Phylum Nanoarchaeum

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This phylum, which may be a deep sister-group to the euryarchaea, is known only from a single cultivated specie and a small number of environmental ssu-rRNA sequences from thermal and hypersaline environments. Because commonly-used primers are not able to amplify ssu-rRNA sequences from this group, they may have been missed in surveys of other environments. Therefore, the phylogenetic and phenotypic diversity of this group remains largely unknown.

Example : Nanoarchaeum equitans

Nanoarchaeum equitans on its host, Ignicoccus hospitalis : H. Huber, M. Hohn, R. Rachel & K.O. Stetter, Univ. Regensburg, Germany

N. equitans is an obligate parasite of the crenarchaeote Ignicoccus. The cells are very small cocci, only about 400nm in diameter, that grow attached to the outside of the host cells (which are also cocci). They cannot be cultivated in the absence of the host, but have been isolated as a pure co-culture from a single Ignicoccus cell harboring a single N. equitans parasite. N. equitans is the only hyperthermophilic symbiont known, and the only archaeal parasite or pathogen. It is also one of the smallest cellular organisms known. The N. equitans genome consists of a single circular molecule of slightly less than 0.5Mbp, and lacks almost all biosynthetic genes. Even the ATPase is a minimal version; it may be used in reverse to generate a proton gradient (at the expense of ATP) for use by active transport pumps. In is probably an energy parasite. Most of the genes that remain are those of information processing; replication, transcription, translation, signal transduction, and the cell cycle.

An interesting aspect of the N. equitans genome is how disorganized it is. In most Bacteria and Archaea, genes are organized in operons, with structurally or functionally related proteins generally encoded together. Not so it N. equitans; not even the ribosomal proteins are grouped together in operons (this is otherwise very highly conserved), nor are the ribosomal RNAs (which are almost always encoded together in the order 16S-23S-5S in Bacteria and Archaea, and the homologous 18S-5.8S-28S rRNA genes in Eukarya). Even the functional domains of some enzymes are encoded separately, and some of the tRNAs are encoded in two pieces that are joined by splicing in trans.