Microheterogeneity of PSI-E subunit of photosystem I in Nicotiana sylvestris.

Microheterogeneity of a photosystem I (PSI) subunit encoded by a nuclear gene psaE was examined in Nicotiana sylvestris, with the aid of cDNA cloning, peptide mapping analysis and protein sequencing. The psaE product of this plant has four isoforms whose mobilities in PAGE are slightly different from each other. We isolated two types of psaE cDNAs from a N. sylvestris cDNA library, and designated the corresponding genes as psaEa and psaEb, respectively. The psaEa and psaEb genes are 77% homologous at DNA level, and their translation products share 80.4% homology for the precursor proteins and 89.1% for the mature forms. Comparative analysis of the four isoproteins and the putative products of the two psaE genes revealed that two isoproteins out of four are derived from psaEa gene, and the difference between these two isoproteins lies in the respective presence or absence of N-terminal alanine. Likewise, the other two proteins are derived from psaEb with similar N-terminal heterogeneity. These results indicate that multi-gene organization and heterogeneous N-terminal formation at post-translational level are two possible causes for PSI subunit polymorphism in isogenic plant lines.