Structure, biosynthesis, and physicochemical properties of archaebacterial lipids.

A series of recent phylogenetic studies, based on 16S ribosomal ribonucleic acid (RNA) composition (2, 33, 84, 104, 105) and other general biochemical features (33, 105) such as RNA polymerase (71, 81, 86, 113, 116), translation system (10, 54, 55, 80, 85), transfer RNA (44, 56, 58, 59), 5S ribosomal RNA (32, 69, 70), cell wall (21, 50-52, 77, 84, 103), and lipids (25, 26, 28, 53, 60, 67, 72, 98), reveals the existence of a new group of microorganisms named archaebacteria. Some features, e.g., their small size and structural simplicity, the absence of a nuclear membrane and organelles, and low deoxyribonucleic acid content, indicate that archaebacteria are procaryotic (33, 84, 105). In contrast, it has been proposed that archaebacteria are closely related to the ancestor eucaryotes (79, 100) since several characteristics, such as the presence of histone-like proteins (42, 87, 96), the nature of their translation system (54, 55, 85), and the sequence of their 5S RNA (46) and that of ribosomal proteins (73), are considered to be typical of eucaryotes. This body of results leads us to consider that archaebacteria are as far from eubacteria as they are from eucaryotes (103). Archaebacteria are quite interesting microorganisms from the point of view of the early evolution of life; indeed, their name was given to underline the hypothesis that these organisms were the dominant inhabitants of the earth in early ancient times (2, 33, 104, 105). In this respect, archaebacteria, as a distinct primary kingdom, are very important as they give us some indications of the early events in the evolution of cells, thus contributing to a better understanding of the universal ancestor. Archaebacteria are characterized by a wide metabolic diversity and a high degree of morphological variability that is roughly comparable to that found in eubacteria. In fact, this group includes aerobes, anaerobes, autotrophs, heterotrophs, thermophiles, acidophiles, phototrophs, cocci, rods, and disk-shaped and pleiomorphic forms (1, 6, 7, 9, 16, 22, 49, 74, 77, 83, 93, 104, 105, 111, 112, 114, 115). Archaebacteria are classified into three major phenotypes;