The δ subunit of DNA polymerase III holoenzyme serves as a sliding clamp unloader in Escherichia coli

In Escherichia coli, the circular beta sliding clamp facilitates processive DNA replication by tethering the polymerase to primer-template DNA. When synthesis is complete, polymerase dissociates from beta and DNA and cycles to a new start site, a primed template loaded with beta. DNA polymerase cycles frequently during lagging strand replication while synthesizing l-a-kilobase Okazaki fragments. The clamps left behind remain stable on DNA (t(1/2) similar to 115 min) and must be removed rapidly for reuse at numerous primed sites on the lagging strand. Here we show that delta, a single subunit of DNA polymerase III holoenzyme, opens beta and slips it off DNA (k(unloading) = 0.011 s(-1)) at a rate similar to that of the multisubunit gamma complex clamp loader by itself (0.015 s(-1)) or within polymerase (pol) III* (0.0065 s(-1)). Moreover, unlike gamma complex and pol III*, delta does not require ATP to catalyze clamp unloading. Quantitation of gamma complex subunits (gamma, delta, delta', chi, psi) in E. coli cells reveals an excess of delta, free from gamma complex and pol III*. Since pol III* and gamma complex occur in much lower quantities and perform several DNA metabolic functions in replication and repair, the delta subunit probably aids beta clamp recycling during DNA replication.