TRUNCATION OF THE D2 PROTEIN IN SYNECHOCYSTIS SP PCC-6803 - A ROLE OFTHE 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 680 3 in order to determine the role of the large hydrophilic C-terminal d omain of D2 in the function and stability of photosystem II (PS II). T he loss of 57 residues from the C-terminal end of D2 (most of the hydr ophilic tail) resulted in the loss of D2 and PS II reaction centers fr om thylakoids. Truncation of 16, 15, 14, or 13 amino acid residues fro m the C-terminus of D2 resulted in a virtual disappearance of oxygen e volution, a loss of photoautotrophic growth, and a decrease in the num ber of PS II centers in thylakoids. The loss of 11 C-terminal amino ac id residues led to a photoautotrophic mutant that grew at one-half the rate of the wild type under photoautotrophic conditions and that show ed a progressive loss of oxygen evolution at high light intensity. Tru ncation of 9 residues from D2 led to a virtual loss of CP43, presumabl y because of interference of the mutation with the overlapping ribosom e-binding site for psbC translation. To delete smaller portions of D2 and yet not interfere with psbC expression, various deletions were mad e between the tenth and twentieth amino acid residues from the C-termi nal end of D2, resulting in the loss of 8, 7, 4, 3, and 2 residues. Th e deletion of 8 or 7 residues from within the C-terminal end of D2 res ulted in photoautotrophic mutants. Surprisingly, the deletion of short er fragments had more pronounced effects: deletion of 4 residues withi n 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 photoheterotrop h, containing functional PS II reaction centers but showing rapid phot oinhibition. The deletion of 2 residues resulted in an obligate photoh eterotrophic mutant with a 10-fold-reduced level of PS II centers. In the mutant lacking the 15 C-terminal residues of D2, fluorescence indu ction 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 r ates 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 acti vity of the PS II complex.