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An unusual promoter controls cell‐cycle regulation and dependence on DNA replication of the Caulobacter fliLM early flagellar operon

Transcription of flagellar genes in Caulobacter crescentus is programmed to occur during the predivisional stage of the cell cycle. The mechanism of activation of Class II flagellar genes, the highest identified genes in the Caulobacter flagellar hierarchy, is unknown. As a step toward understanding this process, we have defined cis‐acting sequences necessary for expression of a Class II flagellar operon, fliLM. Deletion analysis indicated that a 55 bp DNA fragment was sufficient for normal, temporally regulated promoter activity. Transcription from this promoter‐containing fragment was severely reduced when chromosomal DNA replication was inhibited. Extensive mutational analysis of the promoter region from ‐42 to ‐5 identified functionally important nucleotides at ‐36 and ‐35, between ‐29 and ‐22 and at ‐12, which correlates well with sequences conserved between fliLM and the analogous regions of two other Class II flagellar operons. The promoter sequence does not resemble that recognized by any known bacterial sigma factor. Models for regulation of Caulobacter early flagellar promoters are discussed in which RNA polymerase containing a novel Sigma subunit interacts with an activation factor bound to the central region of the promoter.