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
"GTP--RNA guanylyltransferase"
"gtp--rna guanylyltransferase"