The SigB σ Factor Regulates Multiple Salt Acclimation Responses of the Cyanobacterium Synechocystis sp. PCC 6803

Changing of principal sigma (σ) factor in RNA polymerase holoenzyme to a group 2 σ factor redirects transcription when cyanobacteria acclimate to suboptimal environmental conditions. The SigB factor was found to be important for the growth of the cyanobacterium Synechocystis sp. PCC 6803 in high salt (0.7 M NaCl) stress but not in mild heat stress at 43°C although the expression of the sigB gene was similarly highly, but only transiently, upregulated at both conditions. The SigB factor was found to regulate many salt acclimation processes. The amount of glucosylglycerol-phosphate synthase, a key enzyme in the production of the compatible solute glucosylglycerol, was lower in the ∆sigB strain than in the control strain. Addition of the compatible solute trehalose almost completely restored the growth of the ∆sigB strain at 0.7 M NaCl. High salt conditions lowered the chlorophyll and phycobilin contents of the cells while protective carotenoid pigments, especially zeaxanthin and myxoxanthophyll, were upregulated in the control strain. These carotenoids were up-regulated in the ∆sigCDE strain (SigB is the only functional group 2 σ factor) and down-regulated in the ∆sigB strain under standard conditions. In addition, the HspA heat shock protein was less abundant and more abundant in the ∆sigB and ∆sigCDE strains, respectively, than in the control strain in high salt conditions. Some cellular responses are common to heat and salt stresses, but pre-treatment with mild heat did not protect cells against salt shock although protection against heat shock was evident.