S1 destabilization and higher sensitivity to light in metribuzin-resistant mutants.

Mutations in the secondary quinone electron acceptor (QB) pocket of the D1 protein conferring a modification on the donor side of photosystem II (PSII) have been characterized by gene cloning and sequencing in two metribuzin-resistant mutants of Synechocystis PCC 6714. The mutations induce different herbicide resistances: in M30, a point mutation at the codon 248, isoleucine to threonine, results in resistance only to metribuzin; in M35, a single mutation, Ala251Val, confers metribuzin, atrazine, and ioxynil resistance. As with other herbicide-resistant mutants, M30 and M35 present modifications in the electron transfer between the primary quinone electron acceptor (QA) and QB. In addition, they have a modified oscillatory pattern of oxygen emission: 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 oxygen-evolving system more accessible to cell reductants. The mutation Ala251Val also confers to PSII an increased sensitivity to high light. We have already demonstrated that under light stress a double mutant, AzV (Ala251Val, Phe211Ser), lost the ability to recover the PSII activity sooner than the wild type. Here, we confirm that the modification of the alanine-251 is responsible for this specific sensitivity to high light. We conclude that specific mutations of the QB pocket modify the behavior of the cells under light stress and have an effect on the structure of the D1 protein in the other side of the membrane.