The carboxyl-terminal extension of the precursor D1 protein of photosystem II 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 center complex. In the cyanobacterium Synechocystis sp. PCC 6803, D1 is synthesized with a short 16-amino acids-long carboxyl-terminal extension. Removal of this extension is necessary to form active oxygen-evolving photosystem II centers. Our earlier studies have shown that this extension is cleaved by CtpA, a specific carboxyl-terminal processing protease. The amino acid sequence of the carboxyl-terminal extension is conserved among D1 proteins from different organisms, although at a level lower than that of the mature protein. In the present study we have analyzed a mutant strain of Synechocystis sp. PCC 6803 with a duplicated extension, and a second mutant that lacks the extension, to investigate the effects of these alterations on the function of the D1 protein in vivo. No significant difference in the growth rates, photosynthetic pigment composition, fluorescence induction, and oxygen evolution rates was observed between the mutants and the control strain. However, using long-term mixed culture growth analysis, we detected significant decreases in the fitness of these mutant strains. The presented data demonstrate that the carboxyl-terminal extension of the precursor D1 protein is required for optimal photosynthetic performance.