Characterization of chloroplast psbA transformants of Chlamydomonas reinhardtii with impaired processing of a precursor of a photosystem II reaction center protein, D1.

One of the photosystem II reaction center proteins, D1, is encoded by the psbA gene and is synthesized as a precursor form with a carboxyl-terminal extension that is subsequently cleaved between Ala-344 and Ser-345. We have generated three psbA transformants of the green alga Chlamydomonas reinhardtii in which Ala-344 or Ser-345 have been substituted with Pro or Glu (A344P, S345E, and S345P) to understand the effects of the amino acid substitutions on the processing of the precursor D1. S345E grew photoautotrophically and showed PSII activity like the wild type. However, A344P and S345P were unable to grow photoautotrophically and were significantly photosensitive. A344P was deficient in the processing of precursor D1 and in oxygen-evolving activity, but assembled photosystem II complex capable of charge separation. In contrast, both precursor and mature forms of D1 accumulated in S345P cells from the logarithmic phase and the cells evolved oxygen at 18% of wild-type level. However, S345P cells from the stationary phase contained mostly the mature D1 and showed a twofold increase in oxygen-evolving activity. The rate of processing of the accumulated pD1 was estimated to be about 100 times slower than in the wild type. It is therefore concluded that the functional oxygen-evolving complex is assembled when the precursor D1 is processed, albeit at a very low rate. These results suggest the functional significance of the amino acid residues at the processing site of the precursor D1.