample

In-gel digestion was done using Promega MS grade trypsin overnight as reported previously (41), with the addition of the acid-labile surfactant ProteaseMAX (Promega). Prior to digestion, the gel slices were cut into ∼1 × 1-mm cubes, reduced with 5 mm DTT, and alkylated with acrylamide. Peptides were extracted and dried prior to reconstitution and analysis.

DNA Affinity Chromatography Samples

The protein eluates were digested using an acetone precipitation step, reconstituted in 8 m urea/ProteaseMAX, 50 mm ammonium bicarbonate, reduced, and alkylated followed by overnight digestion using trypsin at a 1:100 protease/protein ratio.

Nano-reversed phase HPLC was done using an Eksigent 2D NanoLC system (Eksigent, Dublin, CA) with buffer A consisting of 0.1% formic acid in water and buffer B consisting of 0.1% formic acid in acetonitrile. A fused silica column self-packed with Duragel C18 (Peeke, Redwood City, CA) matrix was used with a linear gradient from 2% B to 40% B at a flow rate of 600 nl/min. The nano-HPLC was interfaced with a Bruker/Michrom Advance Captive spray source for nano-electrospray ionization into the mass spectrometer. The mass spectrometer was an LTQ Orbitrap Velos (Thermo Fisher Scientific), which was set in data-dependent acquisition mode to perform MS/MS on the top 12 most intense multiply charged cations. The .RAW data were searched using Sequest on a Sorcerer platform against the Uniprot database. Data were validated and visualized using Scaffold software.

Genetic Manipulation of EHI_108720

For gene overexpression, the entire coding region for EHI_108720 was PCR-amplified from genomic DNA using the primers 5′-ACACCCGGGATGGAAGAAGATCACGAT-3′ (forward) and 5′-ACACTCGAGTTAATGATAAAATGTTCCTTTACC-3′ (reverse). Myc-tagged EHI_108720 was generated by cloning the full open reading frame into the E. histolytica plasmid pKT3M, downstream of the cysteine synthase promoter and 3× Myc tag (cloned into the SmaI and XhoI restriction sites). The resultant construct was transfected into E. histolytica HM-1:IMSS using the SuperFect (Qiagen) protocol described above. To establish stable transfection, 20 μg of plasmid DNA was used, and 24 h after transfection, drug selection was started with 1 μg/ml G418 added and increased to 3 μg/ml 48 h later. Medium was exchanged as needed until the tube was confluent with stably transfected amoeba. At this stage, the amoebae were passaged, and G418 was gradually increased to a final concentration of 12 μg/ml.

For knockdown of the gene, we used a new RNAi-based approach that we have recently developed.³ Briefly, the entire EHI_108720 coding sequence was cloned downstream of the EHI_197520 small RNA “trigger” fragment in the pKT-3M backbone, using the SmaI and XhoI sites. Stable cell lines were established using the same methodologies as described above. Knockdown efficiency was assessed using RT-PCR (described below).

RNA Isolation and RT-PCR

RNA was isolated from untransfected HM-1:IMSS and EHI_108720 overexpression and knockdown cell lines using the mirVana kit (Ambion), and RNA concentration was measured using a Nanodrop spectrophotometer. RT-PCR was performed as described previously (42). Primers used in RT-PCR were as follows: EHI_108720, 5′-GTGGTATGGGTCAAGCTGGT-3′ (forward) and 5′-CAGTGTCTTGCCAAGTTCCA-3′ (reverse); EHI_199600, 5′-ACGTCATGCTGAATTTGCTG-3′ (forward) and 5′-CCTTTAAGCCCAGCCTTTCT-3′ (reverse); EHI_176810, 5′-CCGGTTTGTGATCCAAGTTT-3′ (forward) and 5′-TTGGAATTTGATTAATTCTTTGAGT-3′ (reverse). cDNA samples were subjected to 29 cycles of PCR amplification.

Cell Monolayer Destruction Assays

Assays were performed as described previously (43). Briefly, 5 × 10⁴ trophozoites were placed on a confluent CHO cell monolayer, centrifuged for 5 min at 50 × g, and incubated for 75 min at 37 °C. Cells were fixed for 10 min with 4% ultrapure formaldehyde, washed twice with PBS, stained with 0.1% methylene blue (OmniPur), diluted in 10 mm borate buffer (pH 8.7), and washed three times with the same buffer. The dye was extracted by adding 1 ml of 0.1 m HCl at 37 °C for 30 min. In order to measure the extracted dye, samples were diluted 1:10 with PBS, and the absorbance was read at 650 nm in a spectrophotometer.

