Abstract
Molecular events associated with the evolution of an enzyme for C4 photosynthesis were investigated. In maize, at least three isozymes of phosphoenolpyruvate carboxylase [EC 4. 1. 1. 31] are known: the C4-form, the C3-form and the root-form, being named according to their physiological roles and pattern of tissue distribution [Ting, I. P. & Osmond, C. B. (1973) Plant Physiol. 51, 448-453]. A cDNA clone which presumably encodes the C3-form isozyme was newly isolated and analyzed. Comparison of the sequences of the C3-form and C4-form isozymes revealed that (i) the homologies in the nucleotide and deduced amino acid sequences were 71 and 77%, respectively, and (ii) the gene for the C4 -form isozyme evolved under strong G/C pressure. The genes for these isozymes were found to be located apart on different chromosomes. A phylogenetic tree was constructed using 8 amino acid sequences of phosphoenolpyruvate carboxylases from various sources. The topology of the tree indicated that, at least in monocots such as maize and sorghum, the genes for the C4-form and C3-form isozymes diverged from their common ancestral gene earlier than the monocot-dicot divergence (about 2×108 yr ago), though the divergence of maize (C4 plant) from wheat (C3 plant) is supposed to have occurred much later (6×107 yr ago).
