The Archaebacteria versus the Eubacteria

Although most common bacteria are eubacteria, there is another group, the archae-bacteria or archaea as they have recently been renamed. Both types of bacteria have microscopic cells without a nucleus. They both have single circular chromosomes and divide in two by simple binary fission. In short, they both conform to the definition of a prokaryotic cell and there was no obvious reason to suspect from their superficial structure that they were so radically different. However, sequence analysis of riboso-mal RNA indicates that there is about as much genetic difference between the eubac-teria and archaea as between either of these two groups and eukaryotic cells.

Of the two groups of prokaryotes, the archaea are probably slightly more closely related to the urkaryote, the primeval ancestor of the eukaryotic nucleus. Some archaebacteria One of the three domains of life comprising the "ancient" bacteria archaea New name for archaebacteria, one of the three domains of life eubacteria One of the three domains of life comprising the "true" bacteria, including the organelles urkaryote The hypothetical primeval ancestor of the eukaryotic nucleus

FIGURE 20.18 The Three Domains of Life

All of today's organisms belong to one of three main divisions based on relationships among ribosomal RNA: the eubacteria, the archaebacteria (or archaea) and the eukaryotes. Mitochondria and chloroplasts have rRNA that most closely resembles eubacteria.

Eukaryotic

Chloroplasts Nucleus Archaea

Eukaryotic

Chloroplasts Nucleus Archaea

FIGURE 20.18 The Three Domains of Life

All of today's organisms belong to one of three main divisions based on relationships among ribosomal RNA: the eubacteria, the archaebacteria (or archaea) and the eukaryotes. Mitochondria and chloroplasts have rRNA that most closely resembles eubacteria.

archaea have their DNA packaged by histone like proteins that show some sequence homology to the true histones of higher organisms. In addition, the details of protein synthesis and the translation factors of archaea resemble those of eukaryotes, rather than eubacteria. These similarities have led to the suggestion that the primeval eukary-ote evolved from an archaeal ancestor.

Archaea differ biochemically from eubacteria in several other major respects. Archaea have no peptidoglycan and their cytoplasmic membrane contains unusual lipids, which are made up from C5 isoprenoid units rather than C2 units as with normal fatty acids (Fig. 20.21). Moreover, the isoprenoid chains are attached to glycerol by ether linkages instead of esters. Some double-length isoprenoid hydrocarbon chains stretch across the whole membrane.

Archaea tend to be found in bizarre environments and many of them are adapted to extreme conditions. They are found in hot sulfur springs, thermal vents in the ocean floor, in the super salty Dead Sea and Great Salt Lake and also in the intestines of cows (and other animals) where they make methane. Some examples of archaea are:

Halobacteria: These are extremely salt-tolerant and grow in up to 5M NaCl but will not grow below 2.5M NaCl (sea water is only 0.6M). They trap energy from sunlight by using bacterio-rhodopsin, a molecule related to the rhodopsin pigment used as a photo-detector in animal eyes.

Methanogens (methane producing bacteria): These are obligate anaerobes and are very sensitive to oxygen. They convert H2 plus CO2 to CH4 (methane). Their metabolism is unique—they contain coenzymes found in no other living organisms but have no typical flavins or quinones.

Sulfolobus: Lives in geothermal springs and grows best at a pH optimum of 2-3 and a temperature of 70-80°C. These archaea oxidize sulfur to sulfuric acid. Many other sulfur-metabolizing archaea are also found living under a variety of extreme conditions.

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