Phosphatidylglycerol (PG) is a ubiquitous component of thylakoid membranes. Experiments with the pgsA mutant of the cyanobacterium Synechocystis sp. PCC6803 defective in biosynthesis of PG have demonstrated an indispensable role of PG in photosynthesis. In the present study, we have investigated the light susceptibility of the pgsA mutant with regard to the maintenance of the photosynthetic machinery. Growth of the mutant cells without PG increased the light susceptibility of the cells and resulted in severe photoinhibition of photosynthesis upon a high-light treatment, whereas the growth in the presence of PG was protected against photoinhibition. Photoinhibition induced by PG deprivation was mainly caused by an impairment of the restoration process. The primary target of the light-induced damage in thylakoid membranes, the D1 protein of photosystem (PS) II was, however, synthesized and degraded with similar rates irrespective of whether the mutant cells were incubated with PG or not. Intriguingly, it was found that instead of the synthesis of the D1 protein, the dimerization of the PSII core monomers was impaired in the PG-deprived mutant cells. Addition of PG to photoinhibited cells restored the dimerization capacity of PSII core monomers. These results suggest that PG plays an important role in the maintenance of the photosynthetic machinery through the dimerization and reactivation of the PSII core complex.

Photoinhibition is light-induced inactivation of PSII. Hypotheses about the photoreceptor(s) of photoinhibition include the Chl antenna of PSII, manganese of the oxygen-evolving complex (OEC), uncoupled Chl and iron–sulfur centres. We measured the action spectrum of photoinhibition in vivo from wild-type Arabidopsis thaliana L. and from the npq1-2 and npq4-1 mutants defective in non-photochemical quenching (NPQ) of excitations of the PSII antenna. The in vivo action spectrum was found to resemble closely the in vitro action spectra published for photoinhibition. We compared the action spectrum with absorbance spectra of model compounds of the OEC complex and other potential photoreceptors of photoinhibition. The comparison suggests that both manganese and Chl function as photoreceptors in photoinhibition. In accordance with the function of two types of photoreceptors in photoinhibition, NPQ was found to offer only partial protection against photoinhibition at visible wavelengths. The low protective efficiency of NPQ supports the conclusion that the Chl antenna of PSII is not the only photoreceptor of photoinhibition. Comparison of the action spectrum of photoinhibition with the emission spectrum of sunlight shows that the UV part of sunlight is responsible for the major part of photoinhibition under natural conditions.

Changing of principal σ factor in RNA polymerase holoenzyme to a group 2 σ 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 43°C 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 ƊsigB than in the control strain. Addition of the compatible solute trehalose almost completely restored the growth of the ƊsigB 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 ƊsigCDE strain (SigB is the only functional group 2 σ factor) and down-regulated in the ƊsigB strain under standard conditions. In addition, the HspA heat shock protein was less abundant and more abundant in the ƊsigB and ƊsigCDE 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.