Western blot analysis
Western blot analysis was performed to confirm the physical linkage of peptides and their cognate mRNA sequences in the form of mRNA-peptide or mRNA/cDNA-peptide conjugates. A control mRNA sequence (Supplementary Table 1) encoding a FLAG epitope (DYKDDDDK) was ligated with puromycin-FITC DNA tag and translated under optimized condition (see previous section). Samples were run on a 10% polyacrylamide SDS-PAGE gel at constant 30 mA until the samples stack and reach the separating gel, and at constant 50 mA for additional 40 minutes. The samples in the gel were semi-dry transferred to a PDVF membrane (Bio-Rad) at constant 110 mA for 60 minutes. Blocking was carried out using the PVDF Blocking Reagent for Can Get Signal (Toyobo) and, the membrane was incubated using Anti DYKDDDDK tag monoclonal antibody peroxidase-conjugated (FUJIFILM Wako Pure Chemical) and Can Get Signal, solution 2 (Toyobo) with a 1: 12,000 ratio, respectively. The membrane was then revealed with chemiluminescence using ECL Select Western Blotting Detection Reagent (GE Healthcare).