THE CONTROL OF ASYMMETRIC GENE-EXPRESSION DURING CAULOBACTER CELL-DIFFERENTIATION

The dimorphic bacterium Caulobacter crescentus provides a simple model for cellular differentiation. Each cell division produces two distinc t cell types: a swarmer cell and a stalked cell. These cells possess d istinct functional morphologies and differential programs of transcrip tion and DNA replication. The synthesis of a single polar flagellum is restricted to the swarmer pole of the predivisional cell by a genetic hierarchy comprising at least 50 genes whose transcription is regulat ed by novel and ubiquitous promoters, cognate sigma factors, and auxil iary transcriptional regulators. Chromosome replication is restricted to the stalked cell by a unique chromosome origin of replication that may be regulated by a novel cell-specific transcriptional control syst em. Phosphorylation signals, DNA methylation, differential chromosome structures, protein targeting, and selective protein degradation are a lso involved in establishing and maintaining cellular asymmetry. The m olecular details of these universal cellular processes in C. crescentu s will provide paradigms applicable to many general aspects of cellula r differentiation.