Characterization of the alternative σ-factors SigD and SigE in Synechococcus sp. strain PCC 7002. SigE is implicated in transcription of post-exponential-phase-specific genes

The sigD and sigE genes, which encode two alternative σ-factors from the unicellular marine cyanobacterium Synechococcus sp. PCC 7002, were cloned and characterized. Strains in which the sigD and sigE genes were insertionally inactivated were viable under standard laboratory conditions, indicating that SigD and SigE are group 2 σ-factors. When stationary-phase cells were diluted into fresh growth medium, it was observed that the sigE mutant strain required longer times to re-establish exponential growth than the wild-type strain. By monitoring the growth rates in such dilution experiments, it was observed that the lag times for the mutant strain became progressively longer as the original cultures progressed towards stationary phase. Transcripts for the sigE gene initially increased and subsequently decreased as cells grew further into stationary phase. It was determined that a functional SigE protein is required for the expression of the starvation-induced protein DpsA/PexB. The results suggest that SigE is involved in the transcription of genes specifically expressed in the post-exponential phase.