Epixenosomes

Epixenosomes are prokaryotes phylogenetically related to the bacterial division Verrucomicrobia. The generic name is derived by the episymbiotic adactation of the organisms (from the ancient Greek = external alien body), indeed they live on the dorsal surface of marine ciliated protozoa of Euplotidium genus. Epixenosomes possesss two distinct developmental phases linked to the host cell cycle. Stage I epixenosomes, are spherical, 0.5 micrometers in diameter and are surrounded by two membranes. They divide by direct binary fission. Stage II epixenosomes, are egg-shaped (2.2 micrometers in length and 1 micrometer in width) and show complex organization (more complicated than the majority of prokaryotic organisms) with different cytoplasmic compartments where an extrusive apparatus within a proteinaceous matrix, although apparently not membrane bounded, differs from the remaining cytoplasm. A functional cell compartmentalization has also been evidenced. The ejection process is involved in the defence of the ciliate host against predators. External signals of unknown origin are detected by the membrane receptors located at the top of the organism. The consequent activation of the adenylate cyclase-cAMP system triggers the ejection of the extrusive apparatus, with the formation of a hollow tube, about 40 micrometes long, terminating in a head mainly consisting of the apical portion of the epixenosome (the region containing DNA). Extrusive apparatus is surrounded by a basket of tubules. Based on experiments with antitubulin drugs and immunocytochemical analyses at the optical and electron microscopical level the possibility that these tubules consist of tubulin (i.e. eukaryotic protein) has been considered. Interestingly the presence of tubulin genes has bee reported in members of free living Verrucomicrobia, in which however tubular structures have not been evidenced.

Sources Rosati G. (1999) Epixenosomes: symbionts of the hypotrich ciliate Euplotidium itoi. Symbiosis,. 26: 1-23

Rosati G., Petroni G., Quochi, S., Modeo L. & Verni F. (1999) Epixenosomes, peculiar Epibionts of the hypotrich Ciliate Euplotidium itoi, defend their host against predators. J. Eukaryotic Microbiol.46: 278-282

Petroni G., Spring S., Schleifer K-H., Verni F. & Rosati G. (2000). Defensive exstrusive ectosymbionts of Euplotidium (Ciliophora) that contain microtubule-like structures are bacteria related to Verrucomicrobia. Proc. Natl. Acad. Sci. 97(4) 1813-1817.

Jenkins C, Samudrala R., Anderson I, Hedlund BP, Petroni G., Michailova N., Pinel N., Overbeek R, Rosati G, Stanley JT (2002) Genes for the cytoscheletal protein tubulin in the bacterial genus Prosthecobacter. Proc. Nat. Acad. Sci. )5: 8550-55. USA