a 5'-end (N(7)-methyl 5'-triphosphoguanosine)-(2'-O-methyl-purine-ribonucleoside)-ribonucleotide in mRNA + S-adenosyl-L-methionine <=> a 5'-end (N(7)-methyl 5'-triphosphoguanosine)-(2'-O-methyl-purine-ribonucleoside)-(2'-O-methyl-ribonucleotide) in mRNA + S-adenosyl-L-homocysteine + H(+)