FIGURE 1.

HrtR is a hemin-binding protein. A, schematic representation of the hrtRBA operon in L. lactis (MG1363). The hrtR (llmg_0626) gene encodes a TetR family transcriptional regulator. The hrtB (llmg_0625) and hrtA (llmg_0624) genes encode a permease and an ATPase, respectively. The locus was named based on protein sequence alignments that identified HrtB and HrtA as orthologs of the heme regulated transporter components first described in S. aureus and present in numerous Gram-positive bacteria (21–26). B, E. coli cells overexpressing HrtR are colored red. E. coli were transformed with the HrtR expression vector pMBP-HrtR. Protein expression was induced with 0.5 mm isopropyl 1-thio-β-d-galactopyranoside (IPTG) (see “Experimental Procedures”). Bacteria were collected by centrifugation and photographed. C, UV-visible absorption spectrum of the MBP-HrtR-hemin complex. Holo-MBP-HrtR exhibits a Soret band at 414 nm and β and α bands at 530 and 558 nm, respectively.