Gene expression changes in group 2 sigma factor mutant, ΔsigCDE, in standard growth conditions and after high-light treatment

Adjustment of gene expression during acclimation to stress conditions, such as bright light, in the cyanobacterium Synechocystis sp. PCC 6803 depends on four group 2 σ factors (SigB, SigC, SigD, SigE). A ∆sigCDE strain containing the stress responsive SigB as an only functional group 2 σ factor appears twice as resistant to photoinhibition of photosystem II (PSII) as the control strain. Microarray analyzes of the ∆sigCDE strain indicated that 77 genes in standard conditions and 79 genes in high light were differently expressed compared to the control strain. Analysis of possible photoprotective mechanisms revealed that high carotenoid content and up-regulation of the photoprotective flavodiiron operon flv4-sll0218-flv2 protected PSII in ∆sigCDE, while up-regulation of pgr5-like, hlipB or isiA genes in the mutant strain did not offer particular protection against photoinhibition. The photoinhibition resistance was lost if ∆sigCDE was grown in high CO2 where carotenoid and Flv4-Sll0218-Flv2 contents were low. Additionally, photoinhibition resistance of the ∆rpoZ strain (lacking the omega subunit of RNA polymerase) with very high carotenoid but only low Flv4-Sll0218-Flv2 content supported the importance of carotenoids in PSII protection. Carotenoids likely protect mainly via quenching of singlet oxygen but efficient non-photochemical quenching in ∆sigCDE might offer some additional protection. Comparison of photoinhibition kinetics in control, ∆sigCDE and ∆rpoZ strains showed that protection by the flavodiiron operon was most efficient during the first minutes of high-light illumination. Cells from cyanobacteria Synechocystis sp. PCC 6803 named as control strain (CS) and group 2 sigma factor mutant ΔsigCDE (A730=1, 40 mL) were harvested directly from standard growth conditions (continuous illumination at the PPFD of 40 µmol m-2s-1, 32°C, ambient CO2) or after 30-min illumination a PPFD 750 µmol m-2s-1. From three to four independent experiments were performed at each conditions.