Electrophoretic Mobility-Shift Assays

In an electrophoretic mobility-shift assay (EMSA, or simply “gel shift”), a 32 P-labeled DNA fragment containing a specific DNA site is incubated with a cognate DNA-binding protein. The protein–DNA complexes are separated from free (unbound) DNA by electrophoresis through a nondenaturing polyacrylamide gel. The protein retards the mobility of the DNA fragments to which it binds. Thus, the free DNA will migrate faster than the DNA–protein complex. An image of the gel is used to reveal the positions of the free and bound radiolabeled DNAs.