Xist RNA Is Confined to the Nuclear Territory of the Silenced X Chromosome throughout the Cell Cycle

Abstract

In mammalian female cells, one X chromosome is inactivated to prevent a dose difference in the expression of X-encoded proteins between males and females. Xist RNA, required for X chromosome inactivation, is transcribed from the future inactivated X chromosome (Xi), where it spreads in cis, to initiate silencing. We have analyzed Xist RNA transcription and localization throughout the cell cycle. It was found that Xist transcription is constant and that the mature RNA remains attached to the Xi throughout mitosis. Diploid and tetraploid cell lines with an MS2-tagged Xist gene were used to investigate spreading of Xist. Most XXXX_MS2 tetraploid mouse embryonic stem (ES) cells inactivate the X_MS2 chromosome and one other X chromosome. Analysis of cells with two Xi's indicates that Xist RNA is retained by the Xi of its origin and does not spread in trans. Also, in XX_MS2 diploid mouse ES cells with an autosomal Xist transgene, there is no trans exchange of Xist RNA from the Xi to the autosome. We propose that Xist RNA does not dissociate from the Xi of its origin, which precludes a model of diffusion-mediated trans spreading of Xist RNA.

SUPPLEMENTAL MATERIAL

Supplemental material for this article may be found at http://mcb.asm.org/ .

ACKNOWLEDGMENTS

We thank Ken Kosik for kindly providing the MS2 repeat sequence and Thasin Stefan Barakat for technical assistance.

This work was supported by HFSP-CDA and NWO-VIDI grants.