Chapter category: RNA
Xist RNA Associates with Chromatin and Causes Gene Silencing
Noncoding RNAs: Molecular Biology and Molecular Medicine
Edited by: Jan Barciszewski and Volker A. ErdmannISBN: 0-306-47835-8
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Chapter authors:
Anton Wutz
The mammalian Xist gene produces a long, spliced and poly-adenylated noncoding RNA that is uniquely distributed in the nucleus. Xist RNA spreads in cis from its site of transcription over the entire X-chromosome and mediates X-inactivation, the transitional silencing of one of the two X-chromosomes of female cells. X-inactivation is required for compensation of the dosage difference of X-linked genes between XY males and XX females. A counting and choosing mechanism ensures that Xist is expressed from all but one X-chromosome in a random manner. Repression of Xist RNA accumulation on the single active X-chromosome is dependent on another large noncoding RNA, the Tsix RNA, transcribed in antisense orientation to Xist. Initiation of Xist expression occurs early in development when cellular differentiation has not yet progressed and involves stabilization of the RNA. Transcriptional repression follows the accumulation of Xist RNA with little delay. At a later stage of cell differentiation Xist expression does no longer trigger silencing suggesting that Xist mediated silencing is restricted to certain cell types which have not yet undergone differentiation. The function of Xist RNA has been studied in mouse and humans. Although, it is unclear at present how Xist RNA interacts with and spreads over chromatin and effects gene silencing recent studies have begun to shed light on the underlying mechanism. A number of chromosomal proteins and histone modifications have been implicated, in Xist mediated gene silencing including variant forms of histone H2A, methylation of H3 and hypoacetylation of H4. The Nova1/2 family of RNA binding proteins has recently been implicated in the function of Xist RNA. Chromosomal association has also been observed with the roX1 and roX2 RNAs in Drosophilaee. In contrast to mammalian Xist RNA these RNAs localize to the fly X-chromosome in a sequence directed manner and mediate enhanced transcription of the single male X. Despite mechanistic differences the mammalian Xist, and the fly roX1 and roX2 RNAs are at present the only RNAs that have been shown to associate with chromatin over the lengths of entire chromosomes and therefore might be viewed as a unique class of chromatin associated RNAs.
Anton Wutz
Institute of Molecular Pathology, Vienna, Austria
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