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 D1 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 D1 showed a marked decrease in the t 1/2 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 pertur bation in the QB- stability and the rate of acceleration in the turnover of D1. These data pro vide a direct evidence for the overlapping binding sites for the plastoquinone B and herbicides in the D1 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 D1. Based on these results and previous work we suggest that destabilization of the semireduced quinone, facilitates a light-induced damage in D1 which triggers its degradation.
A genetically engineered increase in fatty acid unsaturation in Synechococcus sp. PCC 7942 allows exchange of D1 protein forms and sustenance of photosystem II activity at low temperature
