The δ Subunit of DNA Polymerase III Holoenzyme Serves as a Sliding Clamp Unloader in Escherichia coli *

By Frank P. Leu, M. Hingorani, J. Turner, M. O’Donnell. Journal of Biological Chemistry, 2000

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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 polymerase 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 polymerase (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) inE. 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.

Subfields: Genetics, Molecular Biology

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