Bacteria archaebacteria

In the latest taxonomic system based on rRNA sequencing, three domains have been recognized Archaea (the archaebacteria), Bacteria (the eubacteria) and Eukarya (the eukaryotes). When this system is used, bacteria with a lower-case b is generally assumed to refer to all bacteria (archaebacteria + eubacteria). 

Figure 9.9 Unusual di- and tetra-ether lipids from Archae bacteria (archaebacteria). (a) 2,3-Di-O-phylanyl-sw-glycerol (b) Iranf-biphytanyl diglycerol tetra ether.

Most of the bacterial viruses which have been studied in any detail infect bacteria of the enteric group, such as Escherichia coli and Salmonella typhimurium. However, viruses are known that infect a variety of procaryotes, both eubacteria and archaebacteria. A few bacterial viruses have lipid envelopes but most do not. However,... 

A second example of the differences between important biomolecules in archaebacteria and eubacteria is their DNA-dependent RNA polymerase. The enzyme found in archaea resembles that in eukaryotes more than it does those in bacteria ... 

Fig. 5.7. In green sulfur bacteria and in some archaebacteria, a reverse citric acid cycle is used for the assimilation of C02. It must be assumed that this was the original function of the citric acid cycle that only secondarily took over the role as a dissimulatory and oxidative process for the degradation of organic matter. A major enzyme here is 2-oxoglutarate ferredoxin for C02 fixation. Note that it, like several other enzymes in the cycle, uses Fe/S proteins. One is the initial so-called complex I which has eight different Fe/S centres of different kinds but no haem (see also other early electron-transfer chains, e.g. in hydrogenases).

This may explain why this family of bacteria is found in dairy products where the presence of lactoferrin makes iron availability problematic. Class II RNRs is also found in some archaebacteria. 

Sirtuins have been conserved from bacteria to eukaryotes. Notably, they all possess a conserved catalytic core domain flanked by sequence-divergent N- and C-terminal regions. If bacteria and archaebacteria generally possess one or two sirtuins, this number is higher in eukaryotes, with five sirtuins in Saccharomyces cerevisiae and seven in human. The presence of sirtuins in all phyla of life led to a wealth of structural data, not only on eukaryotic enzymes but also on bacterial and archaebacteria enzymes. 

E. coli detects not only sugars and amino acids but also 02, extremes of temperature, and other environmental factors, using this basic two-component system. Two-component systems have been detected in many other bacteria, including gram-positive and gramnegative eubacteria and archaebacteria, as well as in protists and fungi. Clearly this signaling mechanism developed early in the course of cellular evolution and has been conserved. 

The plasma membrane of bacterial cells, other than the wall-less mycoplasmas and some archaebacteria, is surrounded by a multilayered wall which may be separated from the membrane by a thin periplasm (or periplasmic space). This can be seen most clearly in suitably prepared thin sections of cells of E. coli or other gram-negative bacteria as a relatively empty space of 11- to 25-nm thickness (Fig. 8-28).579 581 The volume of this space (which may be filled with gelled material) depends upon the osmotic pressure of the medium. In E. coli it contains 20-40% of the total... 

Other bacterial coats. Archaebacteria not only have unusual plasma membranes that contain phytanyl and diphytanyl groups (Section A,3)608 but also have special surface layers (S-Iayers) that may consist of many copies of a single protein that is anchored in the cell membrane.609 The surface protein of the hypothermic Staphylothermus marius consists of a complex structure formed from a tetramer of 92-kDa rods with an equal number of 85-kDa "arms."610 611 S-layers are often formed not only by archaebacteria but also by eubacteria of several types and with quite varied structures.612 14 While many bacteria carry adhesins on pili, in others these adhesive proteins are also components of surface layers.615 Additional sheaths, capsules, or slime layers, often composed of dextrans (Chapter 4) and other carbohydrates, surround some bacteria. 

Sometimes referred to as the Third Kingdom of Life. the archaebacteria differ markedly from other bacteria. In fact, most scientists do not consider... 

Fig. 2. An evolution diagram illustrating a suggestion of common ancestry of some present-day organisms. The essential features of present-day photosynthesis may have originated in the prebiotic era and is preserved in its most primitive form in (at least some) present-day phototrophs. The heterotrophs may have developed parallel with the aerobic nonphotosynthetic bacteria, some l to 1.5 x 109 years after the emergence of the cyanobacteria. The eukaryotic photosynthetic organisms developed much later, perhaps some 1.5 to 0.5 x 109 years ago. The archaebacteria are primitive organisms that seem to have no evolutionary relation with the present prokaryotes.21 Little is known about their energy metabolism. Tentatively, they are considered as a very early form of cellular life.

This brings a reconstruction of the primeval structure into the realm of the possible. If the sequences of different tRNAs, for example, for E. coli bacteria, yeast cells, or archaebacteria, are compared, then they all show a high GC content, which increases further when the sequences are superimposed. However, it can be seen from the mitochondrial sequences, which are all rich in A and U, that G and C could be displaced in the course of evolution, presumably on account of a rich supply of the metabolite A in mitochondria G and C are thus not demanded by con-... 

Prokaryotes (bacteria and blue-green algae) are the most abundant organisms on earth. A prokaryotic cell does not contain a membrane-bound nucleus. Bacteria are either cocci, bacilli or spirilla in shape, and fall into two groups, the eubacteria and the archaebacteria. 

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