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 nucleocytoplasmic 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 three domains of life. An exhaustive in silico analysis of the noncoding moiety of all known viral genomes now uncovers the unprecedented perfect conservation of an 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, amoebal protists in particular, 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 feature of Mimivirus that again raises the question of its evolutionary origin.