mRNA guanylyltransferase activity https://github.com/geneontology/go-ontology/issues/30193 GTP:mRNA guanylyltransferase activity GO:0004484 Catalysis of the reaction: GTP + (5')pp-Pur-mRNA = diphosphate + G(5')ppp-Pur-mRNA; G(5')ppp-Pur-mRNA is mRNA containing a guanosine residue linked 5' through three phosphates to the 5' position of the terminal residue. Reactome:R-HSA-9815529 GTP--RNA guanylyltransferase activity RHEA:60844 MetaCyc:MRNA-GUANYLYLTRANSFERASE-RXN EC:2.7.7.50 Reactome:R-HSA-9923071 protein lambda2 mRNA capping enzyme activity Reactome:R-HSA-77083 Reactome:R-HSA-77081 RHEA:60836 RHEA:54592 molecular_function messenger RNA guanylyltransferase activity RNA guanylyltransferase activity