Residues of DNA polymerase beta (beta-Pol) that interact with the DNA repair protein XRCC1 have been determined by NMR chemical shift mapping (CSM) and mutagenesis. 15N/(13)C/(2)H/(1)H,(13)C-methyl(Leu,Ile,Val)-labeled beta-Pol palm-thumb domain was used for assignments of the 1H, 15N, and 13C resonances used for CSM of the palm-thumb on forming the 40 kDa complex with the XRCC1 N-terminal domain (NTD). Large chemical shift changes were observed in the thumb on complexation. 15N relaxation data indicate reduction in high-frequency motion for a thumb loop and three palm turn/loops, which showed concomitant chemical shift changes on complexation. A deltaV303-V306 deletion and an L301R/V303R/V306R triple mutation abolished complex formation due to loss in hydrophobicity. In an updated model, the thumb-loop of beta-Pol contacts an edge/face region of the beta sheet of the XRCC1 NTD, while the beta-Pol palm weakly contacts the alpha2 helix.