Flagella expression in Escherichia coli is controlled in a hierarchical manner, in which class-1 gene products, FlhDC, functions as a master regulator to control class-2 genes that encode motility-related genes. fliA, one of the class 2 genes, encodes flagellum-specific sigma factor (FliA/Sigma F/Sigma-28), which is necessary for the expression of class-3 genes. Previously, we carried out transcriptome analyses of all two-component regulatory systems of E. coli, and determined that the arcA mutant showed the motility-defective phenotype. In this study, we characterized the arcA mutant, and we present evidence that ArcA is necessary for the expression of FliA, but not for the master regulators, FlhDC. The phosphorylation site of ArcA is necessary for motility, while a cognate histidine kinase, ArcB, appears not to be involved in motility. This suggests that there must be regulatory factors other than ArcB interacting with ArcA to control flagella genes.

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