The DNA polymerase III holoenzyme contains γ and is not a trimeric polymerase

There is widespread agreement that the clamp loader of the Escherichia coli replicase has the composition DnaX(3)delta delta'chi Psi. Two DnaX proteins exist in E. coli, full length tau and a truncated gamma that is created by ribosomal frameshifting. tau binds DNA polymerase III tightly; gamma does not. There is a controversy as to whether or not DNA polymerase III holoenzyme (Pol III HE) contains gamma. A three-tau form of Pol III HE would contain three Pol IIIs. Proponents of the three-tau hypothesis have claimed that gamma found in Pol III HE might be a proteolysis product of tau. To resolve this controversy, we constructed a strain that expressed only tau from a mutated chromosomal dnaX. gamma containing a C-terminal biotinylation tag (gamma-C-tag) was provided in trans at physiological levels from a plasmid. A 2000-fold purification of Pol III* (all Pol III HE subunits except beta) from this strain contained one molecule of gamma-C-tag per Pol III* assembly, indicating that the dominant form of Pol III* in cells is Pol III2 tau(2) delta delta'chi Psi. Revealing a role for lambda in cells, mutants that express only tau display sensitivity to ultraviolet light and reduction in DNA Pol IV-dependent mutagenesis associated with double-strand-break repair, and impaired maintenance of an F' episome.