Displays methyltransferase, positive regulation of the poly(A) polymerase and transcription elongation activities. Involved in the modification of both mRNA ends and in intermediate and late gene positive transcription elongation. At the mRNAs 5' end, methylates the ribose 2' OH group of the first transcribed nucleotide, thereby producing a 2'-O-methylpurine cap. At the 3' end, functions as a processivity factor which stimulates the activity of the viral poly(A) polymerase VP55 that creates mRNA's poly(A) tail. In the presence of VP39, VP55 does not dissociate from the RNA allowing tail elongation to around 250 adenylates. a 5'-end (N(7)-methyl 5'-triphosphoguanosine)-ribonucleoside in mRNA + S-adenosyl-L-methionine = a 5'-end (N(7)-methyl 5'-triphosphoguanosine)-(2'-O-methyl-ribonucleoside) in mRNA + H(+) + S-adenosyl-L-homocysteine Methyltransferase activity: Monomer, poly(A) polymerase activity: Heterodimer composed of a catalytic component, VP55, and a processivity factor, VP39. Interacts with Rap94 and NPH-I; these interactions might help linking transcription to capping and polyadenylation. Localizes to the virion core. Belongs to the class I-like SAM-binding methyltransferase superfamily. Poxvirus/kinetoplastid 2'-O-MTase family.