Oxygenic photosynthesis without galactolipids

Significance Cyanobacteria, as well as chloroplasts of plants and algae, are the sites of photosynthesis that produces oxygen. Photosynthetic membranes, also known as thylakoid membranes, in these organisms contain galactolipids, without exception, as the major components. Galactolipids are thus believed to be important for photosynthesis or at least for the formation of the flattened shape of thylakoid membranes. The biosynthetic pathway of galactolipids is definitely different in plants and cyanobacteria. Here we identified the final piece of the long-sought gene for the galactolipid synthesis in cyanobacteria, namely monoglucosyldiacylglycerol epimerase, which converts glucolipid to galactolipid. Cyanobacterial mutants in which this gene is disrupted still formed authentic thylakoid membranes without galactolipids and kept the ability to grow photosynthetically, indicating that galactolipids are not essential for either oxygenic photosynthesis or the development of the thylakoid membrane structure.