| Summary |
The CP43 protein, encoded by the psbC gene, is a component of Photosystem II (PSII) in higher plants, algae, and cyanobacteria. Previously, alteration of arginine 305 in the large extrinsic loop (LEL) of CP43 to a serine residue resulted in decreased oxygen-evolving activity and enhanced sensitivity to photoinactivation. These effects were greatly exacerbated when the mutant was grown in chloride-deficient media. A similar chloride effect was also observed when aspartate 306 within the LEL was changed to an asparagine residue. Both the R305 and D306 mutants exhibited phenotypes similar to strains harboring mutations in the psbV gene, which encodes cytochrome c₅₅₀. In the present study, the R305S and D306N mutants were examined and compared to control and psbV deletion strains, as well as to strains harboring both the CP43 mutations and the deletion. Strains were grown in both complete and chloride-deficient media to help characterize the chloride effect. In complete media, the CP43 mutants grew at near control rates, while all [delta]psbV mutants exhibited growth defects. Oxygen evolution rates were highest for the control strain, with the CP43 mutants and the [delta]psbV showing modest reductions. The CP43[delta]psbV mutants evolved oxygen at the lowest rates, with D306N[delta]psbV performing the worst. In chloride-deficient media, all mutant strains exhibited growth defects, with the CP43[delta]psbV strains failing to grow at all. All mutant strains showed severe defects in oxygen-evolving capacity, with the CP43[delta]psbV strains exhibiting the most severe phenotype. Isolated PSII particles from all mutant strands lacked spectrophotometrically detectable levels of cytochrome c₅₅₀. These results were corroborated by TMBZ staining for c-type cytochromes. Collectively, the results from this study indicate that all mutant strains are defective in their ability to utilize chloride to carry out efficient oxygen evolution and that this defect may be partially explained by an interruption in the interaction between the LEL of CP43 and cytochrome c₅₅₀. However, as the CP43[delta]psbV mutants exhibited more severe phenotypes than the mutants harboring the CP43 or psbV mutations alone, it appears that the R305 and D306 residues of the CP43 protein may have an additional function within PSII beyond their participation in the interaction with cytochrome c₅₅₀. |
