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 p sbA gene and is synthesized as a precursor form with a carboxyl-terminal ex tension that is subsequently cleaved between Ala-344 and Ser-345. We have g enerated three psbA transformants of the green alga Chlamydomonas reinhardt ii 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 substitutio ns on the processing of the precursor D1. S345E grew photoautotrophically a nd showed PSII activity like the wild type. However, A344P and S345P were u nable to grow photoautotrophically and were significantly photosensitive. A 344P was deficient in the processing of precursor D1 and in oxygen-evolving activity, but assembled photosystem II complex capable of charge separatio n. 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 wil d-type level. However, S345P cells from the stationary phase contained most ly 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 10 0 times slower than in the wild type. It is therefore concluded that the fu nctional oxygen-evolving complex is assembled when the precursor D1 is proc essed, albeit at a very low rate. These results suggest the functional sign ificance of the amino acid residues at the processing site of the precursor D1.