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 b y characterization of mutations introduced in the D1 protein. Eight no vel point mutations in the gene psbA, which encodes D1, were generated in the cyanobacterium Synechocystis PCC6803 by site-specific mutagene sis in vitro. The effects of the resulting modifications in D1 on elec tron transfer in PSII and on herbicide binding were analyzed. The resu lts extend the structural analogies between the secondary quinone bind ing site in D1 and in subunit L of the photosynthetic reaction center in purple bacteria. The involvement of Phe255, Ser264, and Leu271 of D 1 in plastoquinone binding and electron transfer in PSII was establish ed. An indirect effect of Tyr254 on the binding of Q(B) was demonstrat ed. 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 site s of Q(B) and herbicides.