  The initial analysis of the recently sequenced genome of Acanthamoeba
polyphaga Mimivirus, the largest known double-stranded DNA virus, predicted a
proteome of size and complexity more akin to small parasitic bacteria than to
other nucleo-cytoplasmic large DNA viruses, and identified numerous functions
never before described in a virus. It has been proposed that the Mimivirus
lineage could have emerged before the individualization of cellular organisms
from the 3 domains of life. An exhaustive in silico analysis of the non-coding
moiety of all known viral genomes, now uncovers the unprecedented perfect
conservation of a AAAATTGA motif in close to 50% of the Mimivirus genes. This
motif preferentially occurs in genes transcribed from the predicted leading
strand and is associated with functions required early in the viral infectious
cycle, such as transcription and protein translation. A comparison with the
known promoter of unicellular eukaryotes, in particular amoebal protists,
strongly suggests that the AAAATTGA motif is the structural equivalent of the
TATA box core promoter element. This element is specific to the Mimivirus
lineage, and may correspond to an ancestral promoter structure predating the
radiation of the eukaryotic kingdoms. This unprecedented conservation of core
promoter regions is another exceptional features of Mimivirus, that again
raises the question of its evolutionary origin.
