The E. coli clamp loader. The γ complex is shown here. (a) The five clamp-loading subunits are arranged in a circle, with a gap between parts of two subunits, δ′ and δ. The β clamp (purple) docks onto the clamp loader. (b) The clamp loading mechanism.

ATP binding to the γ subunits powers a conformational change that enables the binding and opening of the β clamp.

The combined γ complex–ATP–β clamp binds primed DNA in a central chamber, and the single-stranded template DNA passes through the gap in the side of the clamp loader.

ATP hydrolysis ejects the clamp loader, allowing β to close again around the DNA.