Catalyzes the last reaction in the mRNA cap formation pathway. a 5'-end (5'-triphosphoguanosine)-(ribonucleoside) in mRNA + S-adenosyl-L-methionine = a 5'-end (N(7)-methyl 5'-triphosphoguanosine)-ribonucleoside in mRNA + S-adenosyl-L-homocysteine Heterodimer of a large and a small subunit. All the enzymes and other proteins required to synthesize early mRNAs are packaged within the virion core along with the DNA genome.