Lipid Biosynthesis and its Regulation in Cyanobacteria

Abstract

According to the endosymbiosis theory, cyanobacteria are assumed to be ancestors of chloroplasts. They are Gram-negative and perform oxygenic photosynthesis via two photosystems, which resemble the photosystems in the chloroplasts of eukaryotic plants. The membranes of cyanobacteria contain four main glycerolipids, three of which are glycolipids, namely, monogalactosyldiacylglycerol (MGDG), digalactosyldiacylglycerol (DGDG), and sulfoquinovosyldiacylglycerol (SQDG), and one of which is the phospholipid phosphatidylglycerol (PG). The lipid composition of cyanobacterial membranes is similar to that of chloroplast membranes, and, in particular, to that of thylakoid membranes, and it is different from that of the membranes of most bacteria, which contain phospholipids as the major glyc-erolipids. The fatty acid composition of the membrane lipids of cyanobacteria differs among strains, and cyanobacteria can be classified into four groups in terms of their fatty acids. Among unsaturated fatty acids, monounsaturated to tetraunsaturated fatty acids have been found in cyanobacteria. In a limited number of cyanobacteria, the fatty acid composition resembles that of chloroplast membranes. The biosynthesis of membrane lipids in cyanobacteria has been studied in vivo by tracer experiments with radio-labeled compounds since the 1980s. However, the characterization and molecular identification of the enzymes involved in the biosynthesis of lipids had to await the cloning of the genes for these enzymes since most of these enzymes are membrane-bound and, thus, their solubilization and subsequent purification are relatively difficult. Genes for the desaturases of cyanobacteria, which introduce double bonds into fatty acids bound to membrane lipids, were cloned in the early 1990s, after identification of the desA gene that was able to complement a mutation that resulted in a defect in desaturation at the Δ12 position. Many genes for enzymes involved in the biosynthesis of various lipid classes were identified in the 2000s with the help of newly available databases of cyanobacterial genomic sequences. Many of the proteins encoded by the identified genes are homologous to their respective counterparts in chloro-plasts, supporting the endosymbiosis theory, but some of them are structurally distinct. This distinction suggests the possibility that original genes were replaced by unrelated genes during the evolution of chloroplasts from cyanobacteria. The biosynthesis of membrane lipids in cyanobacteria is regulated by environmental conditions, indicating that membrane lipids play an active role in adaptive processes. Studies of cyanobacterial lipids have shed light both on the evolution of chloroplasts and the adaptation of photosynthetic organisms to changes in environmental conditions.