APPARENT DESTABILIZATION OF THE S1-STATE RELATED TO HERBICIDE RESISTANCE IN A CYANOBACTERIUM MUTANT

In this work we describe a new phenotype of herbicide-resistant mutants. We have selected and characterized several metribuzin resistant mutants from Synechocystis 6714. We found that an increase in metribuzin resistance involved a cross-resistance with other herbicides. Therefore, the mutants could be classified in three groups: (1) metribuzin resistant; (2) atrazine and metribuzin resistant; (3) DCMU, atrazine and metribuzin resistant. Mutants which did not present cross-resistance were up to 25-fold more resistant to metribuzin than the wild type. We have studied the electron transfer properties of Photosystem 11 in these mutants using several techniques (oxygen emission, fluorescence, and thermoluminescence measurements). They presented modifications in the electron transfer between Q(A) and Q(B), as was generally observed in most herbicide-resistant mutants previously studied. However, unexpectedly, one of these mutants, M30, presented a modified oscillatory pattern of oxygen emission. After dark adaptation the maximum of the oscillation was shifted by one flash. The matrix analysis indicated that the shifted maximum of the oxygen sequence corresponded to an increased S0 concentration in the dark-adapted state. In whole cells S0 and S1 are in equilibrium. This equilibrium is shifted in favor of S0 in the M30 mutant. The mutation renders the S-states more accessible to cell reductants.