Fig. 3. Effect of XRCC1 upon the 3′-dRPase activity of APE1. The 3′-phosphodiesterase activity of APE1 was tested on a 34mer DNA duplex containing a nick with a 3′-unsaturated aldehyde residue and labeled at the 5′ end of the lesion-carrying strand. Limiting amounts of APE1 were incubated in the presence or absence of XRCC1 (0.7 pmol) and the activity was determined by the appearance of a band corresponding to the labeled fragment carrying a 3′-OH at the expense of the substrate carrying a 3′-unsaturated aldehyde residue (dRP). The lower band corresponds to the labeled fragment carrying a 3′-phosphate.