An ArsRC fusion protein enhances arsenate sensing and detoxification

Arsenical resistance (ars) operons encode genes for arsenic resistance and biotransformation. The majority are composed of individual genes, but fusion of ars genes is not uncommon, although it is not clear if the fused gene products are functional. Here we report identification of a four-gene ars operon from Paracoccus sp. SY that has two arsR-arsC gene fusions. ArsRC1 and ArsRC2 are related proteins that consist of an N-terminal ArsR arsenite (As(III))-responsive repressor with a C-terminal ArsC arsenate reductase. The other two genes in the operon are gapdh and arsJ. GAPDH, glyceraldehyde 3-phosphate dehydrogenase, forms 1-arseno-3-phosphoglycerate (1As3PGA) from 3-phosphoglyceraldehyde and arsenate (As(V)), ArsJ is an efflux permease for 1As3PGA that dissociates into extracellular As(V) and 3-phosphoglycerate. The net effect is As(V) extrusion and resistance. ArsRs are usually selective for As(III) and do not respond to As(V). However, the substrates and products of this operon are pentavalent, which would not be inducers of the operon. We propose that ArsRC fusions overcome this limitation by channelling the ArsC product into the ArsR binding site without diffusion through the cytosol, a de facto mechanism for As(V) induction. This novel mechanism for arsenate sensing can confer an evolutionary advantage for detoxification of inorganic arsenate.