The carboxyl-terminal extension of the D1 protein of photosystem II is not required for optimal photosynthetic performance under CO2- and light-saturated growth conditions.

Synthesis of the photosystem II D1 protein as a precursor with a carboxyl-terminal extension occurs in almost all eukaryotic photosynthetic organisms examined so far, as well as in cyanobacteria. Processing of the D1 precursor has been recently postulated to play a regulatory role in the light-dependent migration of photosystem II units from the unstacked to the stacked thylakoids (Bowyer, J. M., Packer, J. C. L., McCormack, B. A., Whitelegge, J. P., Robinson, C., and Taylor, M. A. (1992) J. Biol. Chem. 267, 5424-5433). To test this hypothesis, site-directed mutagenesis and chloroplast transformation have been used to create a "preprocessed" mutant Chlamydomonas strain which synthesizes mature D1 protein directly. We have found that this strain is indistinguishable from wild type in terms of photosynthetic performance and cell doubling time under CO2- and light-saturated photoautotrophic growth conditions.