A temporally controlled σ-factor is required for polar morphogenesis and normal cell division in Caulobacter

Abstract

The transcription of many spatially and temporally controlled flagellar structural genes in Caulobacter requires the RNA polymerase σ54 subunit. Like flagellar biogenesis, stalk formation is an asymmetric polar morphogenesis that occurs once each cell cycle in response to internal cell cycle signals. We have isolated the σ54 gene (rpoN) and describe here a novel role for this alternative σ-factor in cell differentiation: It is required for the biogenesis of both polar structures, and the disruption of the rpoN gene results in aberrant cell division. Surprisingly, the transcription of rpoN is temporally regulated during the cell cycle; it increases 10-fold commensurate with stalk formation and just before the onset of flagellar gene expression. These results suggest that σ54 abundance responds to cell cycle cues and is involved in the global timing of the central events of Caulobacter development, whereas the transcriptional activators of σ54-dependent promoters are responsible for the refined control of the expression of individual or small groups of genes required for each specific event.