gistfile1.txt

Phylogenetic studies indicate that eukaryotic DNA polymerases and some
viral DNA polymerases have a common origin. These studies are not easy to
interpret because only a few of the polymerase domains are conserved and
phylogenetic trees have all been unrooted, so the direction of evolution
cannot be determined. The gene flow between viruses and eukaryotic cells
could have been in either direction. Takemura suggested that
α-polymerases (priming polymerases in eukaryotes) originated from a
pox-like virus. Unlike most other DNA viruses, poxviruses replicate in the
cytoplasm of their hosts, completely independent of the host nucleus.
Vaccinia virus, a well studied poxvirus, encodes protein kinases and
phosphatases that also seem to be homologous with the host nuclear
counterparts, and there are similarities between the structural elements
involved in poxvirus replication and eukaryotic replication. This has led
to the hypothesis that the eukaryotic nucleus arose from a symbiotic
pox-like virus that was engulfed by an early archaeal cell. Other features
of the eukaryotic nucleus, such as linear chromosomes, separation of
transcription and translation, and mRNA capping, have also been attributed
to a viral origin64. This would represent a major evolutionary leap through
symbiogenesis.