Photoinhibition and Recovery of Photosynthesis in psbA Gene-Inactivated Strains of Cyanobacterium Anacystis nidulans.

The susceptibility of photosynthesis to photoinhibition and the rate of its recovery were studied in cyanobacterium Anacystis nidulans strain R2 and its two psbA gene-inactivated mutants R2S2C3 and R2K1. Changes in the fluorescence kinetics at 77K as well as the rate of O(2) evolution were measured when cells were exposed to high photosynthetic photon flux densities in the range of 0 to 2,000 micromoles per square meter per second. The R2S2C3 mutant has an active psbAI gene highly expressed under low and normal light intensities, whereas R2K1 possesses psbAII and psbAIII genes highly expressed under very high light intensities. The level of overall susceptibility of photosynthesis to photoinhibition was more pronounced in the wild type and the mutant R2S2C3 than in the mutant R2K1, especially at higher light intensities. In constrast, all three strains showed an increased but similar sensitivity to photoinhibition after addition of the translational inhibitor streptomycin; mutant R2K1 being slightly less sensitive at lower light intensities. The result is interpreted as demonstrating similar intrinsic susceptibility to photoinhibition of the two different forms of the D1 protein, form I and form II, encoded by the psbAI and psbAII/psbAIII genes, respectively. The increased resistance to photoinhibition of the R2K1 mutant was ascribed to an approximately 3 times higher rate of recovery than the wild type and the mutant R2S2C3. On the basis of our experiments we conclude that the susceptibilities to photoinhibition of the Anacystis nidulans psbA genes mutants studied are regulated mainly by modifying the rate of repair, i.e. the rate of turnover of the D1 protein.