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

单半乳糖二酰甘油(MGDG)和双乳糖二甘油(DGDG)是植物、绿藻和蓝藻等光合作用生物中含量最丰富的膜脂。在植物和绿藻中，MGDG是由MGDG合成酶MGD1合成的。根据对产氧光合细菌Chlorofleus中与MGD1相关的MGDG合成酶的鉴定，结合以往的系统发育分析，认为来自植物/绿藻的MGD1是通过水平基因转移来自Chloroflei的一个古老成员，而不是来自叶绿体的蓝藻前体。我们的系统发育分析揭示了细菌中存在额外的类似MGD1的序列，即在光合作用的紫色细菌Blastoloris viridis和嗜热的Caldiina Aerophila中。绿色芽孢杆菌MGDG合成酶是变形杆菌中第一个MGD1样基因。这些MGDG合酶与植物MGD1的亲缘关系比与绿叶蝉的MGD1序列更近。绿色芽孢杆菌含有两个基因，嗜气假单胞菌含有一个类似MGD1的基因。此外，嗜气隐孢子虫还含有第二个序列，与来自绿僵菌的葡萄糖半乳糖二酰甘油(GlcGalDG)合成酶有关。初步结果表明，这些基因编码糖基转移酶，在大肠杆菌中表达后合成半乳糖脂。在本项目中，我们将对绿色芽孢杆菌和嗜氧芽孢杆菌的四种重组酶的底物特异性进行详细的研究。我们将把两种能够合成DGDG的酶引入半乳脂缺乏的拟南芥突变体(mgd1，dgd1)中进行互补。有趣的是，绿色芽孢杆菌的一种酶编码一种进行性的MGDG合成酶，它可以将二酰甘油转化为DGDG，但不积累MGDG。该基因将被导入拟南芥mgd1突变体中，以产生一个含有DGDG而不是MGDG的品系。因此，我们将收集功能证据来支持这一假说，即植物MGD1是由Caldicina型MGDG合酶进化而来的，我们将通过创建一个新的含有DGDG的不含MGDG的植物品系来研究MGDG在植物中的特定作用。

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.