The gene encodes the flagellum-specific sigma factor σ in The transcription and of this gene was analysed and it was found that there are two promoters for the expression of this gene One is a class 2 promoter which is recognized by σ-RNA polymerase in the presence of the FlhD and FlhC activator proteins. The other is a class 3 promoter which is recognized by σ-RNA polymerase. Therefore, the operon is under dual positive control from FlhD/FlhC and from FliA itself. The nucleotide sequence downstream of the gene was determined. The sequence contains two ORFs following the gene. On the basis of their sequence homology, it is concluded that these two correspond to the and genes of Northern blot analysis revealed that the gene is transcribed from the promoters, whereas the gene is transcribed from both the promoters and its own FlhD/FlhC-independent promoter. A -disruption mutant was constructed by inserting a kanamycin-resistance gene cassette into the gene on the chromosome. The mutant showed poor motility, and introduction of a plasmid into this mutant restored the wild type level of motility. These results suggest that the gene may be required for expression of maximal motility.


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