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. {ECO:0000269|PubMed:21636784, ECO:0000269|PubMed:9473487, ECO:0000269|PubMed:9512541}. This is the function of RNGTT (RNA guanylyltransferase and 5'-phosphatase, Ensembl gene identifier ENSG00000111880).