We describe the first virus-like particle of a hyperthermophilic euryarchaeote which was discovered in a strain of "Pyrococcus abyssi" previously characterized in our laboratory. This particle, named PAV1, is lemonshaped (120 nm ؋ 80 nm), with a short tail terminated by fibers, and resembles the virus SSV1, the type member of the Fuselloviridae, isolated from Sulfolobus shibatae. Sensitivity of the virus-like particle to organic solvents and detergents suggested that the envelope of PAV1 may contain lipids in addition to proteins. It contains a double-stranded circular DNA of 18 kb which is also present in high copy number in a free form in the host cytoplasm. No integrated form of the PAV1 genome could be detected in the host chromosome. Under standard growth conditions, the host cells continuously release PAV1 particles into the culture supernatant without spontaneous lysis, with a maximum reached in the late stationary phase. UV, gamma irradiation, treatment with mitomycin C, and various physiological stresses had no effect on PAV1 production. Screening of a large number of Thermococcales isolates did not permit to find a sensitive host. These results suggest that PAV1 persists in the host strain in a stable carrier state rather than a prophage.The Archaea domain comprises two major phyla, namely, the Crenarchaeota, including the extremely thermophilic sulfur-metabolizing Archaea of the orders Sulfolobales and Thermoproteales, and the Euryarchaeota, containing mainly the methanogens, the extreme halophiles, and the hyperthermophilic order Thermococcales (29). Our knowledge about archaeal viruses is still rather limited, and among known archaeal viruses that have been reported, only few have been studied in detail at the molecular level.All known crenarchaeotal viruses display unusual morphotypes and compose three novel families which were created to account for their unique features, namely, the filamentous Lipothrixviridae (2, 13, 32), the lemon-shaped Fuselloviridae (18,25), and the rod-shaped Rudiviridae (20). The dropletshaped Guttaviridae (3) have not yet been established as an acknowledged virus family. The best studied virus is the lemonshaped SSV1, whose original host is the hyperthermophile Sulfolobus shibatae. SSV1 is temperate and forms stable lysogens by site specifically inserting its 15.5-kb circular genome into the host chromosome (31). Its complete nucleotide sequence has been determined (19). In contrast, all but two of the as-yet-described viruses of extreme halophiles and methanogens have the classical head-and-tail morphology typical of many bacterial phages and have therefore been assigned to the virus families Myoviridae or Siphoviridae (1). There are two known exceptions, both showing a lemon-shaped morphology resembling SSV1, the type member of the Fuselloviridae. The first one was described as a virus-like particle (VLP) isolated from M. voltae strain A3 containing a circular double-stranded DNA of 23 kb, of which an integrated copy was found in the host chromosome (30). The second ex...

Summary We describe a novel virus, TPV1 (Thermococcus prieurii virus 1), which was discovered in a hyperthermophilic euryarchaeote isolated from a deep-sea hydrothermal chimney sample collected at a depth of 2700 m at the East Pacific Rise. TPV1 is the first virus isolated and characterized from the hyperthermophilic euryarchaeal genus Thermococcus. TPV1 particles have a lemon-shaped morphology (140 nm × 80 nm) similar to the structures previously reported for Fuselloviruses and for the unclassified virus-like particle PAV1 (Pyrococcus abyssi virus 1). The infection with TPV1 does not cause host lysis and viral replication can be induced by UV irradiation. TPV1 contains a double-stranded circular DNA of 21.5 kb, which is also present in high copy number in a free form in the host cell. The TPV1 genome encompasses 28 predicted genes; the protein sequences encoded in 16 of these genes show no significant similarity to proteins in public databases. Proteins predicted to be involved in genome replication were identified as well as transcriptional regulators. TPV1 encodes also a predicted integrase of the tyrosine recombinase family. The only two genes that are homologous between TPV1 and PAV1 are TPV1-22 and TPV1-23, which encode proteins containing a concanavalin A-like lectin/glucanase domain that might be involved in virus–host recognition.