a 5'-end diphospho-ribonucleoside in mRNA + GTP + H(+) => a 5'-end (5'-triphosphoguanosine)-ribonucleoside in mRNA + diphosphate
a 5'-end diphospho-ribonucleoside in mrna + gtp + h(+) => a 5'-end (5'-triphosphoguanosine)-ribonucleoside in mrna + diphosphate