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