New members of the ctrA regulon: the major chemotaxis operon in Caulobacter is CtrA dependent

The Caulobacter crescentus che promoter region consists of two divergent pr omoters, directing expression of the major chemotaxis operon and a novel ge ne cagA ((c) under bar hemotaxis (a) under bar ssociated (g) under bar ene (A) under bar). Analyses of start sites by primer extension and alignment o f the divergent promoters revealed significant similarities between them at the -35 promoter region. Both mcpA and cagA are differentially expressed i n the cell cycle, with maximal activation of transcription in predivisional cells. The main difference between the mcpA and cagA promoters is that, in common with the fljK flagellin, cagA is expressed in swarmer cells. A cagA -lacZ promoter fusion that contains 36 bases of untranslated mRNA has suffi cient information to segregate the lacZ transcript to swarmer cells. Expres sion of mcpA and cagA was dependent on DNA replication. Transcriptional epi stasis experiments were performed to identify potential regulators in the f lagellar hierarchy. The sigma factor RpoN, which is required for flagellar biogenesis, is not required for mcpA and cagA expression. Mutations in the genes for the MS-ring and the switch complex (flagellar class II mutants) d o not affect expression of mcpA and cagA. However, CtrA, an essential respo nse regulator of flagellar gene transcription, is required.