The delta 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 s ynthesis is complete, polymerase dissociates from beta and DNA and cycles t o a new start site, a primed template loaded with beta. DNA polymerase cycl es frequently during lagging strand replication while synthesizing l-a-kilo base 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 prim ed sites on the lagging strand. Here we show that delta, a single subunit o f DNA polymerase III holoenzyme, opens beta and slips it off DNA (k(unloadi ng) = 0.011 s(-1)) at a rate similar to that of the multisubunit gamma comp lex 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 r equire ATP to catalyze clamp unloading. Quantitation of gamma complex subun its (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 compl ex occur in much lower quantities and perform several DNA metabolic functio ns in replication and repair, the delta subunit probably aids beta clamp re cycling during DNA replication.