The defining feature of the eukaryotic cell is the possession of a nucleus that uncouples transcription from translation. This uncoupling of transcription from translation depends on a complex process employing hundreds of eukaryotic specific genes acting in concert and requires the 7-methylguanylate (m7G) cap to prime eukaryotic mRNA for splicing, nuclear export, and cytoplasmic translation. The origin of this complex system is currently a paradox since it is not found or needed in prokaryotic cells which lack nuclei, yet it was apparently present and fully functional in the Last Eukaryotic Common Ancestor (LECA). According to the Viral Eukaryogenesis (VE) hypothesis the abrupt appearance of the nucleus in the eukaryotic lineage occurred because the nucleus descends from the viral factory of a DNA phage that infected the archaeal ancestor of the eukaryotes. Consequently, the system for uncoupling of transcription from translation in eukaryotes is predicted by the VE hypothesis to be viral in origin. In support of this hypothesis it is shown here that m7G capping apparatus that primes the uncoupling of transcription from translation in eukaryotes is present in viruses of the Mimiviridae but absent from bona-fide archaeal relatives of the eukaryotes such as Lokiarchaeota. Furthermore, phylogenetic analysis of the m7G capping pathway indicates that eukaryotic nuclei and Mimiviridae obtained this pathway from a common ancestral source that predated the origin of LECA. These results support the VE hypothesis and suggest the eukaryotic nucleus and the Mimiviridae descend from a common First Eukaryotic Nuclear Ancestor (FENA).
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