RESULTS

Identification of Conserved Motifs in the Promoter Regions of H₂O₂-responsive Genes

In a previous study, Vicente et al. (2) identified 184 genes that were up-regulated ≥2-fold in response to hydrogen peroxide in E. histolytica. In order to identify promoter motifs in H₂O₂-responsive genes, we narrowed this list of genes to those that were specifically up-regulated by the H₂O₂ stress. Thus, genes that were also up-regulated by nitric oxide (NO) and/or heat shock were removed from this list (82 genes for NO, 13 genes for heat shock, and 21 genes for both NO and heat shock); in total, 116 genes were removed (2, 29). Additionally, because the microarray data were initially published, the E. histolytica genome has been reannotated (31), resulting in a further small decrease in the number of promoters to be analyzed. Ultimately, 57 promoters of genes that were up-regulated specifically by H₂O₂ (supplemental Table 1) were examined using the MEME program (33) to identify conserved motifs that could be important in the coordinated regulation of gene expression in response to hydrogen peroxide stress. MEME analysis of promoter regions was performed using 300 nucleotides upstream of the start codon and searching for motifs of either 6–10 or 10–14 nucleotides in length. Subsequently, we utilized the MAST program (34) to identify all occurrences of the identified motifs in the promoters of the E. histolytica genome to determine which motifs were significantly enriched in the H₂O₂-responsive subset (supplemental Table 2).

The HRM Specifically Binds Amoebic Nuclear Protein(s)

Motifs identified were prioritized based on (i) enrichment of the motif within the subset of promoters relative to the number of occurrences in the entire promoter set (as determined using the hypergeometric distribution (p < 0.01)), (ii) highly conserved sequence within the promoters of co-regulated genes, and (iii) relatively conserved position in relation to the start codon. The motif CCTCAAT fulfilled all of the criteria listed above. It was identified 15 times within the 57 promoters studied and was enriched within 100 nucleotides of the start codon in 75% of the promoters in which it was present (Fig. 1B). A consensus sequence was obtained by identifying a 15-nucleotide region in each of the promoters that incorporated the CCTCAAT motif (supplemental Table 3). This enabled the identification of nucleotides enriched within the core motif flanking regions. A consensus motif of AAACCTCAATGAAGA was established for this HRM (Fig. 1A) and subsequently tested for its ability to bind nuclear protein(s). EMSA analysis demonstrated that the consensus probe binds to protein(s) present in crude nuclear extract (Fig. 1C) and that this interaction decreased in the presence of a 100-fold excess of cold consensus probe, indicating specificity of the interaction.

FIGURE 1.

Identification of a conserved motif in promoters of E. histolytica genes up-regulated by hydrogen peroxide stress. A, MEME and MAST analysis identified the AAACCTCAATGAAGA motif as being enriched in this subset of H₂O₂-responsive promoters. B, the motif is predominantly located within 60–140 nucleotides of the start codon. C, EMSA analysis demonstrates that amoebic nuclear protein(s) bind to this motif. Competition assays, using a 100-fold excess of cold competitor, demonstrated the specificity of this interaction and identified C7 and A9 as residues essential for protein binding. The arrows indicate major bands that exhibit specific binding. The sequence logo was generated using WebLogo (35).

To determine which residues of the consensus probe were critical to binding, several mutant (M) probes were used as cold competitors (Fig. 1C). The 5′-region of the motif is more relevant to DNA-protein binding because probe M2 could not compete against the wild-type labeled probe, whereas probe M1 could partially compete. Smaller changes, consisting of the switching of nucleotides at positions 7 and 9 (probe M3) and positions 3 and 4 (probe M4), demonstrated that the maintenance of the ACCTCAAT region was critical for DNA-protein binding. In particular, the reversal of positions C7 and A9 (probe M3) was sufficient to prevent this mutant probe from competing against the labeled consensus probe. Similar changes at the 3′-end (nucleotides 10 and 11; probe M5) had no effect on the ability to compete with the labeled probe when compared with the cold consensus competitor.

To determine if the binding of protein(s) to the HRM was altered significantly under stress conditions, EMSA was performed using nuclear extract isolated from amoebae that had been treated with 1 mm H₂O₂ for 1 h (the same stress as used in the transcriptome analyses) (2) (Fig. 2). Nuclear extract from parasites exposed to 1 mm H₂O₂ had decreased binding to the labeled HRM probe. However, binding to a control probe was unchanged, demonstrating that loss of binding was specific to the HRM-interacting protein(s) and not due to the quality of the nuclear extract under stress conditions (data not shown). Thus, the data suggest that under stress conditions, either the abundance of the DNA-binding protein(s) decreases, the protein or protein complex is no longer present in the nucleus, or the protein(s) has been modified in such a way that prevents binding to the HRM.

