Abstract

Regulating the topological state of DNA is a vital function of the enzyme DNA topoisomerase I. However, when acting on damaged DNA, topoisomerase I may get trapped in a covalent complex with nicked DNA (stalled topoisomerase I), that, if unrepaired, may lead to genomic instability or cell death. Here we show that ADP-ribose polymers target specific domains of topoisomerase I and reprogram the enzyme to remove itself from cleaved DNA and close the resulting gap. Two members of the poly(ADP-ribose) polymerase family, PARP-1 and 2, act as poly(ADP-ribose) carriers to stalled topoisomerase I sites and induce efficient repair of enzyme-associated DNA strand breaks. Thus, by counteracting topoisomerase I-induced DNA damage, PARP-1 and PARP-2 act as positive regulators of genomic stability in eukaryotic cells.