Rubisco small and large subunit N-methyltransferases - Bi- and mono-functional methyltransferases that methylate the small and large subunits of Rubisco

Ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco)is methylated at t he alpha-amino group of the N-terminal methionine of the processed form of the small subunit (SS), and at the epsilon-amino group of lysine-14 of the large subunit (LS) in some species. The Rubisco LS methyltransferase (LSMT) gene has been cloned and ex pressed from pea and specifically methylates l ysine-14 of the LS of Rubisco. We determine here that both pea and tobacco Rubisco LSMT also exhibit N-alpha-methyltransferase activity toward the SS of Rubisco, suggesting that a single gene product can produce a bifunctiona l protein methyltransferase capable of catalyzing both N-alpha-methylation of the SS and N-epsilon-methylation of the LS. A homologue of the Rubisco L SMT gene (rbcMT-S) has also been identified in spinach that is closely rela ted to Rubisco LSMT sequences from pea and tobacco. Two mRNAs are produced from rbcMT-S, and both long and short forms of the spinach cDNAs were expre ssed in Escherichia coli cells and shown to catalyze methylation of the alp ha-amino group of the N-terminal methionine of the SS of Rubisco. Thus, the absence of lysine-14 methylation in species like spinach is apparently a c onsequence of a monofunctional protein methyltransferase incapable of methy lating Lys-14, with activity limited to methylation of the SS.