Trypsin digestion
Trypsin digestion was carried out by using 75 ng mRNA-tag sample (with and without translation under optimal condition), 1 mM CaCl2, 50 mM ammonium bicarbonate, and 0.5 µg of Trypsin Gold (Promega). Each reaction was incubated at 37 ℃ for 30 min and resolved in a 3.5% stacking - 10% resolving polyacrylamide SDS-PAGE gel, with 8 M urea added to the resolving gel.