Upon introduction of random mutations in a region of the psbDI gene that encodes the D2 protein in the cyanobacterium Synechocystis sp. PCC 6803, an obligate photoheterotrophic mutant was isolated that contained three mutations: V247M, A249T, and M329I. This mutant evolved oxygen in the absence of added electron acceptors, but oxygen evolution was inhibited by micromolar concentrations of several artificial quinones. Complementation analysis showed that the V247M and/or A249T mutations were responsible for this phenotype. Using fluorescence induction and decay measurements, the site of inhibition by the quinones was found to be at the level of the primary electron-accepting quinone in photosystem II, Q(A). Duroquinone inhibited by blocking reduction of Q(A), and in the presence of other quinones such as 2,5-dichloro-p-benzoquinone, 2,5-dimethyl-p-benzoquinone, and p-benzoquinone, Q(A) could be reduced but could not efficiently transfer an electron to Q(B). To distinguish the effects of the V247M and A249T mutations, single mutants were created. V247M was photoautotrophic and had an essentially normal phenotype. The A249T mutant, although photoautotrophic, was affected by artificial quinones, but less than the mutant carrying both the V247M and A249T changes. The results indicate a decreased plastoquinone affinity at the Q(A) site in the strains carrying a A249T mutation, such that after dark- adaptation a significant percentage of the Q(A) sites is empty or is occupied by an artificial quinone. In light, the percentage of photosystem II centers with plastoquinone bound at the Q(A) site appears to increase, which may be due in part to an increased affinity of the semiquinone versus that of the quinone at the Q(A) site.
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