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

Abstract

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