ACTIVATION OF A TEMPORALLY REGULATED CAULOBACTER PROMOTER BY UPSTREAMAND DOWNSTREAM SEQUENCE ELEMENTS

The flagellar genes of Caulobacter crescentus are expressed under cell -cycle control. Expression is regulated by both flagellar assembly cue s and cell-cycle events. In this paper we define the sequences require d for the expression of the flgF operon, a new class of sigma(54) flag ellar promoter. This promoter type is expressed in the middle portion of the cell cycle and regulates the expression of basal-body genes. DN ase I footprinting and mutagenesis demonstrates that an integration ho st factor (IHF)-binding site is required for maximal levels of transcr iption 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 t ranscriptional start site for maximal levels of gene expression. Cell- cycle experiments indicate that RE-1 and RE-2 contribute equally to th e regulation of temporal transcription. The presence of two intact ele ments in the promoter results in a fourfold increase in promoter activ ity compared with a promoter containing only one intact element, sugge sting that these two elements may function synergistically to activate transcription.