Bifunctional mRNA-capping enzyme exhibiting RNA 5′-triphosphate monophosphatase activity in the N-terminal part and mRNA guanylyltransferase activity in the C-terminal part. Catalyzes the first two steps of cap formation: by removing the gamma-phosphate from the 5′-triphosphate end of nascent mRNA to yield a diphosphate end, and by transferring the GMP moiety of GTP to the 5′-diphosphate terminus of RNA via a covalent enzyme-GMP reaction intermediate.
