Comparative Genomic Analysis Revealed a Gene for Monoglucosyldiacylglycerol Synthase, an Enzyme for Photosynthetic Membrane Lipid Synthesis in Cyanobacteria

Abstract

Cyanobacteria have a thylakoid lipid composition very similar to that of plant chloroplasts, yet cyanobacteria are proposed to synthesize monogalactosyldiacylglycerol (MGDG), a major membrane polar lipid in photosynthetic membranes, by a different pathway. In addition, plant MGDG synthase has been cloned, but no ortholog has been reported in cyanobacterial genomes. We report here identification of the gene for monoglucosyldiacylglycerol (MGlcDG) synthase, which catalyzes the first step of galactolipid synthesis in cyanobacteria. Using comparative genomic analysis, candidates for the gene were selected based on the criteria that the enzyme activity is conserved between two species of cyanobacteria (unicellular [Synechocystis sp. PCC 6803] and filamentous [Anabaena sp. PCC 7120]), and we assumed three characteristics of the enzyme; namely, it harbors a glycosyltransferase motif, falls into a category of genes with unknown function, and shares significant similarity in amino acid sequence between these two cyanobacteria. By a motif search of all genes of Synechocystis, BLAST searches, and similarity searches between these two cyanobacteria, we identified four candidates for the enzyme that have all the characteristics we predicted. When expressed in Escherichia coli, one of the Synechocystis candidate proteins showed MGlcDG synthase activity in a UDP-glucose-dependent manner. The ortholog in Anabaena also showed the same activity. The enzyme was predicted to require a divalent cation for its activity, and this was confirmed by biochemical analysis. The MGlcDG synthase and the plant MGDG synthase shared low similarity, supporting the presumption that cyanobacteria and plants utilize different pathways to synthesize MGDG.

A Bifunctional Glycosyltransferase from Agrobacterium tumefaciens Synthesizes Monoglucosyl and Glucuronosyl Diacylglycerol under Phosphate Deprivation

“…The best studied glycosyltransferases include the MGD/MGlcD and DGD synthases from plants and cyanobacteria (15)(16)(17)(18), the MGlcD and DGlcD synthases from Acholeplasma (7,19), and the processive glycosyltransferases from the Gram-positive bacteria Staphylococcus or Bacillus (20,21). There are only a few glycosyltransferases characterized as promiscuous enzymes, such as Pgt from Agrobacterium or Mesorhizobium, or the processive glycosyltransferases from the human pathogens Mycoplasma pneumoniae and Mycoplasma genitalum using UDP-Glc and UDP-galactose (UDP-Gal) as sugar donors (11,22,23).…”

Configuration of the sugar head of glycolipids in thylakoid membranes

“…Disruption of mgdA is lethal in Synechocystis, probably because MgdA is involved in the synthesis of both MGDG and DGDG (Awai et al, 2006;Shimojima et al, 2009). …”

“…Candidate genes were selected from among the unknown genes using the following criteria: (i) widely conserved in most cyanobacteria and in the chromatophore genome of P. chromatophora, and (ii) encoding the oxidoreductase motif that triggers the epimerase reaction of glucose into galactose. Three candidate genes were knocked out in Synechocystis, which contains a plant-type MGDG synthase from cucumber to prevent a possible lethal effect of the gene disruption (Awai et al, 2006;Shimojima et al, 2009). A knockout mutant of one gene resulted in accumulation of GlcDG.…”

Expression and Characterization of a Mycoplasma genitalium Glycosyltransferase in Membrane Glycolipid Biosynthesis

“…In plants, monogalactosyldiacylglycerol (MGalDAG) synthases belong to family 28 glycosyltransferases (GT28) (9 -13, 15, 16), whereas the digalactosyldiacylglycerol (DGalDAG) synthases are classified in family GT4 (17). In cyanobacteria, MGlcDAG synthases are family GT2 enzymes (18), whereas DGalDAG synthases are tentatively assigned to family GT4 (19). In non-photosynthetic Gram-positive bacteria, sequentially acting GTs are also found, in which a single enzyme transfers the first and second (and eventually a third or more) glycosyl unit.…”