The Archaea (also called Archaebacteria) are a group of prokaryotes that became something of a cause celebre when rDNA analysis by Carl Woese and colleagues discovered a large genetic distance between them and the other prokaryotes. This lead to their promotion as a 'third domain' of life, in addition to the Eubacteria and eukaryotes (Woese, Kandler & Wheelis, 1990). In a number of features, they are more similar to eukaryotes than Eubacteria, as well as having an assortment of unique features of their own.
(from Cavalier-Smith, 2002) Cell membrane of prenyl ether lipids. Flagellar shaft of acid-insoluble glycoproteins related to pilin. DNA binding protein 10b. tRNA modifications, including archaeosine in D-loop and absence of queuine. Tiny large subunit ribosomal protein, LX. No Hsp90 chaperone. RNA polymerase A split into two proteins. Glutamate synthetase split into three proteins.
Recently a third branch has been named, the 'Nanoarchaeota', with a single species, 'Nanoarchaeum equitans' (Huber, Hohn et al., 2002). 'Nanoarchaeum' is a greatly reduced, obligate symbiont of another hyperthermophilic archaeon, Ignicoccus. However, 'Nanoarchaeum' has not yet been validly described under the Bacteriological Code of Nomenclature (which requires, among other things, validation in the International Journal of Systematic and Evolutionary Microbiology and deposition of an isolated culture in at least two repositories).
A fourth branch, the 'Korarchaeota', has only been recognised from environmental DNA sequences. No members of this group have yet been recognised as live organisms.
Cavalier-Smith, T. 2002. The neomuran origin of archaebacteria, the negibacterial root of the universal tree and bacterial megaclassification. International Journal of Systematic and Evolutionary Microbiology 52: 7-76.
Huber, H., M. J. Hohn, R. Rachel, T. Fuchs, V. C. Wimmer & K. O. Stetter. 2002. A new phylum of Archaea represented by a nanosized hyperthermophilic symbiont. Nature 417: 63-67.
Woese, C. R., O. Kandler & M. L. Wheelis. 1990. Towards a natural system of organisms: Proposal for the domains Archaea, Bacteria, and Eucarya. Proceedings of the National Academy of Sciences of the USA 87: 4576-4579.