The amino terminal amino acid sequences of 10 ribosomal proteins from the 30S subunit of the extremely halophilic Archaebacterium Halobacterium cutirubrum have been determined. The sequence data are considerably different from the known sequences of ribosomal proteins from E. coli, B. stearothermophilus, B. subtilis, S. cerevisiae and rat liver.
The archaebacteria currently consist of several distinct subgroups including methanogens, extreme halophiles and certain thermoacidophiles. The lipids of archaebacteria are distinguished from those of other prokaryotes and eukaryotes by the absence of fatty acid glycerol ester lipids and the predominance of nonsaponifiable lipids. The lipid composition of the archaebacteria consists of isoprenoid and hydroisoprenoid hydrocarbons and isopranyl glycerol ether lipids.
The glycerol ethers of archaebacteria, which constitute the hydrophobic residues of the polar lipids and consequently the membrane interior are diphytanylglycerol diethers or dibiphytanyldiglycerol tetraethers. Either or both glycerol ether structures may be present, depending on genus. The tetraethers of the thermoacidophilic archaebacteria are more specialized in that the dibiphytanyl alkyl chains may contain 1 to 4 cyclopentyl rings. As a consequence of the presence of the tetraethers which can span the membrane, some archaebycterial membranes may exist as a lipid “monolayer” rather than the usual lipid bilayer. The structure of some diether-containing polar lipids of archaebacteria have been well established. The extent of the variety of tetraether containing polar lipid structures is still largely unknown, but both the symmetric and asymmetric substitution of polar head groups to the tetraether has been established in some instances. Among neutral lipids, squalenes and isoprenoid hydrocarbons appear to be universal. The exact pathways for the biosynthesis of the lipid components remain a challenge, but clearly the mevalonate pathway for isoprenoid biosynthesis is the major route of lipid synthesis in archaebacteria rather than the malonyl-CoA pathway for fatty acid biosynthesis in prokaryotes and eukaryotes.
The isopranyl glycerol ethers are distinctive, providing a useful taxonomic tool and molecular marker for the identification of archaebacteria. The lipids can also serve as useful biochemical “fossil” evidence for tracing the earlier existence of the organisms. Overall, the discontinuity of archaebacterial lipids formulates a point for delineating early stages of biological evolution and supports the concept that archaebacteria represent a third line of evolutionary descent.
The chemical composition of trypsin-treated cell walls of three strains of Halococcus morrhuae was determined. Neutral sugars (glucose, mannose, galactose), uronic acids (glucuronic and galacturonic acids), amino sugars (glucosamine, galactosamine), gulosaminuronic acid, acetate, glycine and sulfate were found as major constituents. The cell wall of H. morrhuae CCM 859 was studied in more detail. The major cell wall polymer of this strain is a complex heteroglycan which seems to be responsible for the rigidity and stability of the cell wall. The amino groups of the amino sugars are predominantly N-acetylated. A substitution of the amino groups with glycine instead of acetate could be found for part of the glucosamine residues. Sulfate groups are covalently bound as esters to secondary hydroxyl groups in equatorial conformation. Based on periodate cleavage and permethylation studies of the cell wall and analyses of isolated oligosaccharides, the chemical structure of the cell wall polymer can be proposed as follows:
Sulfate groups are linked to hydroxyl groups in positions 2 and/or 3 of uronic acids, galactose and galactosamine residues. Glucose, galactose, galacturonic acid and all amino sugars are 1 → 4 glycosidically linked in the cell wall polymer. A part of the glucose, galactose and to a lesser extent mannose residues possess more than two glycosidic linkages and represent possible branching points. Glycine residues may play a role in connecting glycan strands through peptidic linkages between the amino group of glucosamine and the carboxyl group of an uronic acid or gulosaminuronic acid.
The available information on primary and secondary structure of archaebacterial 5S rRNA is reviewed. The extent of primary sequence diversity is comparable to that seen among eubacterial genera. This is consistent with the view that the archaebacteria represent a taxon of the highest order among entities with procaryotic organization. The sequence information is also used to construct a phylogenetic tree which within the limitations of the data is in reasonable agreement with earlier results. In terms of secondary structure the archaebacterial 5 S rRNAs do not conform to the generally accepted models for either eubacterial or eucaryotic cytoplasmic 5S rRNAs. Instead there may be several classes of structures represented. Each of these would include a mixture of eubacterial features, eucaryotic features and unique features.
