An ArsR/SmtB Family Member Is Involved in the Regulation by Arsenic of the Arsenite Oxidase Operon in Thiomonas arsenitoxydans

The genetic organization of theaioBA operon, encoding the arsenite oxidase of the moderately acidophilic and facultative chemoautotrophic bacteriumThiomonas arsenitoxydans , is different from that of theaioBA operon in the other arsenite oxidizers, in that it encodes AioF, a metalloprotein belonging to the ArsR/SmtB family. AioF is stabilized by arsenite, arsenate, or antimonite but not molybdate. Arsenic is tightly attached to AioF, likely by cysteine residues. When loaded with arsenite or arsenate, AioF is able to bind specifically to the regulatory region of theaio operon at two distinct positions. InThiomonas arsenitoxydans , the promoters ofaioX andaioB are convergent, suggesting that transcriptional interference occurs. These results indicate that the regulation of theaioBA operon is more complex inThiomonas arsenitoxydans than in the otheraioBA containing arsenite oxidizers and that the arsenic binding protein AioF is involved in this regulation. On the basis of these data, a model to explain the tight control ofaioBA expression by arsenic inThiomonas arsenitoxydans is proposed.