Truncation of the D2 Protein in Synechocystis sp. PCC 6803: A Role of the C-Terminal Domain of D2 in Photosystem II Function and Stability

Termination and deletion mutations were introduced near the C-terminal end of the D2 protein in the cyanobacterium Synechocystis sp. PCC 6803 in order to determine the role of the large hydrophilic C-terminal domain of D2 in the function and stability of photosystem II (PS II). The loss of 57 residues from the C-terminal end of D2 (most of the hydrophilic tail) resulted in the loss of D2 and PS II reaction centers from thylakoids. Truncation of 16, 15, 14, or 13 amino acid residues from the C-terminus of D2 resulted in a virtual disappearance of oxygen evolution, a loss of photoautotrophic growth, and a decrease in the number of PS II centers in thylakoids. The loss of 11 C-terminal amino acid residues led to a photoautotrophic mutant that grew at one-half the rate of the wild type under photoautotrophic conditions and that showed a progressive loss of oxygen evolution at high light intensity. Truncation of 9 residues from D2 led to a virtual loss of CP43, presumably because of interference of the mutation with the overlapping ribosome-binding site for psbC translation. To delete smaller portions of D2 and yet not interfere with psbC expression, various deletions were made between the tenth and twentieth amino acid residues from the C-terminal end of D2, resulting in the loss of 8, 7, 4, 3, and 2 residues. The deletion of 8 or 7 residues from within the C-terminal end of D2 resulted in photoautotrophic mutants. Surprisingly, the deletion of shorter fragments had more pronounced effects: deletion of 4 residues within the same domain gave rise to a mutant lacking D2 and PS II centers. A mutant with a deletion of 3 residues was an obligate photoheterotroph, containing functional PS II reaction centers but showing rapid photoinhibition. The deletion of 2 residues resulted in an obligate photoheterotrophic mutant with a 10-fold-reduced level of PS II centers. In the mutant lacking the 15 C-terminal residues of D2, fluorescence induction behavior indicated rapid inactivation at the donor side. In this and similar mutants, stable PS II-mediated electron transport between diphenylcarbazide and dichlorophenolindophenol could be observed at rates proportional to their PS II content. These results indicate that the primary effect of the mutations is on the oxygen-evolving complex. We conclude that domains near the C-terminal end of D2 have a role in oxygen evolution and contribute to determining the stability and activity of the PS II complex.