FIGURE 2.

Binding to the HRM motif decreases in nuclear extracts from hydrogen peroxide-treated parasites. Amoebic cultures were either left untreated or exposed to 1 mm H₂O₂ (1 h) prior to isolation of nuclear fractions. EMSA analysis was performed using 5 μg of crude nuclear extract per binding reaction. Specificity of binding was demonstrated using a 100-fold excess of cold wild type or mutant competitor. Competitor probe was AAACCTCAATGAAGA, and mutant competitor probe was AAACCTAACTGAAGA. The arrows indicate major bands that exhibit specific binding.

Functional Characterization of the HRM

Binding to Promoter-specific Motifs from Three H₂O₂-responsive Genes

In order to determine if the HRM was functionally relevant, three genes were selected that were up-regulated in response to H₂O₂ and whose promoters contain the HRM (Table 1). Genes EHI_029340 and EHI_176810 have low basal expression levels with 3- and 8-fold increase, respectively, following H₂O₂ stress. Gene EHI_134960 is highly expressed at base line and increases 2-fold in response to stress. Despite the differences in basal and regulated mRNA levels and small variations in nucleotide composition, the HRM motif in each of the three promoters bound nuclear protein(s) in a manner similar to the consensus motif (Fig. 3). The binding to labeled probe was competed by excess consensus probe and by promoter-specific probe. Binding to the labeled probe was not affected by the presence of excess mutant competitor, where the nucleotide composition and order only differed by the switching of nucleotides 7 and 9. For all three promoter motifs, H₂O₂ treatment resulted in reduced binding of nuclear protein(s), similar to what was seen with the consensus probe.

TABLE 1.

Identity of H₂O2-responsive promoters that were selected for EMSA and functional characterization

Promoters were selected based on similarity of the promoter HRM to the consensus HRM and proximity of the motif to the start codon (ATG). Gene ID, gene name, and promoter-specific HRMs are shown. Nucleotides that differ from the consensus HRM are underlined and in boldface type. The microarray data originate from Vicente et al, (2). Distance upstream of ATG was determined based on the number of nucleotides from position 15 of the motif sequence to the A of ATG.

Gene ID	Gene name	Sequence	Normalized microarray data			Distance upstream of ATG
			Base line	With H2O2	Change
					-fold	nucleotides
Consensus		AAACCTCAATGAAGA
EHI_029340	Short chain dehydrogenase family protein	AAACCTCAATGTTGA	0.06	0.18	3.15	58
EHI_134960	Skp1-related protein	AAACCTCAATAAACT	34.10	68.90	2.01	44
EHI_176810	Hypothetical protein	AAACCTCATTGTTCA	0.10	0.87	8.68	74

FIGURE 3.

Binding to promoter specific motifs is specific and decreases with H₂O₂. EMSAs were performed using 5 μg of crude nuclear extract from untreated or H₂O₂-stressed (1 mm, 1 h) amoebae and ³²P-labeled probes specific to the promoter motif (see Table 1 for sequence). Competition assays were performed using a 100-fold excess of cold competitor of either the consensus probe, probes that perfectly match the relevant promoter motif (specific competitor), or mutant competitor probes in which the nucleotides at positions 7 and 9 are switched (mutant competitor). The arrows indicate major bands that exhibit specific binding.

Mutation of the HRM in H₂O₂-responsive Gene Promoters Affects Reporter Gene Expression Levels

To ascertain the functional impact of the motif, each promoter was cloned upstream of a luciferase reporter gene. Additionally, the motifs in each promoter were mutated using site-directed mutagenesis to switch the nucleotides at positions 7 and 9, which prevented DNA-protein interaction in EMSA. All three wild type (WT) promoters were able to drive robust luciferase expression, with promoter EHI_134960 driving luciferase expression between 3- and 6-fold higher than the other two promoters (Fig. 4A); of note, this matched the greater mRNA abundance for this gene as noted by microarray analysis (Table 1). Interestingly, although mutations in all promoters affected reporter gene expression, the outcome was variable. In two genes (EHI_029340 and EHI_176810), mutation of HRM decreased luciferase expression; in EHI_134960, mutation of HRM increased luciferase expression.

FIGURE 4.

