Growth Phase- and Cell Division-Dependent Activation and Inactivation of the σ32 Regulon in Escherichia coli▿ †

摘要

Alternative sigma factors allow bacteria to reprogram global transcription rapidly and to adapt to changes in the environment. Here we report on growth- and cell division-dependent σ32 regulon activity in Escherichia coli in batch culture. By analyzing σ32 expression in growing cells, an increase in σ32 protein levels is observed during the first round of cell division after exit from stationary phase. Increased σ32 protein levels result from transcriptional activation of the rpoH gene. After the first round of bulk cell division, rpoH transcript levels and σ32 protein levels decrease again. The late-logarithmic phase and the transition to stationary phase are accompanied by a second increase in σ32 levels and enhanced stability of σ32 protein but not by enhanced transcription of rpoH. Throughout growth, σ32 target genes show expression patterns consistent with oscillating σ32 protein levels. However, during the transition to early-stationary phase, despite high σ32 protein levels, the transcription of σ32 target genes is downregulated, suggesting functional inactivation of σ32. It is deduced from these data that there may be a link between σ32 regulon activity and cell division events. Further support for this hypothesis is provided by the observation that in cells in which FtsZ is depleted, σ32 regulon activation is suppressed.