STRUCTURAL-ANALYSIS OF THE Q(B) POCKET OF THE D1 SUBUNIT OF PHOTOSYSTEM-II IN SYNECHOCYSTIS PCC-6714 AND PCC-6803

Various herbicides inhibit photosynthesis by displacing the second ele ctron acceptor Q(B) from its binding site at the D1 protein. Different amino acid substitutions within this binding site have been found to reduce herbicide affinities, thereby conferring herbicide resistance. In Synechocystis PCC 6714 we have selected 7 single mutants and 6 doub le mutants resistant to various herbicides due to amino acid substitut ions at different positions in the Q(B) Pocket. Characterization of th ese mutants consists in molecular determination of the mutations in th e psbAI genes and in transformation of Synechocystis PCC 6803 by the c loned mutated genes to analyze the role of the mutations in the mutant phenotypes. Comparison with equivalent Chlamydomonas mutants is prese nted. These studies allow us to specify the interactions of several am ino acid residues with herbicides and Q(B) and with each other. Furthe rmore some Synechocystis mutants present additional characteristics su ch as an increased sensitivity to photoinhibition, or resistance to fo rmate or modification of the oscillatory pattern of oxygen evolution. Among the 6 point-mutations giving herbicide resistance, only those lo cated at the limit of the loop and the parallel helix produced additio nal effects on photosystem II function.