Study on the evolution of Archaea and Bacteria from the view points of archaeal polar lipid biosynthesis

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Polar lipids are indispensable constituents of cellular membranes. Because the first cell came into existence at the occurrence of polar lipids, the elucidation of differences in the structure and biosynthetic mechanism between Archaea and Bacteria would give a clue to discuss the origin of Archaea and Bacteria, which show differences in the fundamental biochemistry. Previous studies revealed that one of the most fundamenntal biochemical differences was the stereochemical structure of glycerophosphate backbone of membrane polar lipids, and it was proposed that the segregation of the chiral lipids triggered off differentiation of the Archaea and Bacteria. The present study focused on the evolution of polar groups of membrane polar lipids in relation with the differentiation of Archaea and Bacteria.It was elucidated in the present study that synthases of inositol phospholipid and glycerol phospholipid belonged to the same enzyme family, CDP-alcohol phosphatidyltransferase family, along w … More ith serine phospholipid synthase including ester-type phospholipid synthases. This conclusion was drawn from BLAST search of archaeal and bacterial genomes using the amino acid sequence of archaetidylserine synthase as a query and in vitro studies of archaetidylserine synthase from a methanoarchaeon. Activities of monoglucosylarchaeol synthase and diglucosylarchaeol synthase were detected and partialy characterized. Saturated archaeol was sequentially glucosylated with UDP-glucose by two separate enzymes. Although glycolipid synthases in Archaea have not been phylogenetically elucidated, the outline of the enzymatic mode of reaction of archaeal glycolipid synthesis was the similar to bacterial counterparts. The archaeal monoglucosylarchaeol synthase recognized the stereostructure of the lipid substrate.It was proposed from the above results that the polar lipids of pre-cells proposed by Wachtershauser at the stage before differentiation of Archaea and Bacteria had common and contemporary polar head groups shared by Archaea and Bacteria. Polar lipids with racemic core lipid and common polar groups were segregated into homochiral polar lipids carrying common polar groups. This was the cause of differentiation of Archaea and Bacteria. Homologous enzymes for phospholipid synthesis were carried over into both domains of organisms. Stereospecific glycolipid synthases would be developed after differentiation of Archaea and Bacteria. Less