Roles of Close Homologues SigB and SigD in Heat and High Light Acclimation of the Cyanobacterium Synechocystis sp. PCC 6803

Acclimation of cyanobacterium Synechocystis sp. PCC6803 to suboptimal conditions is largely dependent on adjustments of gene expression, which is highly controlled by the sigma factor subunits of RNA polymerase (RNAP). The SigB and SigD sigma factors are close homologues. Here we show that the sigB and sigD genes are both induced in high light and heat stresses. Comparison of transcriptomes of the control strain (CS), Delta sigB, Delta sigD, Delta sigBCE (containing SigD as the only functional group 2 sigma factor), and Delta sigCDE (SigB as the only functional group 2 sigma factor) strains in standard, high light, and high temperature conditions revealed that the SigB and SigD factors regulate different sets of genes and SigB and SigD regulons are highly dependent on stress conditions. The SigB regulon is bigger than the SigD regulon at high temperature, whereas, in high light, the SigD regulon is bigger than the SigB regulon. Furthermore, our results show that favoring the SigB or SigD factor by deleting other group 2 sigma factors does not lead to superior acclimation to high light or high temperature, indicating that all group 2 sigma factors play roles in the acclimation processes.