Nanoarchaeum equitans

Nanoarchaeum equitans
https://en.wikipedia.org/wiki/Nanoarchaeum_equitans

Nanoarchaeum appears to be an obligate symbiont on the archaeon Ignicoccus; it must be in contact with the host organism to survive. Nanoarchaeum equitans cannot synthesize lipids but obtains them from its host.

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The interaction of Nanoarchaeum equitans with Ignicoccus hospitalis: proteins in the contact site between two cells
Biochem. Soc. Trans. (2009) 37, 127–132
Tillmann Burghardt, et al.

The two archaea Ignicoccus hospitalis and Nanoarchaeum equitans form a unique intimate association, the character of which is not yet fully understood.
Electron microscopic investigations show that at least two modes of cell–cell interactions exist: (i) the two cells are interconnected via thin fibres; and (ii) the two cell surfaces are in direct contact with each other.

N. equitans has a genome of just 0.49 Mb, by far the smallest of all archaeal cells known today.
It apparently lacks genes for the biosynthesis of lipids, nucleotides, many amino acids and cofactors, i.e. molecules essential for the functioning of a physiologically active cell.
This led to the conclusion that N. equitans cells take up important metabolic products from their hosts [5].
Two studies provide evidence that this is indeed the case: the close match of the membrane lipid structure of both micro-organisms can best be explained by the transport of lipids from I. hospitalis to N. equitans [6]. 13C-labelling demonstrated the uptake of I. hospitalis amino acids by N. equitans cells [3].

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