Site-directed mutagenesis of the CP 47 protein of photosystem II: Alteration of conserved charged residues in the domain 364E-444R
Abstract
The intrinsic chlorophyll-protein CP 47 is a component of photosystem II in higher plants, green algae and cyanobacteria. We had shown previously by biochemical methods that the domain 364E-440D of CP 47 interacts with the 33 kDa extrinsic protein of photosystem II [Odom, W. R., & Bricker, T. M. (1992) Biochemistry 31, 5616-5620]. In this study, using oligonucleotide-directed mutagenesis in the cyanobacterium Synechocystis 6803, mutations at 17 conserved charged residues were introduced into the domain 364E-444R of the CP 47 protein. Only mutations introduced at positions 384R and 385R led to a modified PS II phenotype. We previously described a mutation at (RR384385GG) which resulted in a mutant with a defective oxygen-evolving complex [Putnam-Evans, C., & Bricker, T. M. (1992) Biochemistry 31, 11482-11488]. An additional set of mutations, 384R to 384G, 385R to 385G, and 384,385RR to 384,385EE has now been introduced at this site yielding the mutants R384G, R385G, and RR384385EE, respectively. Steady state oxygen evolution measurements and quantum yield measurements demonstrated that these mutants exhibited significant alterations in their ability to evolve oxygen. Total fluorescence yield measurements indicated that all of these mutants contained about 85%-90% of the PS II reaction centers found in the control strain. This decrease was insufficient to explain the oxygen evolution results. Analysis of oxygen flash yield parameters indicated that there was little change in the S-state parameters α, β, γ, or δ. Measurement of the S2 lifetime, however, demonstrated that the S2 lifetime of the mutants was 2-3 times longer than that of the control. Additionally, examination of the risetime of the oxygen signal indicated that there was a significant retardation (6-7-fold) in the rate of oxygen release, suggesting a retarded S3-[S4]-S0 transition. These data reinforce our hypothesis that the positive charge density at positions 384R and 385R in the large extrinsic loop of CP 47 is necessary for its function in water oxidation. We speculate that this positive charge density may be an important factor in establishing the proper interaction between CP 47 and the 33 kDa extrinsic protein.