Mutation of the HRM impacts basal luciferase expression levels and H₂O₂ responsiveness. Promoters for genes EHI_029340, EHI_134960, and EHI_176810 were cloned into an E. histolytica luciferase reporter plasmid. For each construct, site-directed mutagenesis was performed, resulting in the nucleotides at positions 7 and 9 of the HRM being switched. Wild type and mutant promoters were assessed for their ability to drive luciferase expression in transient transfection assays (A) or the response of each promoter to H₂O₂ stress (1 mm, 1 h) (B). −, no stress; +, H₂O₂ stress. -Fold change data from at least three experiments are presented. *, p < 0.01. Error bars, S.D.

The different results seen between promoters could be attributed to the fact that DNA-protein binding and downstream changes in transcriptional response are context-dependent. The EMSA results are only representative of binding to a short oligonucleotide probe, and the presence of the entire promoter region probably influences gene expression, as monitored in luciferase assays. Taken together, these data highlight the complexity of transcriptional regulation and that the protein or protein complex that binds to the HRM may act differently depending upon additional factors. There is precedence for transcription factors in E. histolytica having a dual role, with transcription factor URE3-BP positively influencing expression from the Gal/GalNAc-inhibitable lectin hgl5 promoter but negatively affecting gene expression from the ferredoxin 1 promoter (37, 44).

The Promoter of EHI_176810 Responds to H₂O₂ Stress

We next sought to determine if the WT and mutant promoters were responsive to H₂O₂ stress. Eighteen hours following transient transfection, E. histolytica trophozoites were exposed to 1 mm H₂O₂ for 1 h, and luciferase levels were compared with controls without H₂O₂ exposure. Of the three promoters, only the WT promoter for EHI_176810 demonstrated increased luciferase levels in response to stress (Fig. 4B). The EHI_176810 promoter with the mutated HRM did not drive increased luciferase expression after stress, indicating that it was the HRM specifically that was responsible for directing increased gene expression in response to stress. Luciferase expression driven by promoters from EHI_029340 and EHI_134960 did not change in response to H₂O₂. The reason for the difference in response to oxidative stress for the three promoters is unclear. It is of note, however, that the highest level of mRNA change seen on the microarray data is for gene EHI_176810 (Table 1). The change at the luciferase protein level is less than the 8-fold increase seen at the mRNA level for EHI_176810; thus, it is possible that the smaller changes seen at the mRNA level for genes EHI_029340 and EHI_134960 cannot be recapitulated in the luciferase assay, which relies upon protein activity.

Identification of the HRM-interacting Protein

Enrichment of DNA-binding Proteins by DNA Affinity Chromatography

Having identified the HRM as being enriched in H₂O₂-responsive genes and demonstrated that it can alter luciferase expression levels from three promoters, we utilized the HRM in DNA affinity columns to identify the protein(s) that interacts with it. Biotinylated oligonucleotide probes (consensus and mutant) affixed to Sepharose beads were incubated with nuclear extract and loaded onto a column, and bound proteins were eluted using 500 mm KCl. All flow-through samples and eluates were tested via EMSA (Fig. 5). In the flow-through sample from the column with the mutant oligonucleotide, significant binding was seen in the EMSA; in contrast, in the flow-through from the column with the WT oligonucleotide, no binding was noted, indicating that the protein(s) of interest bound to the oligonucleotide probe containing the HRM. No binding was observed in the 200 mm KCl wash samples from either column. In the 500 mm KCl samples, HRM-binding protein was in the eluate from the WT oligonucleotide column, but no protein binding to HRM was in the eluate from the column with the mutant oligonucleotide. This demonstrates that the protein(s) of interest are enriched in the sample from the WT oligonucleotide column. The 500 mm KCl eluates were therefore examined by mass spectrometry to identify which proteins were unique (or enriched) in the WT samples. MS was performed on three independently generated samples from DNA affinity chromatography columns containing WT or mutant oligonucleotides. Overall, MS analysis of the WT and mutant samples resulted in the identification of 549 proteins that met the cut-off criteria (minimum protein, 95%; minimum number of peptides, 1; minimum peptide, 95%) in at least one of the six samples analyzed (supplemental Table 4).

FIGURE 5.

Enrichment of DNA binding protein(s) from E. histolytica crude nuclear extracts. A DNA affinity chromatography assay was performed to enrich DNA-binding protein(s) that bind to the consensus HRM. Nuclear protein was applied to columns with either the wild-type or mutant consensus HRM, washed with 200 mm KCl EMSA binding buffer, and eluted with 500 mm KCl EMSA binding buffer. 5 μl of the samples from the flow-through, 200 mm wash, and 500 mm elution samples were assessed in EMSA using the ³²P-labeled consensus probe. The arrows indicate major bands that exhibit specific binding.

