THE CAULOBACTER-CRESCENTUS FLBD PROTEIN ACTS AS FTR SEQUENCE ELEMENTSBOTH TO ACTIVATE AND TO REPRESS TRANSCRIPTION OF CELL-CYCLE-REGULATEDFLAGELLAR GENES

The flagellar genes (fla genes) in Caulobacter crescentus are organize d into a regulatory hierarchy of four levels, I-IV, in which transcrip tion of the class III and class IV genes late in the cell cycle from s igma(54)-dependent promoters depends on expression of the class II gen es above them. Timing of fla gene expression has been attributed to se quential activation and repression by specific transcription factors. Here we report that purified FlbD activates transcription in vitro fro m the sigma(54)-dependent class III flbG promoter and represses transc ription from the class II fliF promoter by binding to ftr (flagellar t ranscription regulator) sequence elements required for their transcrip tional regulation in vivo. The FlbD protein makes symmetrical base-spe cific contacts at three highly conserved guanine nucleotides in each h alf 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 repress ion 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 le vel III fla gene expression.