Spatial separation of Xist RNA and polycomb proteins revealed by superresolution microscopy
Cerase A., Smeets D., Tang YA., Gdula M., Kraus F., Spivakov M., Moindrot B., Leleu M., Tattermusch A., Demmerle J., Nesterova TB., Green C., Otte AP., Schermelleh L., Brockdorff N.
Significance Polycomb repressor proteins are recruited to the inactive X chromosome in mammals, and this has been attributed to a biochemical interaction between the non–protein-coding RNA X-inactive specific transcript (Xist), which initiates the X inactivation process, and core polycomb subunits. We have studied this using a combination of genome mapping analysis and 3D structured illumination microscopy (3D-SIM) that allows 3D imaging with eightfold volumetric resolution improvement compared with previous state-of-the-art confocal microscopy. Our findings reveal that Xist-mediated recruitment of polycomb repressors does not correlate well with gene silencing and, moreover, that using 3D-SIM, polycomb proteins and Xist RNA show significant spatial separation. These observations challenge prevailing models and prompt a reappraisal of the role of Xist RNA in polycomb recruitment.