Direct Isolation of the Oligonucleotide Probe and Protein(s) of Interest

As an alternative strategy to the DNA affinity columns, we also directly isolated the protein(s)-probe complex from a polyacrylamide gel. To achieve this, we performed three EMSA binding reactions using nuclear extract and an oligonucleotide probe containing the HRM and resolved them on a polyacrylamide gel. The two flanking samples contained radiolabeled probe, and the middle sample contained unlabeled probe. The gel was exposed to film overnight and then aligned on top of the film, allowing the location of the three samples to be determined. A small section of gel was excised that corresponded to the location of the cold probe-protein complex and was analyzed by MS. MS analysis of this sample identified 127 proteins present in the gel slice. Of these, 32 overlapped with proteins identified in the DNA affinity purification MS analysis (supplemental Table 4).

Identification of Protein EHI_108720

Proteins of interest were required to be present in the MS data from all three of the WT oligonucleotide column samples and absent or greatly reduced in the mutant oligonucleotide column samples. Additionally, proteins of interest were required to be present in the MS data from the gel slice. Only one protein fulfilled all criteria. EHI_108720 is a hypothetical protein that is predicted to be a 50-kDa protein consisting of 444 amino acids. With the exception of homologous proteins in Entamoeba dispar and Entamoeba invadens, BLAST analysis returned no proteins with significant sequence identity. A Pfam search (45) for potential regulatory domains identified a weak match to a helix-turn-helix domain (PF01381) toward the N terminus (e value of 0.18), which is of potential interest as a DNA binding domain.

Previously published microarray data show that EHI_108720 is moderately expressed in the pathogenic strain of E. histolytica, HM1:IMSS (2), and that mRNA expression levels are not significantly altered in response to H₂O₂, dipropylenetriamine-NONOate, or heat shock or during colonic tissue invasion (2, 29, 45). Additionally, mRNA levels are similar in the avirulent E. histolytica Rahman strain (2). These data indicate that EHI_108720 mRNA remains stably expressed under a wide range of conditions.

Characterization of Myc-tagged EHI_108720

In order to characterize EHI_108720, we expressed an N-terminal Myc-tagged version of the protein in E. histolytica trophozoites. Confirmation that the gene was overexpressed in this cell line was achieved by performing RT-PCR (Fig. 7A). We then isolated cytoplasmic and nuclear enriched fractions from the transfected cell line and probed for the Myc-tagged protein by Western blot analysis (Fig. 6A). The EHI_108720 protein was detected at two different sizes, at both the expected 50-kDa size and at 75 kDa, and was present in the nucleus, an important requirement for a potential transcription factor. Additionally, a substantial amount of protein was present in the cytoplasmic fraction. The localization and distribution of EHI_108720 was confirmed using an immunofluorescence assay (data not shown). Cytoplasmic localization is not uncommon for transcription factors; for example, URE3-BP is present in both the nucleus and cytoplasm (46). Changes in the cellular localization of URE3-BP have been linked to a regulatory mechanism for this transcription factor (46). The transfected cell line overexpressing EHI_108720 was exposed to H₂O₂ stress, and a Western blot was performed to determine if there was any change in abundance, size, or localization of EHI_108720. However, there was no reproducible difference in the size, abundance, or localization of EHI_108720 between samples exposed to H₂O₂ stress and those from the unstressed parasites (Fig. 6A).

FIGURE 7.

Knockdown of EHI_108720 results in decreased binding to the consensus HRM. A, overexpression and knockdown of the putative HRM-BP, EHI_108720, was assessed using RT-PCR and compared with untransfected and control cell lines. B, nuclear enriched fractions from untransfected, knockdown, and control cell lines were assessed for specific binding to the consensus HRM. Competition assays were performed using a 100-fold excess of cold competitor of either the consensus probe or mutant probe. C, an alternate oligonucleotide probe was employed as a loading control to ensure that an equal amount of protein was loaded in each sample. The arrows indicate major bands that exhibit specific binding. UT, untransfected control; OX, overexpression cell line; KD, knockdown cell line.

FIGURE 6.

