Fig. 7.

—Comparison of lipid biosynthetic pathways in plastids (top) and cyanobacteria (bottom). All enzymes are color-coded: blue, cyanobacterial or prokaryotic enzymes; green, plastid-localized enzymes; red, ER-localized enzymes. The latter two categories of enzymes are eukaryotic enzymes (of eukaryotic origin), except the TGD complex components that are of cyanobacterial origin (see, e.g., Hori et al. 2016). In plants and green algae, stearoyl-ACP desaturase (FAB2) catalyzes the conversion of 18:0 to 18:1 within the plastid, whereas in red algae, this enzyme is not present, and the final product of fatty acid synthesis within the plastid is 18:0. The C18 acids are transported out of the plastids and used to synthesize the 1-C18-2-C18 lipids in the ER. Desaturation occurs on these lipids, but this is not shown explicitly. The PA or DAG portion of phosphatidylcholine comes back to the plastids and used for the synthesis of the galactolipids. The pathway via ER is a very simplified sketch, because it is not the main topic of this study. Within the plastids (plastid pathway), the 1-C18-2-C16 lipids (sometimes called “prokaryotic lipids”) are synthesized by ATS1 and ATS2, which are not of cyanobacterial origin. MGDG is synthesized by galactosylation of DAG catalyzed by MGD1, and DGDG by galactosylation of MGDG catalyzed by DGD1. In the red algae, C. merolae and Galdieria sulphuraria, the last step is catalyzed by DgdA (or Ycf82), a cyanobacterial enzyme encoded by the plastid genome. Other red algae possess DGD1 but not DgdA. In cyanobacteria, MGDG is synthesized by the epimerization (catalyzed by MgdE) of GlcDG, which is synthesized by the glucosylation of DAG catalyzed by MgdA. DgdA is the sole enzyme catalyzing the production of DGDG in cyanobacteria. ACP, acyl carrier protein; CoA, coenzyme A; FAS, fatty acid synthase; GlcDG, monoglucosyl diacylglycerol; UDP-glc, uridine diphosphate glucose; UDP-gal, uridine diphosphate galactose.