Archaebacterial ether lipids and chemotaxonomy

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Summary The uniqueness of archaebacterial lipids is an important factor in placing these organisms on a separate evolutionary path. Isopranyl glycerol ethers make up the hydrophobic core of the archaebacterial membrane rather than the fatty acid ester-linked glycerol lipids which predominate in the membranes of eubacterial and eukaryotic cells. Furthermore, the optical activity characteristic of the central carbon of glycerol, and the major pathway for lipid biosynthesis in archaebacteria are the opposite to the case in the other two primary kingdoms. These considerations imply important differences in the construction of the archaebacterial membrane. The hydrocarbon chains of the glycerol ethers are isopranoid branched and almost invariably fixed at a chain length of either 20- or 40-carbon atoms. Phytanyl diethers containing two C20 phytanyl chains appear to be universal components in those archaebacteria thus far examined. Dibiphytanyl diglycerol tetraethers that contain two C40 biphytanyl chains ether-linked to two glycerol molecules are also major components although they are not common to all archaebacteria. They occur in many methanogenic, thermophilic and thermoacidophilic archaebacteria but have not yet been found in the extreme halophiles. Thus far, few exceptions to the two basic glycerol ether structures are known to occur in archaebacteria. Haloalkalophilic archaebacteria have been shown to possess diethers that contain a C20 phytanyl and a C25 sesterterpanyl chain and diethers that possess two C25 sesterterpanyl chains, in addition to the usual diethers. One thermophilic methanogen, Methanococcus jannaschi, has been shown to contain a diether possessing a C40 biphytanyl macrocyclic loop as a major component. Two exceptions to the basic diglycerol tetraether structure occur in Sulfolobus species. In addition to the usual tetraethers, these organisms have large amounts of nonitolglycerol tetraethers in which a nine carbon nonitol substitutes for one of the two glycerol molecules. Small amounts of a tetraether containing one C40 bibphytanyl chain and two C20 phytanyl chains are also present. The general invariability of the isopranyl glycerol ethers can serve as a convenient molecular marker for supporting the identification of an organism as an archaebacterium. Utilization of such a marker in attempting to identify new organisms as archaebacteria has also uncovered a variety of unusual new lipids in other organisms which should prove useful in assessing evolutionary relationships among eubacteria.