N(tele)-(N(7)-methylguanosine 5'-phospho)-L-histidyl-[protein] + a 5'-end diphospho-(purine-ribonucleoside) in mRNA + H(+) = a 5'-end (N(7)-methyl 5'-triphosphoguanosine)-(purine-ribonucleoside) in mRNA + L-histidyl-[protein] — reaction