The gamma complex subassembly (gamma delta delta' chi psi) of DNA polymerase III holoenzyme couples ATP to assemble the ring-shaped beta subunit around DNA forming a DNA sliding clamp. This beta clamp is needed for highly processive synthesis by the holoenzyme. Here, the delta and delta' subunits of the gamma complex are studied for their structural and functional interaction with each other and with the gamma subunit. Both delta and delta are monomeric in their native state, and they bind each other tightly to form a 1:1 complex. Neither delta nor delta' alone binds tightly to the gamma subunit. However, as a complex, delta delta' binds gamma tightly to form a gamma delta delta' complex. The fact that all three subunits, gamma, delta, and delta', are needed to form a tight complex correlates well with activity assays which show that gamma and delta are capable but inefficient in assembly of the beta ring onto DNA and delta' is needed for an efficient reaction.