Fig. 2. Effect of XRCC1 on the AP endonuclease activity of APE1. AP endonuclease activity on a 34mer (A) [AP:C] or (B) [F:C] duplex ³²P-labeled at the 5′ end on the lesion-containing strand was measured as described in Materials and methods. A limiting amount (0.03 fmol) of APE1 protein was incubated for 30 min at 37°C with the double-stranded oligonucleotide substrate (50 fmol) and increasing amounts of XRCC1 protein. The products of the reactions were separated by denaturing 20% PAGE. The positions in the gel of the 34mer substrate and the 16mer product of the reaction are indicated. (C) Quantification of the data from (A) and (B).