Viruses infecting Euryarchaeota

The viruses and virus-like particles (VLPs) of the methanogens, extreme halophiles and hyperthermophiles of the archaeal kingdom Euryarchaeota are represented by relatively few isolates, but show a remarkable degree of diversity, with three broad morphological categories. Head-and-tail viruses with similar genome structures to head-and-tail bacteriophages infect both extreme halophiles and methanogens. Of these viruses, the methano-phage 'M1 and relatives show some sequence similarity to Bacteria and bacteriophages; whilst the genomes of haloviruses HF1 and HF2 show little similarity to known sequences, but are closely related to one another and provide evidence of high recombination rates. Haloviruses 'Ch1 and 'H form another related group, even though they originate from different environments and infect different hosts, suggesting that similar viruses are widespread. The spindle-shaped haloviruses such as His1 and His2, and the hyperthermophilic VLP PAV1, morphologically resemble archaeal fuselloviruses such as SSV1, but are genetically distinct and form novel virus groups. Finally, spherical viruses such as halovirus SH1 contain an internal lipid layer, similar to tectiviruses (including bacteriophage PRD1) and the crenarchaeal virus STIV, but share no sequence similarity with these viruses and little similarity with other known sequences. Ongoing studies of these unusual viruses continue to reveal unexpected and interesting characteristics.