On the origin of plant MGD1, the glycosyltransferase responsible for the synthesis of the major chloroplast lipid monogalactosyldiacylglycerol

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The two galactolipids monogalactosyldiacylglycerol (MGDG) and digalactosyldiacylglycerol (DGDG) are the most abundant membrane lipids in photosynthetic organisms including plants, green algae and cyanobacteria. In plants and green algae, MGDG is synthesized by the MGDG synthase MGD1. Based on the identification of MGD1-related MGDG synthases in the anoxygenic photosynthetic bacterium Chloroflexus together with previous phylogenetic analyses, it was suggested that MGD1 from plants/green algae originated from an ancient member of Chloroflexi by horizontal gene transfer, but not from the cyanobacterial progenitor of chloroplasts. Our phylogenetic analyses revealed the presence of additional MGD1-like sequences in bacteria, i.e. in the photosynthetic purple bacterium Blastochloris viridis and the thermophilic Caldilinea aerophila. B. viridis MGDG synthase represents the first MGD1-like gene from any proteobacterium. These MGDG synthases are closer related to plant MGD1 than the Chloroflexus MGD1 sequences. B. viridis contains two and C. aerophila one MGD1-like genes. In addition, C. aerophila contains a second sequence related to the glucosylgalactosyldiacylglycerol (GlcGalDG) synthase from Chloroflexus. Preliminary results showed that these genes encode glycosyltransferases synthesizing galactolipids after expression in E. coli. In the present project, the substrate specificities of the four recombinant enzymes from B. viridis and C. aerophila will be characterized in detail. We will introduce two of the enzymes capable of synthesizing DGDG into galactolipid deficient Arabidopsis mutants (mgd1, dgd1) for complementation. Interestingly, one enzyme from B. viridis encodes a processive MGDG synthase which can convert diacylglycerol to DGDG, but without accumulating MGDG. This gene will be introduced into the Arabidopsis mgd1 mutant to generate a line containing DGDG, but not MGDG. Thus we will collect functional evidence for the hypothesis that plant MGD1 is evolutionary derived from a Caldilinea type MGDG synthase, and we will study the specific role of MGDG in plants by creating a novel MGDG-free plant line containing DGDG.