CLONING OF THE GENE FOR MONOGALACTOSYLDIACYLGLYCEROL SYNTHASE AND ITSEVOLUTIONARY ORIGIN

Monogalactosyldiacylglycerol (MGDG) synthase (UDPgalactose:1,2-diacylg lycerol 3-beta-D-galactosyltransferase; EC 2.4.1.46) catalyzes formati on of MGDG, a major structural lipid of chloroplast. We cloned a cDNA for the synthase from cucumber cDNA library. The full-length cDNA clon e was 2142 bp, and it contains a 1575-bp open reading frame encoding 5 25 aa. The open reading frame consists of the regions for a mature pro tein (422 aa; M(r) of 46,552) and transit peptide to chloroplast (103 aa). Although the molecular weight of mature protein region matched th at purified from cucumber cotyledons, it was quite different from thos e purified from spinach (approximate to 20 kDa) reported by other grou ps. The mature region of the protein was expressed in Escherichia coli as a fusion protein with glutathione S-transferase. The expression in E. coli showed that the protein catalyzed MGDG synthesis very efficie ntly. Therefore, we concluded that the cDNA encodes MGDG synthase in c ucumber. In addition, the deduced amino acid sequence of the MGDG synt hase cDNA showed homology with MurG of Bacillus subtilis and E. coli, which encode a glycosyltransferase catalyzing the last step of peptido glycan synthesis in bacteria. This sequence homology implies that the machinery of chloroplast membrane biosynthesis is evolutionarily deriv ed from that of cell wall biosynthesis in bacteria. This is consistent with the endosymbiotic hypothesis of chloroplast formation.