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...

Only one virus-like particle (VLP) has been reported from hyperthermophilic Euryarchaeotes. This VLP, named PAV1, is shaped like a lemon and was isolated from a strain of "Pyrococcus abyssi," a deep-sea isolate. Its genome consists of a double-stranded circular DNA of 18 kb which is also present at a high copy number (60 per chromosome) free within the host cytoplasm but is not integrated into the host chromosome. Here, we report the results of complete analysis of the PAV1 genome. All the 25 predicted genes, except 3, are located on one DNA strand. A transcription map has been made by using a reverse transcription-PCR assay. All the identified open reading frames (ORFs) are transcribed. The most significant similarities relate to four ORFs. ORF 180a shows 31% identity with ORF 181 of the pRT1 plasmid isolated from Pyrococcus sp. strain JT1. ORFs 676 and 678 present similarities with a concanavalin A-like lectin/glucanase domain, which could be involved in the process of host-virus recognition, and ORF 59 presents similarities with the transcriptional regulator CopG. The genome of PAV1 displays unique features at the nucleic and proteinic level, indicating that PAV1 should be attached at least to a novel genus or virus family.