The \u3ci\u3eCaulobacter crescentus\u3c/i\u3e FlbD protein acts at \u3ci\u3eftr\u3c/i\u3e sequence elements both to activate and to repress transcription of cell cycle-regulated flagellar genes

The flagellar genes (fla genes) in Caulobacter crescentus are categorized into a regulatory hierarchy of four levels, I-IV, in which transcription of the class III and class IV genes late in the cell cycle from σ54-dependent promoters depends on expression of the class II genes above them. Timing of fla gene expression has been attributed to sequential activation and repression by specific transcription factors. Here we report that purified FlbD activates transcription in vitro from the σ54-dependent class III flbG promoter and represses transcription from the class II fliF promoter by binding to ftr (flagellar transcription regulator) sequence elements required for their transcriptional regulation in vivo. The FlbD protein makes symmetrical base-specific contacts at three highly conserved guanine nucleotides in each half site of ftr1 and ftr1* at flbG and the single ftr4 site at fliF. The dual function of FlbD in activation of class III genes and repression of the class II fliF promoter is consistent with a central role of FlbD as a switch protein mediating the transition from level II to level III fla gene expression