Identification of a novel Escherichia coli gene whose expression is dependent on the flagellum-specific sigma factor, FliA, but dispensable for motility development

FliA is an alternative sigma factor specific for class 3 flagellar operons. Using a promoter-probe vector, we randomly cloned Escherichia coli DNA fragments, which showed FliA-dependent promoter activities. Among the DNA fragments cloned, one was found to be derived from a non-flagellar region. Hybridization analysis with the Kohara E. coli library indicated that this DNA fragment is located at around 35.4 min on the E. coli chromosome where no flagellar gene has been reported yet. DNA sequence analysis revealed that it contains an FliA-dependent promoter-like sequence followed by an open reading frame (ORF) that can encode a 110-amino-acid protein. A rho-independent terminator-like sequence follows this ORF. This putative gene was named flxA. A gene disruptant was constructed by inserting the kan gene cassette into the flxA gene on the chromosome. This mutant was found to be actively motile, suggesting that this gene is unlikely to be involved in the motility phenotype of E. coli.