Role of the flaR gene in flagellar hook formation in Salmonella spp.

Flagellar filaments were reconstituted by polymerization with exogenously supplied flagellin monomers at the tips of normal hooks on Salmonella cells which were missing the filaments because of mutations in either the flaL or flaU gene or the flagellin genes H1 and H2. Reconstitution did not occur at the tips of polyhooks of the flaR mutant cells. Thus, the absence of flagellar filaments in the flaR mutant cells was probably caused by the inability of the polyhooks to work as polymerization nuclei for flagellin. A Phf+ mutant which produced polyhooks with flagellar filaments was isolated from a flaR polyhook mutant. Genetic analysis of the Phf+ mutant showed that it carried an intracistronic suppressor mutation of the original flaR mutation. This result indicated that the flaR gene regulates hook length and initiates flagellin formation.