The recent cryo-electron microscopy
(cryo-EM) and 3D image reconstruction of Haloarcula
sinaiiensis tailed virus 1 (HSTV-1) at subnanometer reso-
lution showed that archaeal head-tailed viruses also adopt
the HK97 fold (Figure 3). Interestingly, this HK97 fold
is also characteristic of herpesviruses and is indicative of a
common ancestry linking archaeal and bacterial headtailed
viruses together with eukaryotic herpesviruses. At the genome level, remarkable diversity can
be found among the head-tailed viruses. Their linear
genomes are mosaics composed of variable and conserved
genomic regions, which have evolved by recombination
and have been derived from a common global pool of genes.
All described tailless icosahedral viruses infecting archaea
have a membrane under the capsid shell and share
their virion morphology with the bacterial counterparts
without any sequence similarity. All these viruses have
either a linear or circular dsDNA genome and their protein-
rich membranes follow the icosahedral shape of
the capsid.