Sen1 and Rrm3 ensure permissive topological conditions for replication termination.

Replication forks terminate at TERs and telomeres. Forks that converge or encounter transcription generate topological stress. Combining genetics, genomics, and transmission electron microscopy, we find that Rrm3 and Sen1 helicases assist termination at TERs; Sen1 specifically acts at telomeres. rrm3 and sen1 genetically interact and fail to terminate replication, exhibiting fragility at termination zones (TERs) and telomeres. sen1rrm3 accumulates RNA-DNA hybrids and X-shaped gapped or reversed converging forks at TERs; sen1, but not rrm3, builds up RNA polymerase II (RNPII) at TERs and telomeres. Rrm3 and Sen1 restrain Top1 and Top2 activities, preventing toxic accumulation of positive supercoil at TERs and telomeres. We suggest that Rrm3 and Sen1 coordinate the activities of Top1 and Top2 when forks encounter transcription head on or codirectionally, respectively, thus preventing the slowing down of DNA and RNA polymerases. Hence Rrm3 and Sen1 are indispensable to generate permissive topological conditions for replication termination.