Resistance to arsenic compounds in microorganisms

Abstract: Arsenic ions, frequently present as environmental pollutants, are very toxic fl~r most microorganisms. Some microbial strains possess genetic determinants hat confer resistance. In bacteria, these determinants are often found on plasmids, which has facilitated their study at the molecular level. Bacterial plasmids conferring arsenic resistance encode specific effiux pumps able to extrude arsenic from the cell cytoplasm thus lowering the intracellular concentration of the toxic ions. In Gram-negative bacteria, the effiux pump consists of a two-component ATPase complex. ArsA is the ATPase subunit and is associated with an integral membrane subunit, ArsB. Arsenate is enzymatically reduced to arsenite (the substrate of ArsB and the activator of ArsA) by the small cytoplasmic ArsC polypeptide. In Gram-positive bacteria, comparable arsB and arsC genes (and proteins) are found, but arsA is missing. In addition to the wide spread plasmid arsenic resistance determinant, a few bacteria confer resistance to arsenitc with a separate determinant for enzymatic oxidation of more-toxic arsenite to less-toxic arsenate. In contrasl to the detailed information on the mechanisms of arsenic resistance in bacteria, little work has been reported on this subject in algae and fungi