Transfer of Pol III from an old β clamp to a new β clamp. In these experiments, the Pol III holoenzyme (just the β clamp and Pol III core are shown for simplicity) is assembled onto a primed donor DNA circle (M13mp18), then the Pol III–DNA is mixed with two competing primed acceptor DNA circles of different sizes, only one of which includes a β clamp. In the experiment shown on the left side, the M13Gori DNA has a β clamp and the other (φX174) DNA does not. In the experiment on the right, φX174 DNA has a β clamp and the other (M13Gori) does not. Replication is initiated and timed. Aliquots of the reaction mixtures are analyzed on an agarose gel. The acceptor DNA with a β clamp is replicated in preference to the acceptor DNA without a β clamp.