PLANT PHYSIOLOGY , Vol 102, Issue 4 1259-1267, Copyright © 1993 by American Society of Plant Biologists
Molecular Heterogeneity of Photosystem I (psaD, psaE, psaF, psaH, and psaL Are All Present in Isoforms in Nicotiana spp.)
J. Obokata, K. Mikami, N. Hayashida, M. Nakamura and M. Sugiura
Department of Botany, Faculty of Science, Hokkaido University, Sapporo 060, Japan (J.O., K.M., M.N.)
The protein composition of photosystem I (PSI) was examined in Nicotiana
spp. by high-resolution polyacrylamide gel electrophoresis, N-terminal
amino acid sequencing, and immunoblot analysis. Five PSI proteins show
polymorphism in an amphidiploid species, Nicotiana tabacum, but not in its
ancestral diploid species, Nicotiana sylvestris and Nicotiana
tomentosiformis. These Nicotiana spp. appear to have at least 18 PSI
proteins per genome that range in molecular mass from 3 to 20 kD. They
include the products of nuclear genes psaD, psaE, psaF, psaG, psaH, psaK,
and psaL, the product of chloroplast gene psaC, N-terminally blocked
proteins of 4.5 and 3.0 kD, and an unidentified protein of 12.5 kD. The
psaD, psaF, psaH, and psaL products have two isoforms each that are
distinguished by different mobilities in polyacrylamide gel
electrophoresis, and the psaE product has four isoforms. The two isoforms
of 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 amino acid
sequence, indicating that at least two psaE genes are present in the
genome. To examine whether the polymorphic nature of PSI is peculiar to
Nicotiana spp., we carried out immunoblot analysis of the psaD and psaE
products in isogenic lines of tomato (Lycopersicon esculentum), Arabidopsis
thaliana, red bean (Vigna angularis), and corn (Zea mays). Two
electrophoretically distinct isoforms were found for the psaD products of
tomato, A. thaliana, and corn, and two isoforms of psaE products were
detected in tomato, A. thaliana, and red bean. These results suggest that
the nuclear-encoded subunits of PSI, except for the psaG and psaK products,
generally have two isoforms.