Xeroderma pigmentosum complementation group A protein acts as a processivity factor

We have previously shown that endonucleases present in a protein complex, w hich has specificity for cyclobutane pyrimidine dimers, locate sites of dam age in DNA by a processive mechanism of action in normal human lymphoblasto id cells. In contrast, the endonucleases present in this complex from xerod erma pigmentosum complementation group A (XPA) cells locate damage sites by a distributive or significantly less processive mechanism. Since the XPA p rotein has been shown to be responsible for the DNA repair defect in XPA ce lls, this protein was examined for involvement in the mechanism of target s ite location of these endonucleases. A recombinant XPA protein, produced by expression of the normal XPA cDNA in E. coli, was isolated and purified. T he results show that the recombinant XPA protein was able to correct the de fect in ability of the XPA endonucleases to act by a processive mechanism o f action on WC irradiated DNA. These studies indicate that the XPA protein, in addition to a role in damage recognition or damage verification, may fu nction as a processivity factor. (C) 2000 Academic Press.