Genetic engineering of the processing site of D1 precursor protein of pholosystem II reaction center in Chlamydomonas reinhardtii

The D1 protein (D1) of photosystem II (TSII) reaction center is synhtesized as a precursor (pD1) and then processed at its carboxyl terminus to establish the function of water cleavage. The amino acid sequence of the carboxyl terminal extension excised by this process is poorly conserved except for a residue after the cleavage site at position of 345. We have constracted a vector for site-directed the green of the chloroplast psbA gene encoding D1 of the green alga, Chlamydomonas reinhardtii. The vector enables one to transfrom the chloroplasts of a psbA deletion mutant (Fud7) and directiy select transformants for resistance to spectinomycin. Using this transforming vector, we have substituted Ser345 to Gly, Cys, Val and Phe in order to investigate effects of the amino acid side chain at this position ona the processing rate. All of the resulting transformants exhibited the PSII activity as wild type and grew normally under photoautotrophic conditions even under strong light where rapid turnover of D1 protein is expected to occur. Western blotting analysis demonstrated that mature D1 accumulates in these transformants at wild type level. Pulse and chase labeling of chloroplast-encoded proteins using [³⁵S]sulfate revealed that the processing of D1 precursor protein occurs in all four transformants as efficiently as in wild type, at least under the experimental conditions examined. The results suggest that either the amino acid side chain at position of 345 (+1 position) is aol crucial to the enzymatic cleavage of pD1 in vivo or the apparent rate of processiag in vivo is not limited by the enzymatic cleavage.