a 5'-end (N(2),N(2),N(7)-trimethyl 5'-triphosphoguanosine)-purine ribonucleoside in mRNA + H2O = (N(2),N(2),N(7))-trimethyl-GMP + a 5'-end diphospho-(purine-ribonucleoside) in mRNA + 2 H(+) — reaction