Prokaryotic Arsenic Resistance - Studies in Bacillus subtilis

Abstract: Arsenic is a toxic metalloid which is found all over the globe. Due to its toxicity and wide abundance, all living organisms have evolved intrinsic arsenic resistance systems. In this study, three proteins that provide arsenic resistance in the Gram-positive bacterium Bacillus subtilis have been investigated. Acr3 is a trans-membrane protein that extrudes arsenite to the cell exterior. It belongs to the ACR family of arsenite transporters. Experimental data on the topology of Acr3, the first ever for a member of the ACR family, is provided and show that Acr3 has 10 trans-membrane helices. Both the N- and C-terminal ends of Acr3 are located to the cytoplasm and the protein has unusually short loops connecting its helices. ArsR is an arsenite sensitive transcription regulator that controls the expression of genes encoding arsenic resistance proteins. Experiments show that the operator site for ArsR from B. subtilis consists of a 6-6-6 inverted repeat and that DNA binding by ArsR involves formation of higher order multimers of the protein. The ArsK (former YqcK) protein has an unknown function. The present study shows that the arsK gene contributes to resistance towards both arsenite and arsenate. The results indicate that the function of ArsK is important in an aerobic environment and that it decreases the inhibitory effect that arsenite has on the sporulation efficiency of B. subtilis. A function of ArsK that involves an enzymatic addition of low molecular weight thiols to arsenic is proposed.