*4.1. Protein Expression and Purification*

Recombinant human APE1, its N-terminally truncated forms (APE1N∆35 and APE1N∆61), and rat Polβ were produced by expression in *Escherichia coli* BL21(DE3)pLysS (BL21 E. coli strain carrying the lambda DE3 lysogen and the pLysS plasmid, which expresses T7 lysozyme) and purified as described previously [67–69]. Human PARP1 and human protein XRCC1 were produced by expression in *E. coli* Rosetta (DE3) and purified as described [70,71]. APE1 and Polβ expression vectors were kindly provided by S.H. Wilson (National Institute of Health, North Carolina, USA). The plasmid constructs used to express APE1N∆35 and APE1N∆61 were kindly provided by A.A. Ishchenko (Groupe Réparation de l'ADN, UMR 8126 CNRS, Univ Paris-Sud, Institut Gustave Roussy, France). The XRCC1 expression vector was a generous gift from J.P. Radicella (UMR217 CNRS/CEA, France). The plasmid construct used to express PARP1 was kindly provided by M. Satoh (Laval University, Quebec, Canada). The purified proteins were dialyzed against a solution containing 50 mM Tris-HCl, pH 8.0, 100 mM NaCl, 5 mM DTT (dithiotheitol), and 40% glycerol, and stored at −30 ◦C.
