A MUTATION THAT UNCOUPLES FLAGELLUM ASSEMBLY FROM TRANSCRIPTION ALTERS THE TEMPORAL PATTERN OF FLAGELLAR GENE-EXPRESSION IN CAULOBACTER-CRESCENTUS

The transcription of flagellar genes in Caulobacter crescentus is regu lated by cell cycle events that culminate in the synthesis of a new fl agellum once every cell division. Early flagellar gene products regula te the expression of late flagellar genes at two distinct stages of th e flagellar trans-acting hierarchy. Here we investigate the coupling o f early flagellar biogenesis with middle and late flagellar gene expre ssion. We have isolated mutants (bfa) that do not require early class II flagellar gene products for the transcription of middle or late fla gellar genes. bfa mutant strains are apparently defective in a negativ e regulatory pathway that couples early flagellar biogenesis to late f lagellar gene expression. The bfa regulatory pathway functions solely at the level of transcription. Although flagellin promoters are transc ribed in class II/bfa double mutants, there is no detectable flagellin protein on immunoblots prepared from mutant cell extracts. This findi ng suggests that early flagellar biogenesis is coupled to gene express ion by two distinct mechanisms: one that negatively regulates transcri ption, mediated by bfa, and another that functions posttranscriptional ly. To determine whether bfa affects the temporal pattern of late flag ellar gene expression, cell cycle experiments were performed in bfa mu tant strains. In a bfa mutant strain, flagellin expression fails to sh ut off at its normal time in the cell division cycle. This experimenta l result indicates that bfa may function as a regulator of flagellar g ene transcription late in the cell cycle, after early flagellar struct ures have been assembled.