Practice question

Supershift assay extends conventional EMSA to identify specific protein within DNA-protein complex. Binding reaction initially shows retarded band corresponding to protein-DNA interaction. Addition of antibody specific to suspected transcription factor binds exposed epitope of protein while still attached to DNA, forming ternary antibody-protein-DNA complex of even larger size and more positive charge. Consequently electrophoretic mobility decreases further, producing super-shifted band migrating above original shift near well. Control immunoglobulin lacking specificity does not produce this additional retardation. Supershift therefore confirms identity of DNA-binding protein and demonstrates protein participation in specific complex formation.