The DNA polymerase III holoenzyme (EC:2.7.7.7) is the polymerase responsible for the replication of the Escherichia coli chromosome. The holoenzyme is composed of the DNA polymerase III core, the sliding clamp, and the DnaX clamp loading complex. The DnaX complex contains either either the tau or gamma product of gene dnax, complexed to delta.delta' and to chi psi. Chi forms a 1:1 heterodimer with psi. The chi psi complex functions by increasing the affinity of tau and gamma for delta.delta' allowing a functional clamp-loading complex to form at physiological subunit concentrations. Psi is responsible for the interaction with DnaX (gamma/tau), but psi is insoluble unless it is in a complex with chi [1].

1: DnaX complex of Escherichia coli DNA polymerase III holoenzyme. The chi psi complex functions by increasing the affinity of tau and gamma for delta.delta' to a physiologically relevant range. Olson MW, Dallmann HG, McHenry CS; J Biol Chem 1995;270:29570-29577. PMID:7494000