Methanogens, archaebacterial

The presence of the enzyme and cofactor are co-incident, indicating that lipoic acid may indeed be the trae substrate of the archaebacterial dihydrolipoamide dehydrogenase. Interestingly, their presence can be correlated with the organisms phylogenetic positions within the archaebacteria. That is, the archaea comprise two main divisions - the methanogens, extreme halophiles. Thermoplasma and Thermococcus in one, and the remaining sulphur-dependent thermophiles in the other [66,72]. The enzyme and/or the cofactor have been detected in all the phenotypes of the former division but neither have yet been discovered in the latter. Further analyses are required to test this correlation as the data are incomplete. 

Shioda et al. [43,44] visualized by electron microscopy both regions of naked DNA and of DNA covered with particles in the chromosome of Halobacterium salinarium isolated from gently lysed cells. In a control experiment, they did not detect such particles in E. coli. They also reported the existence of nucleosome-like structures in S. acidocaldarius and methanogens (unpublished results cited in ref. [43]). The size of the particles detected in H. salinarium (9.5 nm) is similar to that of eukaryotic nucleosomes (10.3 nm) however, this putative archaebacterial chromatin is not as regular as eukaryotic chromatin, since not all of the DNA is covered with nucleosomes and since the length of the DNA spacer between the particles is not uniform. In contrast to these results, Bohrmann and coworkers [45] did not visualize nucleosome-like structures in isolated chromosome fibers of Thermoplasma acidophilum. These authors also reported that in situ the nucleoid of T. acidophilum appears to be highly dispersed in the cytoplasm. 

The study of methanogenic bacteria from the archaebacterial kingdom revealed an impressive number of hitherto unknown coenzymes, including the deazariboflavin derivative, coenzyme F42o- In methanogenic bacteria, they serve an important role in the conversion of CO2 and of low molecular weight organic acids into methane. More recently, deazaflavin coenzymes were also found in Streptomycetes and in mycobacteria. In Streptomycetes, they function as antenna chromophore of DNA photolyase. 

Figure 2. The eoc>te tree. A Shape of the large (above) and small (below) ribosomal subunits of Eubacteria, Halobacteria, Eocytes and Eukaryotes (from Lake et al. 1984). B Rooted evolutionary tree for five groups of extant organisms Eubacteria, Halobacteria, Methanogens, Eocytes (Sulfobacteria) and Eukaryotes (from Lake 1988). C The archaebacterial tree and the eocyte tree, both unrooted (from Lake et al.l984)...

Chen, M. and Poulter, C.D. (2010) Characterization of thermophilic archaeal isopentenyl phosphate kinases. Biochemistry, 49, 207-217. Chong, P.L.-G. (2010) Archaebacterial bipolar tetraether lipids physicochemical and membrane properties. Chem. Phys. Lipids, 163, 253-265. Comita, P.B. and Gagosian, R.B. (1983) Membrane lipid from deep-sea hydrothermal vent methanogen a new macrocydic glycerol diether. Science, 222,1329-1331. 

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