MOLECULAR HETEROGENEITY OF PHOTOSYSTEM-I - PSAD, PSAE, PSAF, PSAH, AND PSAL ARE ALL PRESENT IN ISOFORMS IN NICOTIANA SPP

The protein composition of photosystem I (PSI) was examined in Nicotia na spp. by high-resolution polyacrylamide gel electrophoresis, N-termi nal amino acid sequencing, and immunoblot analysis. Five PSI proteins show polymorphism in an amphidiploid species, Nicotiana tabacum, but n ot in its ancestral diploid species, Nicotiana sylvestris and Nicotian a tomentosiformis. These Nicotiana spp. appear to have at least 18 PSI proteins per genome that range in molecular mass from 3 to 20 kD. The y include the products of nuclear genes psaD, psaE, psaF, psaG, psaH, psaK, and psaL, the product of chloroplast gene psaC, N-terminally blo cked proteins of 4.5 and 3.0 kD, and an unidentified protein of 12.5 k D. The psaD, psaF, psaH, and psaL products have two isoforms each that are distinguished by different mobilities in polyacrylamide gel elect rophoresis, and the psaE product has four isoforms. The two isoforms o f the psaD product have distinct amino acid sequences, indicating that they are encoded by different genes within the genome. Four isoforms of the psaE products can be classified into two groups by N-terminal a mino acid sequence, indicating that at least two psaE genes are presen t in the genome. To examine whether the polymorphic nature of PSI is p eculiar to Nicotiana spp., we carried out immunoblot analysis of the p saD and psaE products in isogenic lines of tomato (Lycopersicon escule ntum), Arabidopsis thaliana, red bean (Vigna angularis), and corn (Zea mays). Two electrophoretically distinct isoforms were found for the p saD products of tomato, A. thaliana, and corn, and two isoforms of psa E products were detected in tomato, A. thaliana, and red bean. These r esults suggest that the nuclear-encoded subunits of PSI, except for th e psaG and psaK products, generally have two isoforms.