Regulation of the Caulobacter flagellar gene hierarchy; not just for motility

The Caulobacter crescentus flagellum serves not only as a motility apparatus, but also as a key landmark in the differentiation of this asymmetrically dividing bacterium. A distinctive aspect of flagellum biosynthesis is the periodic expression of the flagellar genes during the cell cycle in a sequence corresponding to the order of gene product assembly into the growing flagellum. This program of gene expression is achieved in part by the organization of flagellar genes into a four-tiered regulatory hierarchy that controls their expression at both the transcriptional and post-transcriptional levels. Because of the close interconnection of the developmental program to the asymmetric cell-division cycle in C. crescentus, studies of flagellar gene regulation and motility have also begun to reveal basic mechanisms responsible for control of the cell cycle itself. Here, we review recent work on regulation of the flagellar gene hierarchy in C. crescentus and consider regulatory mechanisms that are distinct from those described in Escherichia coli and Salmonella typhimurium.