RNA polymerase II depletion from the inactive X chromosome territory is not mediated by physical compartmentalization.
Sub-nuclear compartmentalization has been proposed to play an important role in gene regulation by segregating active and inactive parts of the genome in distinct physical and biochemical environments, where transcription and epigenetic factors are either concentrated or depleted. The inactive X chromosome offers a paradigm for studying sub-nuclear compartmentalization. When the non-coding Xist RNA coats the X chromosome, it recruits repressors and chromatin factors that trigger gene silencing, and forms a dense body of heterochromatin from which the transcription machinery appears to be excluded. Phase separation has been proposed to be involved in X-chromosome inactivation (XCI) and might explain exclusion of the transcription machinery by preventing its diffusion into the Xist-coated territory. Here, using quantitative fluorescence microscopy and single particle tracking, we show that RNA polymerase II (RNAPII) freely accesses the Xist territory during initiation of XCI, and that its diffusion is not prevented by biophysical constraints. Instead, the apparent depletion of RNAPII is due to the loss of its chromatin bound fraction. These findings demonstrate that initial exclusion of RNA Pol2 from the inactive X is a consequence of its reduced binding rate at the chromatin and gene level, rather than the biophysical compartmentalization of the inactive X heterochromatin domain. The Xist silent compartment is thus a biochemical rather than a biophysical compartment, at least during initiation of XCI.