Myc-tagged EHI_108720 localizes to both the nucleus and cytoplasm and specifically binds the consensus HRM. A, distribution of Myc-tagged EHI_108720 was assessed via Western blot analysis. α-Histone H3 and α-actin antibodies were utilized as loading controls between nuclear (NE) and cytoplasmic (CE) fractions, respectively (stress = 1 mm H₂O₂, 1 h). B, EMSA analysis demonstrated that overexpressed protein binds to the HRM consensus motif. Competition assays, using a 100-fold excess of cold competitor, and supershift assays (α-Myc and α-actin antibodies) confirmed the specificity of this interaction. The arrows indicate major bands that exhibit specific binding. *, supershifted bands. C, all four major bands decrease proportionally with increasing amounts of α-Myc antibody, confirming that the Myc-tagged protein is present in each protein-probe complex. α-Actin antibody at the lowest and highest concentrations did not impact binding of the four major bands. *, supershifted bands.

Specific Interaction between EHI_108720 Protein and the HRM

Overexpressed Myc-tagged EHI_108720 Protein Specifically Binds the HRM

In order to confirm that the overexpressed protein was able to bind to the HRM motif, nuclear enriched fractions were isolated from EHI_108720 overexpression cell lines (under untreated and stress conditions) and assessed for the presence of HRM-binding protein using EMSA (Fig. 6B). Four distinct bands were present, all of which were absent when cold competitor was provided in excess. The addition of mutant competitor did not affect binding, indicating that each of these bands represents a specific interaction between protein and probe. Nuclear extract from amoebae overexpressing HRM-BP and H₂O₂ stressed showed a decrease in three of the four EMSA bands. This decrease in binding was also observed when using nuclear enriched fractions from wild-type H₂O₂-stressed amoebae (Fig. 2). Of note, the decreased binding occurs despite the lack of observable differences in protein distribution or quantity observed via Western blot (Fig. 6A).

In order to confirm that bands on the EMSA represented a specific interaction between the Myc-tagged EHI_108720 protein and the labeled probe, we performed supershift assays using α-Myc antibody; an α-actin antibody was also used to confirm the specificity of the changes (Fig. 6, B and C). Increasing concentration of the α-Myc antibody resulted in the four bands decreasing to below detectable levels and resulted in two new bands in the uppermost portion of the gel, representing the supershifted complex. The control α-actin antibody had no effect on binding and did not result in a supershift. Overall, this analysis confirmed a specific interaction between the Myc-tagged EHI_108720 and the consensus HRM motif.

Knockdown of EHI_108720 Leads to Abrogation of Binding to the HRM

Knockdown of EHI_108720 was achieved using an RNAi-based method with significant reduction in gene-specific mRNA compared with controls (Fig. 7A). EMSA analysis with nuclear extract from the knockdown cell line demonstrated decreased binding to the labeled probe despite equal binding to a control probe (Fig. 7, B and C). Taken together, the genetic manipulation of EHI_108720, by overexpression and knockdown experiments, and associated changes in binding as shown by EMSA confirm that EHI_108720 is an HRM-BP.

Genetic Manipulation of EHI_108720 Influences Gene Expression from an H₂O₂-responsive Promoter

Because a specific interaction between the HRM and EHI_108720 was confirmed, we addressed the impact of manipulating HRM-BP on expression from the H₂O₂-responsive promoter of EHI_176810. RT-PCR analysis confirmed that in the untransfected control cell line (UT) the levels of EHI_176810 mRNA increased following the application of H₂O₂ stress (Fig. 8A). In the HRM-BP overexpression cell line, EHI_176810 mRNA levels were greater than those in the control sample, but H₂O₂ exposure had no effect on mRNA levels. In the HRM-BP knockdown cell line, the mRNA levels of EHI_176810 still demonstrated an increase following stress.

FIGURE 8.

Overexpression or knockdown of EHI_108720 influences gene expression from an H₂O₂-responsive promoter. A, the effects of HRM-BP manipulation on EHI_176810 mRNA levels were assessed via RT-PCR. Untransfected control, overexpression, and knockdown cell lines were examined before and after exposure to H₂O₂ (1 mm, 1 h). B, untransfected, overexpression, and knockdown cell lines were transiently transfected with the EHI_176810 promoter-luciferase construct. Untreated or H₂O₂-exposed (1 mm, 1 h) parasites were assessed for luciferase activity. Fold change data from five experiments are presented. *, p value < 0.05. Error bars, S.E.; UT, untransfected control; KD, knockdown; OX, overexpression. −, no stress; +, H₂O₂ stress.

