A SIGMA(54) TRANSCRIPTIONAL ACTIVATOR ALSO FUNCTIONS AS A POLE-SPECIFIC REPRESSOR IN CAULOBACTER

The differential localization of proteins in the Caulobacter predivisi onal cell leads to the formation of two distinct progeny cells: a moti le swarmer cell and a sessile stalked cell. Pole-specific transcriptio n in the predivisional cell is one mechanism responsible for protein l ocalization. Here we show that the sigma(54) transcriptional activator FlbD, which activates swarmer pole-specific transcription of a subset of late flagellar genes, is also capable of functioning as a pole-spe cific repressor of the early flagellar fliF operon. DNase I footprinti ng and methylation interference assays indicate that FlbD binds to reg ions of the fliF promoter at regions that would be likely to interfere with the binding of RNA polymerase. A mutation that abolishes FlbD bi nding results in up to a fourfold increase in fliF promoter expression . This mutation alters both the spatial and temporal pattern of fliF e xpression resulting in the inappropriate expression of the fliF operon in the swarmer pole of the predivisional cell. These results demonstr ate that FlbD represses early flagellar gene expression in the swarmer pole of the Caulobacter predivisional cell. This is the first instanc e in which a protein specifically involved in pole-specific repression has been identified in Caulobacter. The restriction of FlbD activity to the swarmer pole accomplishes two regulatory missions by simultaneo usly activating late flagellar gene expression and repressing early fl agellar genes.