POSTIMPORT METHYLATION OF THE SMALL-SUBUNIT OF RIBULOSE-1,5-BISPHOSPHATE CARBOXYLASE IN CHLOROPLASTS

Electron impact mass spectronomy analysis of the amino-terminal amino acid of the small subunit (SSU) of ribulose-1,5-bisphosphate carboxyla se (Rubisco) showed that the amino-terminal methionine residue is post -translationally modified to N-methyl-methionine. Modification of the amino-terminal methionine residue was found in mature SSU proteins fro m the dicotyledonous plants pea and spinach as well as the monocotyled onous plants barley and corn, SSU methyltransferase is a soluble prote in in the chloroplast stroma and accepts heterologously expressed non- methylated SSU as a substrate using S-adenosylmethionine as methyl-gro up donor, We show that this modification occurs after post-translation al uptake of the precursor form of SSU into chloroplasts and processin g to its mature size, This reaction represents a new step in the impor t and assembly pathway of Rubisco holoenzyme. (C) 1997 Federation of E uropean Biochemical Societies.