  The discovery and genome analysis of Acanthamoeba polyphaga Mimivirus, the
largest known DNA virus, challenged much of the accepted dogma regarding
viruses. Its particle size (>400 nm), genome length (1.2 million bp) and huge
gene repertoire (911 protein coding genes) all contribute to blur the
established boundaries between viruses and the smallest parasitic cellular
organisms. Phylogenetic analyses also suggested that the Mimivirus lineage
could have emerged prior to the individualization of cellular organisms from
the three established domains, triggering a debate that can only be resolved by
generating and analyzing more data. The next step is then to seek some evidence
that Mimivirus is not the only representative of its kind and determine where
to look for new Mimiviridae. An exhaustive similarity search of all Mimivirus
predicted proteins against all publicly available sequences identified many of
their closest homologues among the Sargasso Sea environmental sequences.
Subsequent phylogenetic analyses suggested that unknown large viruses
evolutionarily closer to Mimivirus than to any presently characterized species
exist in abundance in the Sargasso Sea. Their isolation and genome sequencing
could prove invaluable in understanding the origin and diversity of large DNA
viruses, and shed some light on the role they eventually played in the
emergence of eukaryotes.
