Membrane Lipid Biosynthesis in Purple Bacteria

Abstract

Membranes are essential to all living cells. They provide the boundary to the surrounding environment, allow the controlled exchange of compounds through membrane transporters, and serve as a matrix for membrane associated enzymes and protein complexes involved in the generation of energy or communication with the environment. Biomembranes are built from amphipathic, polar lipids that either on their own or in mixtures with other lipids spontaneously form a bilayer in aqueous solutions. Proteins are embedded into this lipid matrix serving many different functions. Photosynthetic purple bacteria have a very rich complement of membrane lipids including phospholipids not commonly found in bacteria such as phosphatidylcholine, glycolipids typical for plant chloroplasts such as and the betaine and ornithine lipids. These latter lipids lack phosphorus presumably allowing purple bacteria to outcompete other organisms in a phosphorusdepleted environment. Advances in the genetic analysis of lipid metabolism of purple bacteria and related bacteria of the α-proteobacteria group have provided us with many genes encoding enzymes for the biosynthesis of polar membrane lipids beyond those described for Escherichia E. coli . Lipid genes discovered first in Rhodobacter Rba. sphaeroides include the sqd genes required for biosynthesis and the bta genes required for betaine lipid biosynthesis. Similar approaches in the closely related bacterium Sinorhizobium Sr. meliloti in combination with genome comparison provided the genes of purple bacteria encoding proteins of ornithine lipid biosynthesis and a new pathway for phosphatidylcholine biosynthesis. Advances in the analysis of membrane associated protein complexes gave new insights into specific interactions of lipids with these complexes.