Molecular genetics of the flgI region and its role in flagellum biosynthesis in Caulobacter crescentus.

The differentiating bacterium Caulobacter crescentus has been studied extensively to understand how a relatively simple life form can govern the timing of expression of genes needed for the production of stage-specific structures. In this study, a clone containing the 5.3-kb flaP region was shown to contain the flgI, cheL, and flbY genes arranged in an operon with transcription proceeding from flgI to flbY. The predicted flgI polypeptide shows remarkable identity (44%) to the flagellar basal body P-ring protein encoded by the flgI gene of Salmonella typhimurium. flgI mutations case a reduction in the levels of flagellin production and the overproduction of the hook proteins. Therefore, the flgI-encoded P-ring protein is required for normal flagellin and hook protein synthesis, suggesting that basal body assembly may play a role in the regulation of flagellar gene expression. The flbY gene probably is a basal body component as well, since flbY mutants have flagellin and hook protein synthesis patterns similar to those exhibited by other basal body mutants. The smaller cheL gene complements a mutant that is unable to respond to chemotactic signals despite possessing a functional flagellum. This is the first example of an operon containing both flagellar and chemotaxis genes in C. crescentus.