The Fur-Iron Complex Modulates Expression of the Quorum-Sensing Master Regulator, SmcR, To Control Expression of Virulence Factors in Vibrio vulnificus

The gene vvpE, encoding the virulence factor elastase, is a member of the quorum-sensing regulon in Vibrio vulnificus and displays enhanced expression at high cell density. We observed that this gene was repressed under iron-rich conditions and that the repression was due to a Fur (ferric uptake regulator)-dependent repression of smcR, a gene encoding a quorum-sensing master regulator with similarity to luxR in Vibrio harveyi. A gel mobility shift assay and a footprinting experiment demonstrated that the Fur-iron complex binds directly to two regions upstream of smcR (-82 to -36 and -2 to +27, with respect to the transcription start site) with differing affinities. However, binding of the Fur-iron complex is reversible enough to allow expression of smcR to be induced by quorum sensing at high cell density under iron-rich conditions. Under iron-limiting conditions, Fur fails to bind either region and the expression of smcR is regulated solely by quorum sensing. These results suggest that two biologically important environmental signals, iron and quorum sensing, converge to direct the expression of smcR, which then coordinates the expression of virulence factors.