CTCF governs chromatin boundary permeability through rapid, millisecond-timescale sampling of discrete conformational states—wrapped and unwrapped—that are tuned by DNA sequence, CpG methylation, and nucleosome positioning to probabilistically gate cohesin capture, with PDS5A both priming CTCF into the cohesin-capturing unwrapped conformation prior to encounter and subsequently stabilizing captured cohesin complexes into mechanically robust, orientation-dependent loop anchors.

Genomes are organized into chromatin loops through cohesin-mediated extrusion, with CTCF acting as a polar boundary element. As cohesin approaches CTCF at kilobase-per-second speeds, it must rapidly choose whether to stall or bypass. How CTCF encodes this probabilistic decision within a brief encounter window has remained unclear. Here we show that CTCF governs this probabilistic outcome by rapidly sampling a dynamic ensemble of conformations generated by spontaneous rearrangements of its DNA-binding zinc fingers. This ensemble is tuned by DNA sequence, CpG methylation, nearby nucleosomes, and the cohesin regulator PDS5A before cohesin engagement. Upon cohesin binding, PDS5A enhances loop-anchor mechanical stability, reinforcing orientation-dependent boundaries. These findings establish conformational ensemble tuning, rather than static occupancy, as a regulatory principle linking base pair–scale motions to megabase-scale genome organization.

Chromatin boundary function is governed not by CTCF occupancy alone, but by a tunable ensemble of DNA-bound conformations that probabilistically gates cohesin capture.