Poly(ADP-ribose) polymerase-1 (PARP-1) modifies various proteins, including itself, with ADP-ribose polymers (automodification). Polymer synthesis is triggered by binding of its zinc finger 1 (Zn1) and 2 (Zn2) to DNA breaks and is followed by inactivation through automodification. The multiple functional domains of PARP-1 appear to regulate activation and automodification-mediated inactivation of PARP-1. However, the roles of these domains in activation-inactivation processes are not well understood. Our results suggest that Zn1, Zn2, and a domain identified in this study, the double-stranded DNA binding (DsDB) domain, are involved in DNA break-dependent activation of PARP-1. We found that binding of the DsDB domain to double-stranded DNA and DNA break recognition by Zn1 and Zn2, whose actual binding targets are likely to be single-stranded DNA, lead to the activation of PARP-1. In turn, the displacement of single-and double-stranded DNA from Zn2 and the DsDB domain caused by ADP-ribose polymer synthesis results in the dissociation of PARP-1 from DNA breaks and thus its inactivation. We also found that the WGR domain is one of the domains involved in the RNA-dependent activation of PARP-1. Furthermore, because zinc finger 3 (Zn3) has the ability to bind to single-stranded RNA, it may have an indirect role in RNA-dependent activation. PARP-1 functional domains, which are involved in oligonucleic acid binding, therefore coordinately regulate PARP-1 activity depending on the status of the neighboring oligonucleic acids. Based on these results, we proposed a model for the regulation of PARP-1 activity.Poly(ADP-ribose) polymerase-1 (PARP-1) 2 is an enzyme that catalyzes post-translational modifications with ADPribose polymers (1-4). This modification plays roles in the regulation of various fundamental cellular processes, including DNA repair (5-8), chromatin remodeling (9 -11), and transcription (9,(12)(13)(14). PARP-1 regulates these processes by modifying proteins and enzymes, including histones and high mobility group protein (15-18), through its interaction with other enzymes, e.g. topoisomerase I and XRCC1 (19 -22), by recruiting DNA repair enzymes to DNA damage sites (6 -8) and by binding to transcriptional promoters (9,23).It has been demonstrated that binding of PARP-1 to double strand DNA breaks (DSB) or single strand DNA breaks (SSB) triggers ADP-ribose synthesis (1, 2). PARP-1 activation also occurs through its binding to the linker DNA of nucleosomes and upon activation of transcription (11,12,24,25). In addition to DNA, PARP-1 is capable of binding to RNA (13, 26). Thus, PARP-1 could have the ability to recognize diverse oligonucleic acid structures. ADP-ribose synthesis leads to PARP-1 inactivation through automodification (27). These complex regulations of PARP-1 activity are carried out by at least six functional domains of PARP-1.PARP-1 is a 110-kDa enzyme with a modular architecture of multiple functional domains (see Fig. 1A). The most N-terminal end of PARP-1 is the DNA break binding (DBD) domain...