Abstract
A candidate for a gene encoding MGDG synthase was identified from the green sulfur bacterium, Chlorobaculum tepidum (formerly called Chlorobium tepidum), through a newly developed computer-added gene discovery system. This gene, mgdA, encodes a 49-kDa protein that belongs to a beta-glycosyltranferase family. The primary structure of MgdA shows no significant similarity to any MGDG synthases identified so far from other organisms. MgdA expressed in E. coli showed MGDG synthase activity in vitro by use of UDP-galactose, but not UDP-glucose, as a substrate. mgdA is essential for this bacterium; only heterozygous mgdA mutant could be isolated. We performed complementation analysis of Arabidopsis MGDG synthase (MGD1) mutant, mgd1-2, by heterologous expression of MgdA that contains the chloroplast transit peptide of MGD1 at its N-terminus. The complementing line showed almost normal levels of MGDG, although mgd1-2 mutation causes about 98% reduction in MGDG content. From these observations, we conclude that MdgA is a novel MGDG synthase, which has been established and conserved in Chlorobi phylum. The physiological function of MgdA will be discussed.
