S-1 DESTABILIZATION AND HIGHER SENSITIVITY TO LIGHT IN METRIBUZIN-RESISTANT MUTANTS

Mutations in the secondary quinone electron acceptor (Q(B)) pocket of the DI protein conferring a modification on the donor side of photosys tem II (PSII) have been characterized by gene cloning and sequencing i n two metribuzin-resistant mutants of Synechocystis PCC 6714. The muta tions induce different herbicide resistances: in M(30), a point mutati on at the codon 248, isoleucine to threonine, results in resistance on ly to metribuzin; in M(35), a single mutation, Ala(251)Val, confers me tribuzin, atrazine, and ioxynil resistance. As with other herbicide-re sistant mutants, M(30) and M(35) present modifications in the electron transfer between the primary quinone electron acceptor (Q(A)) and Q(B ) In addition, they have a modified oscillatory pattern of oxygen emis sion: after dark adaptation, the maximum oscillation is shifted by one flash. Both mutants have a higher concentration of the redox state in the dark adapted state than the wild type. The mutations render the o xygen evolving system more accessible to cell reductants. The mutation Ala(251)Val also confers to PSII an increased sensitivity to high lig ht. We have already demonstrated that under light stress a double muta nt, AzV (Ala(251)Val, Phe(211)Ser), lost the ability to recover the PS II activity sooner than the wild type. Here, we confirm that the modif ication of the alanine-251 is responsible for this specific sensitivit y to high light. We conclude that specific mutations of the Q(B) pocke t modify the behavior of the cells under light stress and have an effe ct on the structure of the D-1 protein in the other side of the membra ne.