Solution structure of Domains IVa and V of the τ subunit of Escherichia coli DNA polymerase III and interaction with the α subunit

The solution structure of the C-terminal Domain V of the τ subunit of E. coli DNA polymerase III was determined by nuclear magnetic resonance (NMR) spectroscopy. The fold is unique to τ subunits. Amino acid sequence conservation is pronounced for hydrophobic residues that form the structural core of the protein, indicating that the fold is representative for τ subunits from a wide range of different bacteria. The interaction between the polymerase subunits τ and α was studied by NMR experiments where α was incubated with full-length C-terminal domain (τ(C)16), and domains shortened at the C-terminus by 11 and 18 residues, respectively. The only interacting residues were found in the C-terminal 30-residue segment of τ, most of which is structurally disordered in free τ(C)16. Since the N- and C-termini of the structured core of τ(C)16 are located close to each other, this limits the possible distance between α and the pentameric δτ(2)γδ′ clamp–loader complex and, hence, between the two α subunits involved in leading- and lagging-strand DNA synthesis. Analysis of an N-terminally extended construct (τ(C)22) showed that τ(C)14 presents the only part of Domains IVa and V of τ which comprises a globular fold in the absence of other interaction partners.