The Herbicide-Resistant Species of the Cyanobacterial Dl Protein Obtained by Thorough and Random in vitro Mutagenesis

Random mutations were introduced into the DNA fragment of thepsbA2 gene of Synechocystis sp. PCC 6803, which encodes thecarboxyl-terminal 178 amino acid region of the Dl protein ofthe PSII reaction center, by in vitro random mutagenesis toobtain Dl species resistant to herbicides and to understandthe protein-herbicide interactions. The mutants were screenedon the criterion of resistance to either 1 µM DCMU or10 µM atrazine. In these mutants, amino acid substitutionswere distributed throughout the entire area of the targetedregion in the Dl protein. However, in every mutant, except forone case, the substitution was present in the region describedas the "herbicide-binding niche", i.e., between Phe211 and Leu275,although some amino acid substitutions which were not previouslydescribed were found at residues known to be involved with herbicideaffinity. Thus, the result of random mutagenesis basically supportsthe validity of the proposed structural model for the Dl protein,as well as of the herbicide-binding niche. Preliminary characterizationof the herbicide-resistant mutants obtained in this study hasalso been conducted. (Received December 8, 1997; )