A reduction-secretion system contributes to roxarsone (V) degradation and efflux in Brevundimonas sp. M20.

Roxarsone (V) (Rox(V)) is an organoarsenical compound that poses significant risks to aquatic ecosystems and various diseases. Reducing trivalent 3-amino-4-hydroxyphenylarsonic acid (HAPA(III)) offers a competitive advantage; however, it leads to localized arsenic contamination, which can disrupt the soil microbiome and impede plant growth. Three genes, BsntrA, arsC2, and BsexpA, encoding nitroreductase, arsenate reductase, and MFS transporter, respectively, were identified in the Rox(V)-resistant strain Brevundimonas sp. M20. A three-step approach, including nitroreduction, As(V) reduction, and HAPA(III) secretion, which is responsible for roxarsone(V) resistance, was subsequently confirmed. Moreover, the flavonoid compound baicalin occupied the HAPA(III) delivery space and grabbed the R127 residues via stronger interactions. This steric hindrance prevented the transportation of HAPA(III) by BsexpA to the extracellular space. These results demonstrate a new Rox(V) reduction pathway, providing a potential efflux pump inhibitor to trap more toxins.