Poly(ADP-ribose) polymerase (PARP) 1, whose primary role is initiation of DNA repair, is activated by damaged DNA and uses NAD+ to automodify itself and recruit other proteins involved in DNA repair. Due to its role in DNA repair, PARP-1 inhibition has been long targeted for treatment of different cancer types. By now there are already several different clinical APRP-1 inhibitors used in treatment of ovarian and breast cancers, and many others are under clinical trials for other types of cancer, such as prostate cancer, pancreatic cancer, blood cancer and others. PARP-1 inhibition has also been demonstrated to have promising effect for treatment of some cardiovascular conditions. Extensive DNA damage caused by number of cardiovascular conditions, such as a stroke or heart attack, can result in PARP-1's hyper-activation, leading to depletion of cellular NAD+ and subsequent cell death. It has been demonstrated that inhibition of PARP-1's activity using small molecules can prevent apoptosis and necrosis in such cells. Studies in animal models have indeed shown that inhibition of PARP-1 can have beneficiary effects for treatment of various cardiovascular conditions, such as ischemic stroke, cerebral ischemia, diabetic cardiomyopathy and others. Despite growing number of PARP-1 inhibitors, their molecular mechanism of action is not well understood. The overall objective of my project is to define the molecular mechanisms of activation and silencing of PARP-1. My central hypothesis is that the structural and dynamic changes occurring in PARP-1 upon DNA binding play key roles in the regulation of protein activation and dictate relative efficiency of PARP-1 inhibitors. Three specific aims are pursued in this project: 1. To define how PARP-1 is silenced through auto-modification and released from single-strand break (SSB) DNA, 2. To measure the effect of inhibitors on PARP1 structural dynamics for those that trap it at a SSB versus those that don't, 3. To define the organization and dynamics of the PARP- 1/nucleosome complex in conjunction with the housekeeping role of PARP-1 in transcriptional regulation. My proposed experiments will reveal key insights on the precise molecular mechanisms of PARP-1 activation and inhibition, aiding in the design of new PARP-1 inhibitors to improve outcomes in patients with various diseases.