The unstructured C-terminus of the τ subunit of Escherichia coli DNA polymerase III holoenzyme is the site of interaction with the α subunit
AbstractAbstract The τ subunit of Escherichia coli DNA polymerase III holoenzyme interacts with the α subunit through its C-terminal Domain V, τ C 16. We show that the extreme C-terminal region of τ C 16 constitutes the site of interaction with α. The τ C 16 domain, but not a derivative of it with a C-terminal deletion of seven residues (τ C 16Δ7), forms an isolable complex with α. Surface plasmon resonance measurements were used to determine the dissociation constant ( K D ) of the α−τ C 16 complex to be ∼260 pM. Competition with immobilized τ C 16 by τ C 16 derivatives for binding to α gave values of K D of 7 μM for the α−τ C 16Δ7 complex. Low-level expression of the genes encoding τ C 16 and τ C 16▵7, but not τ C 16Δ11, is lethal to E. coli . Suppression of this lethal phenotype enabled selection of mutations in the 3′ end of the τ C 16 gene, that led to defects in α binding. The data suggest that the unstructured C-terminus of τ becomes folded into a helix–loop–helix in its complex with α. An N-terminally extended construct, τ C 24, was found to bind DNA in a salt-sensitive manner while no binding was observed for τ C 16, suggesting that the processivity switch of the replisome functionally involves Domain IV of τ. © 2007 The Author(s) This is an Open Access article distributed under the terms of the Creative Commons Attribution Non-Commercial License (http://creativecommons.org/licenses/by-nc/2.0/uk/) which permits unrestricted non-commercial use, distribution, and reproduction in any medium, provided the original work is properly cited.