Probably catalyzes the second reaction in the mRNA cap formation pathway (By similarity). Forms a covalent complex with GTP (By similarity). a 5'-end triphospho-(purine-ribonucleoside) in mRNA + H2O = a 5'-end diphospho-(purine-ribonucleoside) in mRNA + H(+) + phosphate a 5'-end diphospho-ribonucleoside in mRNA + GTP + H(+) = a 5'-end (5'-triphosphoguanosine)-(ribonucleoside) in mRNA + diphosphate 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 mRNA processing; mRNA capping. Part of the viral DNA-directed RNA polymerase that consists of 8 polII-like subunits (RPB1, RPB2, RPB3, RPB5, RPB6, RPB7, RPB9, RPB10), a capping enzyme and a termination factor. Found in association with viral nucleoid. Expressed in the early phase of the viral replicative cycle. The N-terminus contains the mRNA 5'-triphosphatase domain, the central part contains the mRNA guanylyltransferase, and C-terminus contains the methyltransferase domain. In the N-terminal section; belongs to the dsDNA virus mRNA guanylyltransferase family. In the C-terminal section; belongs to the class I-like SAM-binding methyltransferase superfamily. mRNA cap 0 methyltransferase family.