Fig. 4. Effect of XRCC1 upon the in vitro activities of the bacterial EndIV and the interaction mutant APE^136–318. Reactions were carried out with (A) 34mer [AP:C] or (B) [F:C] duplexes as described in Figure 2 except that only one amount (0.7 pmol) of XRCC1 protein was assayed for each AP endonuclease. The products of the reactions were separated by denaturing 20% PAGE. The positions in the gel of the substrate (34mer) and the product (16mer) of the reaction are indicated.