Activation of a temporally regulated Caulobacter promoter by upstream and downstream sequence elements

The flagellar genes of Caulobacter crescentus are expressed under cell-cycle control. Expression is regulated by both flagellar assembly cues and cell-cycle events. In this paper we define the sequences required for the expression of the flgF operon, a new class of sigma 54 flagellar promoter. This promoter type is expressed in the middle portion of the cell cycle and regulates the expression of basal-body genes. DNase I footprinting and mutagenesis demonstrates that an integration host factor (IHF)-binding site is required for maximal levels of transcription of the flgF promoter. In addition to containing a conventional upstream enhancer element (RE-1), this promoter is unusual in that it also requires sequences (element RE-2) immediately downstream of the transcriptional start site for maximal levels of gene expression. Cell-cycle experiments indicate that RE-1 and RE-2 contribute equally to the regulation of temporal transcription. The presence of two intact elements in the promoter results in a fourfold increase in promoter activity compared with a promoter containing only one intact element, suggesting that these two elements may function synergistically to activate transcription.