Abstract
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.