Abstract
DNA replication is frequently perturbed by intrinsic, as well as extrinsic, genotoxic stress. At damaged forks, DNA replication and repair activities require proper coordination to maintain genome integrity. We show here that PARI antirecombinase plays an essential role in modulating the initial response to replication stress in mice. PARI is functionally dormant at replisomes during normal replication, but upon replication stress, it enhances nascent-strand shortening that is regulated by RAD51 and MRE11. PARI then promotes double-strand break induction, followed by new origin firing instead of replication restart. Such PARI function is apparently obstructive to replication but is nonetheless physiologically required for chromosome stability in vivo and ex vivo Of note, Pari-deficient embryonic stem cells exhibit spontaneous chromosome instability, which is attenuated by differentiation induction, suggesting that pluripotent stem cells have a preferential requirement for PARI that acts against endogenous replication stress. PARI is a latent modulator of stalled fork processing, which is required for stable genome inheritance under both endogenous and exogenous replication stress in mice.