FIGURE 4.

Bacterial resistance to arsenic. (As) enter the bacterial cell using Phosphate-Specific Transporters (Pst) and Type III Transporters (PiT) in the case of As⁵⁺ and Aquaglycerolporins in the case of As³⁺ (Paez-Espino et al., 2009). The ars operon harbors 3 co-transcribed core genes that confer resistance not only to As³⁺ and As⁵⁺, but also to Antimony (Sb³⁺). arsR encodes a Transcriptional Repressor, arsC encodes a Cytoplasmic Arsenate Reductase, and arsB encodes a membrane bound Arsenite Efflux Pump. Two additional genes may be present within the ars operon, arsA and arsD. The former encodes an intracellular ATPase which binds ArsB to form an ArsA-ArsB ATPase Efflux Pump, while the latter is a metallochaperone that binds and delivers As³⁺ and Sb³⁺ to ArsA-ArsB complex for efflux, in addition to its role as a trans-activating co-repressor of the ars operon along with ArsR (Silver and Phung le, 2005; Paez-Espino et al., 2009; Hobman and Crossman, 2014). Moreover, some microorganisms escape As toxicity by methylation thus, leading to the production of less toxic and volatile derivatives that diffuse outside the bacterial cell (Paez-Espino et al., 2009). Besides As toxicity, bacteria belonging to the Shewanella spp., Sulfurospirillum spp., Clostridium spp., and Bacillus spp., use As⁵⁺ as a final electron acceptor during anaerobic respiration by reducing it to As³⁺, while other bacteria use As³⁺ as an electron donor and oxidize it to As⁵⁺ during aerobic oxidation (Paez-Espino et al., 2009). The oxidation/reduction processes are mediated by the Respiratory Arsenate Reductase and Respiratory Arsenite Oxidase that are encoded by the arrAB operon and asoAB genes respectively (Silver and Phung le, 2005; Paez-Espino et al., 2009). Adapted and modified with permission from Paez-Espino et al. (2009).