FliT acts as an anti-FlhD₂C₂ factor in the transcriptional control of the flagellar regulon in Salmonella enterica serovar typhimurium

Flagellar operons are divided into three classes with respect to their transcriptional hierarchy in Salmonella enterica serovar Typhimurium. The class 1 gene products FlhD and FlhC act together in an FlhD₂C₂ heterotetramer, which binds upstream of the class 2 promoters to facilitate binding of RNA polymerase. Class 2 expression is known to be enhanced by a disruption mutation in a flagellar gene, fliT. In this study, we purified FliT protein in a His-tagged form and showed that the protein prevented binding of FlhD₂C₂ to the class 2 promoter and inhibited FlhD ₂C₂-dependent transcription. Pull-down and far-Western blotting analyses revealed that the FliT protein was capable of binding to FlhD₂C₂ and FlhC and not to FlhD alone. We conclude that FliT acts as an anti-FlhD₂C₂ factor, which binds to FlhD₂C₂ through interaction with the FlhC subunit and inhibits its binding to the class 2 promoter.