Site-specific mutations localized in the D-E loop of the D1 protein of photosystem II affect phototolerance in Synechocystis sp. PCC 6803 containing psbAII gene

Photosynthetic characteristics along with phototolerance and photoinhibition of photosystem II (PS II) were monitored in Synechocystis sp. PCC 6803 wild type (KC) and its psbAII mutants viz., I6 (N322I, I326F, and F328S), G6 (N267Y), and H7 (Y254C and I314V) that have up to three point mutations, localized in the D-E loop of the D1 polypeptide of PSII reaction centre. These strains exhibited entirely different growth trends upon shifting from 30 micormol m(-2)s(-1) to high irradiance (500 micromol m(-2)s(-1) , 30 degrees C). The I6 and H7 cells grew well, whereas KC and G6 cells showed inability for cell multiplication. The photosynthetic efficiency demonstrated about 50% loss in chlorophyll fluorescence of variable yield (Fv/Fm) within 20-30 min in all mutants, whereas the wild type (KC) cells could reach the same level of loss in 2 hr. I6 and H7 cells showed continuous cell growth and maintenance under long-term exposure of high light compared to G6 mutant and wild type cells. The wild type cells showed slow decrease in their photochemical activity and Fv/Fm values, compared to mutant cells. The recovery seemed to be almost identical, and also stimulated by growth light, inspite of differential photoinhibitory behaviours. Darkness and translational inhibitor lincomycin both were found to be unassociated with the restoration of photoinhibited process of PS II.