The carboxyl-terminal extension of the precursor D1 protein of photosystemII is required for optimal photosynthetic performance of the cyanobacterium Synechocystis sp PCC 6803

The D1 protein is an integral component of the photosystem II reaction cent er complex. In the cyanobacterium Synechocystis sp. PCC 6803, D1 is synthes ized with a short 16-amino acids-long carboxyl-terminal extension. Removal of this extension is necessary to form active oxygen-evolving photosystem I I centers. Our earlier studies have shown that this extension is cleaved by CtpA, a specific carboxyl-terminal processing protease. The amino acid seq uence of the carboxyl-terminal extension is conserved among D1 proteins fro m different organisms, although at a level lower than that of the mature pr otein. In the present study we have analyzed a mutant strain of Synechocyst is sp. PCC 6803 with a duplicated extension, and a second mutant that lacks the extension, to investigate the effects of these alterations on the func tion of the D1 protein in vivo. No significant difference in the growth rat es, photosynthetic pigment composition, fluorescence induction, and oxygen evolution rates was observed between the mutants and the control strain. Ho wever, using long-term mixed culture growth analysis, we detected significa nt decreases in the fitness of these mutant strains. The presented data dem onstrate that the carboxyl-terminal extension of the precursor D1 protein i s required for optimal photosynthetic performance.