IDENTIFICATION OF THE FLII AND FLIJ COMPONENTS OF THE CAULOBACTER FLAGELLAR TYPE-III PROTEIN SECRETION SYSTEM

Caulobacter crescentus is motile by virtue of a polar flagellum assemb led during the predivisional stage of the cell cycle. Three mutant str ains in which flagellar assembly was blocked at an early stage were is olated. The mutations in these strains mapped to an operon of two gene s, fliI and fliJ, both of which are necessary for motility. fliI encod es a 50-kDa polypeptide whose sequence is closely related to that of t he Salmonella typhimurium FliI protein, an ATPase thought to energize the export of flagellar subunits across the cytoplasmic membrane throu gh a type III protein secretion system. fliJ encodes a 16-kDa hydrophi lic protein of unknown function. Epistasis experiments demonstrated th at the fliIJ operon is located in class II of the C. crescentus flagel lar regulatory hierarchy, suggesting that the gene products act at an early stage in flagellar assembly. The expression of fliIJ is induced midway through the cell cycle, coincident,vith other class II operons, but the FliI protein remains present throughout the cell cycle. Subce llular fractionation showed that FIII is present both in the cytoplasm and in association with the membrane. Mutational analysis of FliI sho wed that two highly conserved amino acid residues in a bipartite ATP b inding motif are necessary for flagellar assembly.