The gyr genes of Salmonella enterica serovar Typhimurium are repressed by the factor for inversion stimulation, Fis

The DNA sequence of the gyr genes from Salmonella enterica serovar Typhimurium revealed strong similarity between gyrB and its counterpart in Escherichia coli. However, the gyrA gene showed similarity to the E. coli homologue only downstream from the Pribnow box of the promoter, with the sequence upstream diverging markedly. Since this region encompasses the binding sites for the Fis DNA binding protein in E. coli, we investigated the possibility that the gyrA genes in the two species might differ in their responses to this regulatory protein. Fis was found to act as a transcriptional repressor of both gyr genes in S. enterica. In electrophoretic mobility shift assays, Fis was found to bind to both the gyrA and gyrB promoters of S. enterica, despite the strong divergence from the E. coli sequence on the part of the former. The binding sites were mapped by DNase I protection assays, and the results are consistent with conservation of the mechanism of Fis-mediated repression between the two bacterial species.