The SigB s Factor Regulates Multiple Salt Acclimation Responses of the Cyanobacterium Synechocystis

Changing of principal s factor in RNA polymerase holoenzyme to a group 2 s factor redirects transcription when cyanobacteria acclimate to suboptimal environmental conditions. The group 2 sigma factor SigB 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 43C although the expression of the sigB gene was similarly highly, but only transiently up-regulated 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 inactivation strain DsigB than in the control strain. Addition of the compatible solute trehalose almost completely restored the growth of the DsigB 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 up-regulated in the control strain. These carotenoids were up-regulated in the DsigCDE strain (SigB is the only functional group 2 s factor) and down-regulated in the DsigB strain under standard conditions. In addition, the HspA heat shock protein was less abundant and more abundant in the DsigB and DsigCDE strains, respectively, than in the control strain in high-salt conditions. Some cellular responses are common to heat and salt stresses, but pretreatment with mild heat did not protect cells against salt shock although protection against heat shock was evident. Different cyanobacterial species have adapted to ecological niches varying from fresh water to hypersaline environments. Research of salt acclimation is important because large-scale cultivation of cyanobacteria, for example for bioenergy production, would best occur in brackish water or seawater, as fresh water supplies of the Earth are limited. Salt acclimation processes in cyanobacteria are already fairly well known (for recent review, see Hagemann, 2011) but the sensing and transmission of salt signals are not yet well understood. Salt stress acclimation and signaling processes are best characterized in a model cyanobacterium Synechocystis sp. strain PCC 6803 (hereafter Synechocystis). Synechocystis is a unicellular, nontoxic, nonnitrogenfixing freshwater cyanobacterium that is moderately