Anton Wutz, Theodore P. Rasmussen & Rudolf Jaenisch
Whitehead Institute for Biomedical Research, Nine Cambridge Center, Cambridge, Massachusetts 02142,USA.
Correspondence should be addressed to R Jaenisch. e-mail: jaenisch@wi.mit.edu
The gene Xist initiates the chromosomal silencing process
of X inactivation in mammals. Its product, a noncoding RNA, is expressed
from and specifically associates with the inactive X chromosome in female
cells. Here we use an inducible Xist expression system in
mouse embryonic stem cells that recapitulates long-range chromosomal silencing
to elucidate which Xist RNA sequences are necessary for chromosomal
association and silencing. We show that chromosomal association and spreading
of Xist RNA can be functionally separated from silencing by specific
mutations. Silencing requires a conserved repeat sequence located at the
5' end of Xist. Deletion of this element results in Xist
RNA that still associates with chromatin and spreads over the chromosome
but does not effect transcriptional repression. Association of Xist
RNA with chromatin is mediated by functionally redundant sequences that
act cooperatively and are dispersed throughout the remainder of Xist
but show little or no homology.
1. Luikenhuis S, Wutz A, and Jaenisch R, "Antisense Transcription Through the Xist Locus Mediates Tsix Function in Embryonic Stem Cells".
2. Stavropoulos N, Lu N, and Lee JT, "A Functional Role for Tsix Transcription in Blocking Xist RNA Accumulation but Not in X-Chromosome Choice".
3. Kelley RL, and Kuroda MI, "The Role of Chromosomal RNAs in Marking the X for Dosage Compensation".
4. Eddy SR, "Non-Coding RNA Genes and the Modern RNA World".
5. Wutz A, and Jaenisch R, "A Shift from Reversible
to Irreversible X Inactivation is Triggered during ES Cell Differentiation".