PARP may be classified as a posttranslational protein modification enzyme, but unlike most other such enzymes, PARP mainly modifies itself. Thus, poly ation seems to represent an autoregulatory mechanism of PARP activity, although other proteins, including histones and topoisomerases, also are known to be acceptors for poly ation.In vivo, such breaks can be generated by exposure to ionizing radiation or in the process of base excision repair of damaged DNA bases.This activation may be explained by the binding of PARP to intracellularly generated DNA breaks, and such activation of PARP is believed to modulate the DNA break rejoining processes.These observations have been taken to indicate that PARP may stimulate the DNA break rejoining process by, for example, recruiting DNA repair enzymes to damaged sites.However, direct experimental evidence for this model has been lacking.Using a cellfree DNA repair assay system, we found that yray induced DNA single strand breaks are enzymatically rejoined, and NAD strongly stimulates this activity in association with PARP activation. Furthermore, inhibition of PARP activity with aminobenzamide suppresses NAD promoted DNA break rejoining ac tivity.Thus, consistent with in vivo results, poly alion is apparently involved in the break rejoining reaction.Surprisingly, however, when PARP was removed from cellfree extracts by affinity chromatography, the depleted extracts could rejoin DNA breaks effi ciently even without the addition of NAD. These results indicate that in the absence of NAD, or in the presence of aminobenamide with NAD, PARP binds tightly to yrayinduced DNA breaks and inhibits DNA break repair by the prevention of access of DNA repair enzymes to the damaged sites.NAD dependent repair associated with PARP automodification occurs. In particular, repair of bleomycininduced DNA breaks seems totally dependent on PARP activation.Repair of modified bases generated by alkylating agents also is promoted by PARP activation.Such altered bases arc known to be corrected by base excision repair, which is initiated by the elimination of damaged bases by DNA glycosylases.DNA polymerization, and ligation.During the repair of the damage.PARP is not activated, nor docs NAD stimulate the repair.s REPAIR OF OXIDATIVE DNA DAMAGE inated by nucleotide excision repair, which requires formation of multiprotein repair complexes before DNA incision; such com plexes may prevent the binding of PARP to DNA breaks. PARP is involved only in base excision repair and in repair of DNA breaks induced by ionizing radiation but not in nucle otide excision repair. There are seven genetic complementation groups from A through G, and the defective gene products in these groups are involved in nucleotide excision repair, particularly in damage recognition and incision processes.Typical clinical features of XP are skin lesions, including frequent carcinomas and melanomas, and hypersensitivity to sunlight.In ad dition, severe cases of XP exhibit neurological abnormalities as well as increased frequency of endogenous tumor formation.The origins of the latter symptoms have been unclear, although formation of DNA damage by endogenous agents has been proposed as a possible explanation. Digestion of the poly returns PARP to its original form, while DNA repair enzymes complete DNA strand break rejoining.This process requires the formation of repair protein complexes which may inhibit binding of PARP to the DNA breaks.