Archaeal proviruses TKV4 and MVV extend the PRD1-adenovirus lineage to the phylum Euryarchaeota

The viral lineage hypothesis predicting a common origin for viruses that infect hosts residing in different domains of life gains more support as data on viral structures accumulates. One such lineage is the PRD1-adenovirus lineage, which unites icosahedral dsDNA viruses with large facets and a double β-barrel trimer coat protein. This lineage is represented by a number of viruses infecting bacteria and eukaryotes. However, only one member of the lineage, Sulfolobus turreted icosahedral virus, infecting a crenarchaeal host, has been identified in the domain Archaea. In this study we characterize the genomic sequences of two archaeal proviruses, TKV4 and MVV, integrated into the 5′- and 3′-distal regions of tRNA genes of the euryarchaeal species Thermococcus kodakaraensis KOD1 and Methanococcus voltae A3, respectively. Bioinformatic approaches allowed placement of TKV4 and MVV into the PRD1-adenovirus lineage, thus extending the lineage to the second archaeal phylum, Euryarchaeota.