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Vaccinia virus poly(A) polymerase(vp55) can add poly(A) tail on 3mRNA and is the only known polymerase that can translocate independently with respect to single stranded nucleic acid.Like most DNA polymerase, the vp55 has three conserved aspartate, D204, D202 and D253 at catalytic center which identified by crystal structure and mutagenesis analysis.