Two novel regulatory genes, fliT and fliZ, in the flagellar regulon of Salmonella

The flagellar operons of Salmonella are divided into three classes with res pect to their transcriptional hierarchy Expression of the class 2 operons r equires the class 1 gene products, FlhD and FlhC, and is increased by mutat ion in the flgM gene, which encodes a class S-specific anti-sigma factor. H ere we report the identification of two novel regulatory genes for class 2 transcription. Presence of the fliZ and fliT genes on multicopy plasmids en hanced and inhibited, respectively, transcription from a chromosomal class 2 promoter. Disruption of the fliZ and fliT genes on the chromosome decreas ed and increased, respectively, class 2 expression. These results suggest t hat the fliZ and fliT genes may encode positive and negative regulatory fac tors, respectively, for class 2 expression. Enhancement of class 2 expressi on by the flgM mutation was cancelled by the coexisting fliZ mutation, indi cating that FliZ is essential for this enhancement.