Looking for the Mechanism of Arsenate Respiration of Sp. Strain 3D3, Independent of ArrAB

The literature has reported the isolation of arsenate-dependent growing microorganisms which lack a canonical homolog for respiratory arsenate reductase, ArrAB. We recently isolated an arsenate-dependent growing bacterium from volcanic arsenic-bearing environments in Northern Chile, sp. strain 3D3 () and studied the arsenic metabolism in this Gram-positive isolate. Features of deduced from genome analysis and comparative analysis with other arsenate-reducing microorganisms revealed the lack of ArrAB coding genes and the occurrence of two genes encoding for putative cytoplasmic arsenate reductases named ArsC-1 and ArsC-2. Interestingly, ArsC-1 and ArsC-2 belong to the thioredoxin-coupled family (because of the redox-active disulfide protein used as reductant), but they conferred differential arsenate resistance to the WC3110 Δ strain. PCR experiments confirmed the absence of genes and results obtained using uncouplers revealed that growth is linked to the proton gradient. In addition, harbors ferredoxin-NAD oxidoreductase (Rnf) and electron transfer flavoprotein () coding genes. These are key molecular markers of a recently discovered flavin-based electron bifurcation mechanism involved in energy conservation, mainly in anaerobic metabolisms regulated by the cellular redox state and mostly associated with cytoplasmic enzyme complexes. At least three electron-bifurcating flavoenzyme complexes were evidenced in , some of them shared in conserved genomic regions by other members of the genus. These physiological and genomic findings permit us to hypothesize the existence of an uncharacterized arsenate-dependent growth metabolism regulated by the cellular redox state in the genus.