Fig. 9. Cell cycle control of a novel cluster of pilus biogenesis genes. (A) The Caulobacter pilus assembly gene cluster is similar to the flp–rcp–tadA region found in the human oral pathogen A.actinomycetemcomitans. The gene order and predicted function of the Caulobacter (Cc) and Actinobacillus (Aa) clusters are conserved. (B) Model for the cell cycle control of Caulobacter pilus assembly. DNA replication and temporally controlled transcription events are shown above a schematic of the cell cycle. Morphological changes are indicated below. CtrA∼P represses the initiation of DNA replication, activates the expression of the Class II flagellar (fla) genes, and late in the cell cycle, activates the transcription of the pilA gene. The genes encoding putative components of the pilus secretion machinery (cpaA–cpaF) are transcriptionally induced before the pilA gene.