Having ascertained the effects of manipulating HRM-BP on the expression of EHI_176810 mRNA, we next sought to determine the effects of HRM-BP on reporter protein expression. To achieve this, the EHI_176810 promoter-luciferase fusion constructs (WT and mutant) were transiently transfected into either HRM-BP overexpression, HRM-BP knockdown, or untransfected control cell lines. Luciferase expression levels were determined for all cell lines before and after exposure to H₂O₂ (Fig. 8B). As expected, oxidative stress resulted in an ∼3-fold increase in luciferase levels from the WT promoter in the control cells (p < 0.05), similar to previous observations (Fig. 4B). In the HRM-BP-overexpressing cell line, there was an ∼2-fold increase in luciferase levels from the unstressed WT promoter compared with the luciferase levels observed in the control cell line (p < 0.05). As a consequence of increased basal luciferase expression, the effect of H₂O₂ exposure was muted and no longer significantly different (∼1.4-fold increase) (Fig. 8B). In the HRM-BP knockdown cell line, the basal levels of luciferase expression were not significantly different from the basal levels seen in the control cell line. Additionally, exposure to H₂O₂ no longer significantly increased levels of the reporter gene (Fig. 8B). In all three cell lines, transfection with the mutant promoter constructs resulted in decreased luciferase expression compared with the WT counterparts and were unaffected by exposure to H₂O₂.

These data demonstrate that manipulation of the HRM-BP similarly alters both mRNA and reporter gene protein levels. Importantly, overexpression of this protein significantly raises basal transcription levels and negates the effect of exposure to hydrogen peroxide. Given that overexpression increased basal luciferase expression, it could be hypothesized that knockdown of EHI_108720 would negatively affect basal levels. However, this was not observed, which may be due to incomplete knockdown of EHI_108720 and low levels of protein persisting, as suggested by the EMSA analysis using extract from the knockdown cell line (Fig. 7B). However, knockdown of EHI_108720 did negate the ability of the WT promoter to significantly increase luciferase expression in response to H₂O₂ exposure (Fig. 8B). Results from the mutant promoters confirm that EHI_108720 requires the presence of an intact HRM to influence reporter gene expression from this promoter. Taken together, these findings confirm that EHI_108720 impacts gene expression and that it mediates its effect via the HRM.

Overexpression of HRM-BP Increases Parasite Virulence

Given that overexpression of the HRM-BP has the potential to impact basal gene expression via the HRM, we examined this cell line for phenotypic changes. Comparisons against a control cell line (maintained at an identical drug concentration) determined that overexpression of the HRM-BP had no effect on growth rate or on cell survival following exposure to H₂O₂ (data not shown).

Monolayer destruction assays were performed to determine if overexpression of the HRM-BP resulted in changes in cytotoxicity (Fig. 9). Compared with the control cell line, the HRM-BP overexpression cell line destroyed a significantly greater amount of the target cells (p ≤ 0.05), indicating an increase in virulence. Exposure to H₂O₂ resulted in decreased cytotoxicity in both the control and the HRM-BP overexpression cell lines when compared with the unstressed parasites.

FIGURE 9.

Overexpression of EHI_108720 increases parasite virulence. The cytotoxicity of HRM-BP overexpression and control cell lines (UT) was determined using a monolayer destruction assay. H₂O₂-stressed (1 mm, 1 h) or untreated parasites were added to confluent CHO monolayers. The level of monolayer destruction was assessed using 0.1% methylene blue, dye extraction, and absorbance at 650 nm. Relative absorbance levels are shown for four experiments performed in duplicate. *, p < 0.05. Error bars, S.D.

Overall, these data suggest not only that overexpression of the HRM-BP affects basal expression levels of a subset of genes but that these changes result in increased cytotoxicity and that HRM-BP significantly impacts the virulence potential of E. histolytica.

DISCUSSION

Invasive pathogens are frequently exposed to reactive oxygen species as a mechanism of host defense, and an organism's ability to survive this aspect of host immunity is critical to establishing infection. In many systems, the transcriptional machinery that regulates gene expression in response to ROS is well characterized (12, 13, 47–52). In the pathogenic protist E. histolytica, increased virulence has been linked to the parasite's ability to survive exposure to ROS (21), and the transcriptional response to hydrogen peroxide stress has been characterized (2). However, the regulatory pathways involved in controlling gene expression in response to ROS have not been identified, and homologues of transcription factors that respond to ROS are absent from the amoebic genome (30, 31). In this study, we utilized a bioinformatics approach to identify promoter motifs enriched in genes up-regulated in response to hydrogen peroxide. We identified AAACCTCAATGAAGA as a hydrogen peroxide regulatory motif, used EMSA to show that amoebic nuclear proteins bind specifically to this motif, demonstrated that substitution of two core nucleotides (C7 and A9) is sufficient to abrogate DNA-protein binding, and established the biological importance of the HRM using reporter gene assays. Furthermore, we employed DNA affinity chromatography and mass spectrometry to identify the HRM-binding protein (EHI_108720). Confirmation of this interaction and the biological significance of HRM-BP was achieved by assessing amoebic cell lines in which the protein was either overexpressed or knocked down. Overall, this work represents the first identification of a transcription factor that controls the coordinated regulation of oxidative stress response genes in E. histolytica and has important ramifications for understanding the molecular basis of stress response in an important human pathogen.

