preferred_name: a 5'-end (N(7)-methyl 5'-triphosphoguanosine)-(purine-ribonucleoside) in mRNA + S-adenosyl-L-homocysteine => a 5'-end (5'-triphosphoguanosine)-(purine-ribonucleoside) in mRNA + S-adenosyl-L-methionine