The unstructured C-terminus of the [tau] subunit of Escherichia coli DNA polymerase III holoenzyme is the site of interaction with the [alpha] subunit

• Published in: Nucleic acids research 2007-05, Vol.35 (9), p.2813
• Main Authors: Jergic, Slobodan, Ozawa, Kiyoshi, Williams, Neal K, Xun-Cheng, Su, Scott, Daniel D, Hamdan, Samir M, Crowther, Jeffrey A, Otting, Gottfried, Dixon, Nicholas E
• Format: Article
• Language: English
• ISSN: 0305-1048; 1362-4962
• DOI: 10.1093/nar/gkm079

The τ subunit of Escherichia coli DNA polymerase III holoenzyme interacts with the α subunit through its C-terminal Domain V, τC 16. We show that the extreme C-terminal region of τC 16 constitutes the site of interaction with α. The τC 16 domain, but not a derivative of it with a C-terminal deletion of seven residues (τC 16Δ7), forms an isolable complex with α. Surface plasmon resonance measurements were used to determine the dissociation constant (KD ) of the α-τC 16 complex to be ∼260 pM. Competition with immobilized τC 16 by τC 16 derivatives for binding to α gave values of KD of 7 μM for the α-τC 16Δ7 complex. Low-level expression of the genes encoding τC 16 and τC 16[white triangle up]7, but not τC 16Δ11, is lethal to E. coli . Suppression of this lethal phenotype enabled selection of mutations in the 3[variant prime] end of the τC 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, τC 24, was found to bind DNA in a salt-sensitive manner while no binding was observed for τC 16, suggesting that the processivity switch of the replisome functionally involves Domain IV of τ.