Synergistic Coordination of Chromatin Torsional Mechanics and Topoisomerase Activity
The material properties of eukaryotic chromatin fibers partition supercoiling ahead of progressing replication forks, illustrating that chromatin provides a buffer against torsional stress and that its unique mechanical properties help to facilitate replication and minimize genome instability. Â© 2019 Elsevier Inc. DNA replication in eukaryotes generates DNA supercoiling, which may intertwine (braid) daughter chromatin fibers to form precatenanes, posing topological challenges during chromosome segregation. The mechanisms that limit precatenane formation remain unclear. By making direct torque measurements, we demonstrate that the intrinsic mechanical properties of chromatin play a fundamental role in dictating precatenane formation and regulating chromatin topology. Whereas a single chromatin fiber is torsionally soft, a braided fiber is torsionally stiff, indicating that supercoiling on chromatin substrates is preferentially directed in front of the fork during replication. We further show that topoisomerase II relaxation displays a strong preference for a single chromatin fiber over a braided fiber. These results suggest a synergistic coordinationâ€”the mechanical properties of chromatin inherently suppress precatenane formation during replication elongation by driving DNA supercoiling ahead of the fork, where supercoiling is more efficiently removed by topoisomerase II. Video Abstract: Â© 2019 Elsevier Inc.