Cis- and trans-acting elements required for regulation of flagellar gene transcription in the bacterium Caulobacter crescentus.

The flagellar genes of the bacterium Caulobacter crescentus are organized into a regulatory hierarchy that consists of four classes or levels of genes, and expression of these genes is restricted to a discrete interval in the cell cycle that begins just prior to flagellum assembly. This paper summarizes data on the promoters and other cis-acting elements that are required for transcription of the level II gene fliF and the level III genes flaN and flbG. Regulated expression of flaN and flbG requires sigma 54 promoters, enhancer-like sequences called ftr, and sequences called ihf that conform to the consensus binding sequence for Escherichia coli integration host factor protein. The fliF regulatory region contains a new type of promoter sequence that is different from other known promoter motifs, and it has a sequence called ftr4 that is a site of negative regulation. ftr4 appears to function as part of a developmental switch that turns fliF transcription off at the correct time in the cell cycle. flbD, the last gene in the fliF operon is a negative regulator of fliF and an activator of both flaN and flbG expression. Evidence that FlbD protein plays a direct role as a transcriptional regulator comes from the finding that it has a DNA binding activity within its carboxy terminus that specifically recognizes ftr4 in fliF and four ftr elements in the flaN-flbG promoter region.