REGULATION OF CELLULAR-DIFFERENTIATION IN CAULOBACTER-CRESCENTUS

In Caulobacter crescentus, asymmetry is generated in the predivisional cell, resulting in the formation of two distinct cell types upon cell division: a motile swarmer cell and a sessile stalked cell. These pro geny cell types differ in their relative programs of gene expression a nd DNA replication. In progeny swarmer cells, DNA replication is silen ced for a defined period, but stalked cells reinitiate chromosomal DNA replication immediately following cell division. The establishment of these differential programs of DNA replication may be due to the pola r localization of DNA replication proteins, differences in chromosome higher-order structure, or pole-specific transcription. The best-under stood aspect of Caulobacter development is biogenesis of the polar fla gellum. The genes encoding the flagellum are expressed under cell cycl e control predominantly in the predivisional cell type. Transcription of flagellar genes is regulated by a trans-acting hierarchy that respo nds to both flagellar assembly and cell cycle cues. As the flagellar g enes are expressed, their products are targeted to the swarmer pole of the predivisional cell, where assembly occurs. Specific protein targe ting and compartmentalized transcription are two mechanisms that contr ibute to the positioning of flagellar gene products at the swarmer pol e of the predivisional cell.