A similar structure of the herbicide binding site in photosystem II of plants and cyanobacteria is demonstrated by site specific mutagenesis of the psbA gene.

Many herbicides inhibit the photosynthetic electron transfer in photosystem II by binding to the polypeptide D1. A point mutation in the chloroplast gene psbA, which leads to a change of the amino acid residue 264 of D1 from serine to glycine, is responsible for atrazine resistance in higher plants. We have changed serine 264 to glycine in Synechococcus PCC7942 and compared its phenotype to a mutant with a serine to alanine shift in the same position. The results show that glycine at position 264 in D1 gives rise to a similar phenotype in cyanobacteria and in higher plants, indicating a similar structure of the binding site for herbicides and for the quinone QB in the two systems. A possible mode of binding of phenyl-urea herbicides to D1 is predicted from the difference in herbicidal cross-resistance between glycine and alanine substitutions of serine 264.