RNA Pol II antagonises mitotic chromatin folding and chromosome segregation by condensin

Abstract

Condensin shapes mitotic chromosomes by folding chromatin into loops but whether it does so by DNA-loop extrusion remains speculative. While loop-extruding cohesin is stalled by transcription, no conclusive evidence has been provided regarding the impact of transcription on condensin despite its conserved enrichment at highly expressed genes. Using degrons of Rpb1 or the torpedo nuclease Dhp1XRN2, we depleted or displaced RNAP2 on chromatin in fission yeast metaphase cells. We show that RNAP2 does not load condensin on DNA but instead retains condensin and hinders its ability to fold mitotic chromatin and to support chromosome segregation, consistent with the stalling of a loop-extruder. Transcription termination by Dhp1 limits such a hindrance. Our results shed a new light on the integrated functioning of condensin and we argue that a tight control of transcription underlies mitotic chromosome assembly by loop-extruding condensin.