Amino Acid Substitutions in the D1 Protein of Photosystem II Affect Qb” Stabilization and Accelerate Turnover of D1

Abstract

Isogenic strains of Synechococcus PCC 7942 were genetically engineered so that copy I of the gene psbA was mutated at specific sites. These mutations resulted in replacements of Ser 264 by Gly or Ala and of Phe 255 by Tyr or Leu in the D 1 protein. The mutants were resistant to herbicides inhibiting electron transfer in photosystem II. All mutants exhibited alterations in the stability of QB” as demonstrated by a temperature downshift, to various extents, of the in vivo thermoluminescence emission. Measurements of the light-dependent turnover of D 1 showed a marked decrease in the 11/2 of this protein in the mutants as compared to wild-type, under low to medium light intensities. A correlation was found between the degree of perturbation in the QB~ stability and the rate of acceleration in the turnover of D 1. These data provide a direct evidence for the overlapping binding sites for the plastoquinone B and herbicides in the D 1 protein. In addition these data indicate a close link between QB“ destabilization in reaction center II and the mechanism controlling the light-dependent turnover of D 1. Based on these results and previous work we suggest that destabilization of the semireduced quinone, facilitates a light-induced damage in D 1 which triggers its degradation. © 1990, Walter de Gruyter. All rights reserved.