Molecular Evolution of Phosphoenolpyruvate Carboxylase for C4 Photosynthesis in Maize: Comparison of Its cDNA Sequence with a Newly Isolated cDNA Encoding an Isozyme Involved in the

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 2X 10s yr ago), though the divergence of maize (C4 plant) from wheat (C3 plant) is supposed to have occurred much later (6X107 yr ago)