SITE-SPECIFIC MUTATIONS IN THE D1-POLYPEPTIDE AFFECT THE SUSCEPTIBILITY OF SYNECHOCYSTIS-6803 CELLS TO PHOTOINHIBITION

Photoinhibition of photosystem II in the cyanobacterium Synechocystis 6803 was followed after site-specific mutagenesis of the DI polypeptide. Mutations were created in the stromal/cytosolic loop connecting helices D and E. Two mutations E243K and CA1, a deletion of the three glutamates 242-244 and a substitution Q241H, were made in the putative cleavage area of the Dl polypeptide. A third mutation E229D was made in the PEST-like sequence. Mutants and control cells were illuminated and F(V)/F(M) was recorded. Compared to the control, the mutants were less photoinhibited. Fluorescence relaxation after a single flash was delayed in CA1. Restoration of F(V)/F(M) after photoinhibition in the mutants was totally dependent on protein synthesis while control cells were able to recover partially also when protein synthesis was inhibited. In addition, the protein synthesis-dependent recovery of CA1 was slowed down. Our results indicate a correlation between the mutated amino acids and photoinhibition of photosystem II.