Abstract
Many widely used herbicides are inhibitors of photosynthesis. Their mechanism of action was found, in the case of triazine and urea herbicides, to involve binding of the herbicide molecule to a thylakoid-membrane polypeptide of 32000 dalton (the QB protein) and blocking the photosynthetic electron transport at the second stable electron acceptor of photosystem II (PSII) (1, 2). The QB protein is part of PSII complex and it is encoded by the chloroplast gene psbA. It has been shown previously (3), that a point mutation in psbA, resulting in a serine to glycine substitution at position 264 of QB, is responsible for the resistance to the herbicide atrazine (a triazine derivative) in two herbicide resistant mutants of higher plants. The same serine residue is changed to alanine in herbicide resistant mutants of Clamydomonas (4) and Cyanobacteria (5). Two other mutations in psbA confering herbicide resistance, have been identified in Chlamydomonas (6). In order to study the structure and function of QB, we have isolated and characterized herbicide resistant mutants in the Cyanobacterium Synechococcus R2 (Anacystis nidulans R2).
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