These findings are discussed as they relate to the evolutionary position of the archaebacteria. It is pointed out that lateral gene transfer may have been far more frequent in the earliest stages of evolution and it is argued that this could profoundly effect the way one interprets data pertaining to the phylogenetic position of the archaebacteria. It is also observed that future studies would benefit greatly by inclusion of a phylogenetically diverse selection of archaebacteria.
The extreme acidophile Thermoplasma acidophilum is about an order of magnitude less sensitive to certain H+ ionophores than is a bacillus that lives in similar conditions. Thermoplasma is also totally insensitive to DCCD, a specific inhibitor of the H+-trans-locating ATPase, and a potent poison to virtually all other organisms that have been examined. Thus, the unusual metabolic properties of Thermoplasma membranes are of particular interest.
The membranes were purified from Thermoplasma by means of sonication and differential centrifugation. They contained respiratory enzymes capable of reducing O2, using either NADH, D-lactate, or succinate as an electron donor. According to the reduced/ oxidized difference spectrum of the purified membranes, the respiratory chain appeared to consist of only a b-type cytochrome plus a quinone.
The purified membranes also contained a phosphatase activity capable of hydrolyzing ATP, ADP, and inorganic PPi, but not organic monophosphates such as AMP and 1-glycerol phosphate. This activity was specifically stimulated by MgS04, and was not inhibited by either DCCD or ouabain. It is proposed that this enzyme is a sulfate-exporting trans-locase, and that its activity may be the source of the inside-positive electrical potential of these cells. There was no evidence of any other membrane-bound ATPase. Presumably, the only mechanism for expelling H+ is by the respiratory chain described above, which could account for the observation that T. acidophilum is obligately aerobic.
Two novel species of anaerobic thermophilic archaebacteria isolated from acidic hot springs of Iceland, Desulfurococcus mucosus and Desulfurococcus mobilis, representing a second family, termed Desulfurococcaceae, of the order Thermoproteales are described.
They utilize yeast extract or casein or its tryptic digest, but not casamino acids, as carbon sources, by sulfur respiration with the production of H2S and CO2, or by fermentation.
The pH optimum of growth is pH 5.5 to 6, the temperature optimum 85 °C.
The archaebacterial nature of the Desulfurococcaceae is evident from their insensitivity towards vancomycin, streptomycin and chloramphenicol, the lack of a murein cell wall, the presence of phytanol and polyisoprenoid dialcohols in the lipids, and the composition and the properties of the DNA dependent RNA polymerase.
They are closely related to the recently described anaerobic thermoacidophilic sulfur-respiring Thermoproteus tenax representing the first family, Thermoproteaceae of the Thermoproteales. Of the other divisions of archaebacteria, Sulfolobus is the nearest relative.
Desulfurococcus mucosus has a slimy polymer attached to its envelope. Desulfurococcus mobilis possesses flagellae in monopolar polytrichous arrangement.
Most methanogenic bacteria can grow on H2 plus CO2 as sole energy source; CO2 is reduced to methane: 4 H2 + CO2 → CH4 + 4 H2O. The exergonic step in this process is probably the reduction of methyl CoM to methane. This reaction is catalyzed by the methyl CoM reductase, the prosthetic group of which is factor F430. This factor contains nickel and has been shown to have a nickel tetrapyrrole structure. The involvement of a nickel tetrapyrrole in methane formation explains why growth of all methanogens is dependent on nickel.
A tentative scheme of methanogenesis from CO2 is presented. It involves the participation of methanopterin, coenzyme FA, coenzyme M and the activated form of coenzyme M. CO2 is bound to the enamine group of methanopterin and reduced to the formyl level by an intramolecular redox reaction. Coenzyme M is activated by binding to factor F430, a nickel-tetrapyrrole present in methylcoenzyme M reductase.
Results from studies to elucidate the protein composition of archaebacterial ribosomes and their immunological relatedness are summarized. In vivo and in vitro investigations on the susceptibility of translation in archaebacteria to inhibition by ribosome-targeted antibiotics are also discussed.
Pseudomurein, the cell wall polymer found in all species of the order Methanobacteriales, is composed of glycan strands cross-linked by peptide subunits. The glycan strands are composed of alternating N-acetyl-D-glucosamine or N-acetyl-D-galactosamine and N-acetyl-D-talosaminuronic acid residues in β-1.3-linkage. In most species the cross-linking peptide subunits consist of alanine, glutamic acid and lysine. However, alanine is completely or partly replaced by threonine in Methanobrevibacter ruminantium. In Methano-brevibacter smithii ornithine was found as an additional amino acid residue attached by its (δ-amino group to the α-carboxyl group of the terminal glutamyl residue.
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