The mechanism by which the HRM-binding protein (EHI_108720) regulates gene expression through the HRM is not yet known, but our data suggest several intriguing features. Functional analysis of three HRM-containing promoters determined that this motif asserts activating or repressing effects, with divergent roles noted in a promoter-specific manner. Of interest, a similar observation was noted with mutational analysis of the E. histolytica URE3 motif in the promoters of the hgl5 and ferredoxin 1 genes (37, 44). The variable role that the HRM plays in modulating transcription could be explained by a number of factors, including the attributes of the surrounding DNA, small changes in the nucleotide composition of the HRM itself, or properties of the HRM-binding protein. From the original microarray study, we know that the basal transcription levels and the magnitude of the response to stress varies for each promoter. This indicates that other components, specific to each promoter, influence gene expression. It is also plausible that the small changes seen in the nucleotide composition of the promoter-specific HRMs influence the role it plays in regulating transcription. This scenario is exemplified by the dual role observed for the transcription factor Pit-1, where Pit-1 activity is influenced by the presence or absence of an additional two nucleotides within its DNA binding motif (53). The conformational change induced in the Pit-1 dimer by the inclusion of the two nucleotides enables the recruitment of the transcriptional co-repressor N-CoR and the reversal of its activating role (53).

Alternatively, the variable outcome of mutating the HRM could be dependent on the intrinsic properties of the protein and/or the regulatory co-factors that it recruits. Given that ∼65% of the genes annotated in the E. histolytica genome encode hypothetical proteins (31), it was not surprising that a protein of unknown function was identified as the HRM-BP. Unfortunately, the lack of identifiable domains makes it difficult to immediately predict how this protein differentially regulates transcription. Overexpression of the HRM-BP revealed that binding to the consensus motif results in four distinct bands, each of which contains the HRM-BP. Whether these bands represent concatemers of the HRM-BP or whether the increased size is due to additional protein co-factors being recruited to bind to the HRM is not known at present. Therefore, whether HRM-BP activates or represses expression from a given promoter may depend upon the components of the protein complex bound at each promoter motif.

Given that expression or localization of HRM-BP does not change in response to oxidative stress, it is not readily apparent how this protein is regulated in response to hydrogen peroxide. In E. coli and yeast, the OxyR and YAP-1 proteins direct increased expression of a subset of genes in response to hydrogen peroxide (1, 6, 7, 12, 13). In Bacillus subtilis, one of the key transcription factors involved in up-regulating gene expression in response to hydrogen peroxide is PerR, a transcriptional repressor (5, 48). In all three cases, the protein itself is altered in response to hydrogen peroxide exposure (increased DNA binding affinity (OxyR) (14, 15), increased nuclear retention (YAP-1) (16), and impaired DNA binding (PerR) (54)). In each situation, the protein alteration was key to the transcriptional regulation that occurs in these organisms. Given the lack of change in mRNA levels observed for EHI_108720, the multiple bands observed in EMSA analysis, and the decreased binding seen following exposure to stress, it is interesting to speculate that changes in the HRM-BP protein itself may be central to its behavior.

In addition to understanding how HRM-BP is regulated, it is important to consider the role it plays in pathogenesis. The initial microarray study looking at global transcriptional changes in response to H₂O₂ observed that many genes known to be involved in mitigating the effects of ROS exposure were already highly expressed and were not further affected by stress. Consistent with this observation, the phenotypic studies performed in this study demonstrated that overexpression of HRM-BP does not confer increased resistance to H₂O₂. However, the overexpression of HRM-BP does increase the cytotoxicity of E. histolytica and suggests that at least a portion of the genes regulated in response to H₂O₂ exposure are involved in other aspects of amoebic biology, such as virulence.

HRM-BP represents a novel transcription factor that is involved in a fundamentally important aspect of E. histolytica biology, and its identification is a significant first step in understanding how the parasite coordinates gene expression in response to hydrogen peroxide stress. The link between susceptibility to ROS and virulence underscores the importance of this area of research. Further characterization of this protein and its binding partners will help to fully elucidate its role in parasite virulence.