Structure and expression of the fliA operon of Salmonella typhimurium

The fliA gene encodes the flagellum-specific sigma factor σ²⁸ in Salmonella typhimurium. The transcription in vivo and in vitro 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 σ⁷⁰-RNA polymerase in the presence of the FlhD and FlhC activator proteins. The other is a class 3 promoter which is recognized by σ²⁸-RNA polymerase. Therefore, the fliA operon is under dual positive control from FlhD/FlhC and from FliA itself. The nucleotide sequence downstream of the fliA gene was determined. The sequence contains two ORFs following the fliA gene. On the basis of their sequence homology, it is concluded that these two correspond to the fliZ and fliY genes of Escherichia coli. Northern blot analysis revealed that the fliZ gene is transcribed from the fliA promoters, whereas the fliY gene is transcribed from both the fliA promoters and its own FlhD/FlhC-independent promoter. A fliZ-disruption mutant was constructed by inserting a kanamycin-resistance gene cassette into the fliZ gene on the chromosome. The mutant showed poor motility, and introduction of a fliZ⁺ plasmid into this mutant restored the wildtype level of motility. These results suggest that the fliZ gene may be required for expression of maximal motility.