Flagellar Biogenesis of Xanthomonas campestris Requires the Alternative Sigma Factors RpoN2 and FliA and Is Temporally Regulated by FlhA, FlhB, and FlgM

In prokaryotes, flagellar biogenesis is a complicated process involving over 40 genes. The phytopathogen Xanthomonas campestris pv. campestris possesses a single polar flagellum, which is essential for the swimming motility. A sigma(54) activator, FleQ, has been shown to be required for the transcriptional activation of the flagellar type III secretion system (F-T3SS), rod, and hook proteins. One of the two rpoN genes, rpoN2, encoding sigma(54), is essential for flagellation. RpoN2 and FleQ direct the expression of a second alternative sigma FliA (sigma(28)) that is essential for the expression of the flagellin FliC. FlgM interacts with FliA and represses the FliA regulons. An flgM mutant overexpressing FliC generates a deformed flagellum and displays an abnormal motility. Mutation in the two structural genes of F-T3SS, flhA and flhB, suppresses the production of FliC. Furthermore, FliA protein levels are decreased in an flhB mutant. A mutant defective in flhA, but not flhB, exhibits a decreased infection rate. In conclusion, the flagellar biogenesis of Xanthomonas campestris requires alternative sigma factors RpoN2 and FliA and is temporally regulated by FlhA, FlhB, and FlgM.