The ChrA homologue from a sulfur-regulated gene cluster in cyanobacterial plasmid pANL confers chromate resistance

The cyanobacterium Synechococcus elongatus strain PCC 7942 possesses pANL, a plasmid rich in genes related to sulfur metabolism. One of these genes, srpC, encodes the SrpC protein, a homologue of the CHR chromate ion transporter superfamily. The srpC gene was cloned and expressed in Escherichia coli and its role in relation to sulfate and chromate was analyzed. srpC was unable to complement the growth of an E. coli cysA sulfate uptake mutant when sulfate was utilized as a sole sulfur source, suggesting that SrpC is not a sulfate transporter. Expression of srpC in E. coli conferred chromate resistance and caused diminished chromate uptake. These results suggest that the S. elongatus SrpC protein functions as a transporter that extrudes chromate ions from the cell's cytoplasm, and further demonstrate the close relationship between sulfate and chromate metabolism in this organism.