FIGURE 2.

MAs(III): a primordial antibiotic. In communities of soil microbes some bacteria such as Rhodopseudomonas palustris carry the arsM gene for the As(III) SAM methyltransferase, producing highly toxic MAs(III). This trivalent organoarsenical has antibiotic-like properties. Other soil bacteria carry genes for MAs(III) resistance. Some, such as Bacillus MD1, have the arsI gene for the ArsI C-As lyase enzyme that confers resistance to MAs(III) by degrading it into As(III) and formaldehyde. Yet other soil bacteria such as Pseudomonas putida have a gene encoding ArsH, a flavoprotein that uses NADP+ to oxidize MAs(III) to MAs(V), thus conferring resistance. Finally, other bacteria such as Campylobacter jejuni, which inhabits the intestinal track of poultry and other farm animals, carry the arsP gene. ArsP is a MAs(III) efflux permease that extrudes trivalent organoarsenicals from cells, conferring resistance. The crystal structures of the relevant enzymes are shown next to their reactions (Li et al., 2016b).