GO:0004484:   mRNA guanylyltransferase activity
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.