Arsenic is a toxic element prevalent in the environment since the origin of life on Earth. Bacteria evolved in an arsenic-rich environment, where they developed ways to both overcome arsenic toxicity and harness it to compete with other organisms. These mechanisms include chemical modifications (e.g. oxidation, methylation), degradation, and efflux. The goal of this dissertation is to better characterize these mechanisms, illuminating the arsenic biogeocycle and allowing us to harness organoarsenical toxicity for novel antibiotics. A goal of my research was to elucidate the antibiotic properties of MAs(III), which is synthesized by bacteria to thrive over other bacteria, by identifying cellular targets involved in its toxicity. I identified MurA, a key enzyme in bacterial cell wall synthesis, as a potential target. I determined that MurA is inhibited by organoarsenicals, but not by inorganic As(III), suggesting that these antibiotics kill bacteria by inhibiting cell wall formation. I also determined that MAs(III) inhibits MurA differently than fosfomycin, the conventional MurA-inhibitory antibiotic, highlighting the potential that organoarsenicals have as antibiotics. The most efficient bacterial resistance mechanism to MAs(III) is efflux from cells, catalyzed by the ArsP permease. While ArsP can transport aromatic organoarsenicals like trivalent roxarsone (Rox(III)), it transports MAs(III) most efficiently. To further characterize ArsP, I probed its substrate binding site by mutagenesis. I found that several conserved residues hypothesized to be required for transport are in fact not required for resistance to organoarsenicals. Furthermore, I determined that the higher affinity of Rox(III) for cysteine pairs does not play a role in selectivity for MAs(III). Aromatic arsenicals are widely used as growth promoters in animal husbandry and understanding their degradation to As(III) by soil bacteria is critical to minimizing their environmental hazards. We characterized their biotransformation by Sinorhizobium meliloti, and identified an enzyme, MdaB, that catalyzes the first step of aromatic organoarsenical degradation. Overall, we show how S. meliloti can “activate” benign pentavalent aromatic arsenicals into more toxic species. Characterizing enzymes and transporters involved in organoarsenical toxicity and resistance will help us design novel arsenic bioremediation strategies, as well as develop effective arsenic-based antibiotics that will contribute to human health and safety.
