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RNA polyadenylate polymerase of nucleocytoplasmic large DNA viruses This model represents the poly(A) polymerases (PAPs) from nucleocytoplasmic large DNA viruses (NCLDV), a group of giant eukaryotic double-stranded DNA viruses that make up the phylum Nucleocytoviricota. They are referred to as nucleocytoplasmic because they are often able to replicate in both the host's cell nucleus and cytoplasm. PAPs catalyze the attachment of adenylates to the 3' ends of messenger RNA and other RNAs, forming poly(A) tails. PAP acts as a nucleic acid template-independent NMP-transferase, preferentially utilizing a single species of NTP, namely ATP. The polyadenylation state of an mRNA may correlate with the efficiency of its translation. This group includes PAPs from the Poxviridae and Mimiviridae family of viruses. In Vaccinia virus, from the Poxviridae family, polyadenylation is crucial for virion maturation and is carried out by a heterodimer, formed by the catalytic subunit VP55 and the processivity factor (VP39), which is required for the formation of long poly(A) tails. PAPs from Acanthamoeba polyphaga mimivirus and Megavirus chiliensis, which belong to the Mimiviridae family, are homodimeric and intrinsically self-processive, generating >700 nucleotides long poly(A) tails. Homodimerization is required for PAP activity; monomers are able to bind RNA but are enzymatically inactive. Thus, while other PAPs form heterodimers with processivity factors, the Mimiviridae PAPs become processive upon homodimerization. The catalytic subunit of NCLDV PAPs contains two topologically identical subdomains with a nucleotidyltransferase fold, suggesting that an ancestral duplication was at the origin of these viral PAPs. |
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