Structural dynamics of DNA strand break sensing by PARP-1 at a single-molecule level

Single stranded breaks (SSBs) are the most frequent DNA lesions threatening genomic integrity. A highly kinked DNA structure in complex with human PARP-1 led to the proposal that SSB sensing in Eukaryotes relies on dynamics of both the broken DNA double helix and PARP-1s multi-domain organization. Here, we directly probe this fundamental yet poorly understood process at the single-molecule level. Quantitative SM-FRET and structural ensemble calculations reveal how PARP-1 binding converts DNA SSBs from a largely unperturbed conformation into the highly kinked state. Binding of the second N-terminal zinc finger yields an intermediate DNA conformation that can be recognized by the first zinc finger. Thus, the data is inconsistent with a conformational selection model, instead an induced fit mechanism via a multi-domain assembly cascade drives SSBs sensing. Interestingly, a clinically used PARP-1 inhibitor niraparib shifts the equilibrium towards the unkinked state, whereas the inhibitor EB47 stabilizes the kinked state.