Topological stress resolution during replication fork progression and conflicts with transcription

Abstract

DNA transactions generate topological constraints that are resolved by specialized toposiomerase enzymes. Template DNA unwinding during replication generates an increase in chromosomal linking number, primarily accommodated as positive supercoiling of unreplicated chromosomal regions or precatenane crossings between replicated duplexes. This topological stress is timely resolved by eukaryotic DNA topoisomerase I and II. We found that both enzymes are recruited in close proximity to replication forks to support helicase function and thus prevent fork collapse and chromosomal breakage. Replication fork stability is also sustained by modulation of transcribed chromatin topology. Sites were chromatin attaches Nuclear Pore Complexes (NPCs) have been proposed to act as topological domain barriers limiting topological stress diffusion. We found that the replication checkpoint, a specialized branch of the DNA damage response monitoring replication problems, counteracts topology-driven fork collapse by phosphorylating key nucleoporins and releasing transcribed genes from NPCs. In addition to this mechanism, DNA topoisomerase II is recruited to transcribed genes in S-phase to mediate a yet poorly understood function preventing chromosomal breakage. We will discuss how DNA topology is modulated to promote replication fork stability and how topological stress resolution might be achieved during replication of transcribed units.