Mutations in the D1 subunit of photosystem II distinguish between quinone and herbicide binding sites

The structure-activity relationships of the plastoquinone Q_B binding domain in the D1 subunit of photosystem II (PSII) were investigated by characterization of mutations introduced in the D1 protein. Eight novel point mutations in the gene psbA, which encodes D1, were generated in the cyanobacterium Synechocystis PCC6803 by site-specific mutagenesis in vitro. The effects of the resulting modifications in D1 on electron transfer in PSII and on herbicide binding were analyzed. The results extend the structural analogies between the secondary quinone binding site in D1 and in subunit L of the photosynthetic reaction center in purple bacteria. The involvement of Phe²⁵⁵, Ser²⁶⁴, and Leu²⁷¹ of D1 in plastoquinone binding and electron transfer in PSII was established. An indirect effect of Tyr²⁵⁴ on the binding of Q_B was demonstrated. Changes in binding of herbicides and Q_B to D1 as a result of the mutations revealed specific interactions between amino acid residues in D1 and the plastoquinone and distinguished between the binding sites of Q_B and herbicides.