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XIST (gene)
Xist (X-inactive specific transcript) is an RNA gene on the X chromosome of the placental mammals that acts as a major effector of the X inactivation process.〔(【引用サイトリンク】 url = http://www.ncbi.nlm.nih.gov/sites/entrez?Db=gene&Cmd=ShowDetailView&TermToSearch=7503 )〕 It is a component of the ''Xic'' - X-chromosome inactivation centre - along with two other RNA genes (''Jpx'' and ''Ftx'') and two protein genes (''Tsx'' and ''Cnbp2''). The Xist RNA, a large (17 kb in humans) transcript, is expressed on the inactive chromosome and not on the active one. It is processed in a similar way to mRNAs, through splicing and polyadenylation. However, it remains untranslated. It has been suggested that this RNA gene evolved at least partly from a protein coding gene that became a pseudogene. The inactive X chromosome is coated with this transcript, which is essential for the inactivation. X chromosomes lacking Xist will not be inactivated, while duplication of the Xist gene on another chromosome causes inactivation of that chromosome.
== Function ==
X inactivation is an early developmental process in mammalian females that transcriptionally silences one of the pair of X chromosomes, thus providing dosage equivalence between males and females (see dosage compensation). The process is regulated by several factors, including a region of chromosome X called the X inactivation center (XIC). The XIST gene is expressed exclusively from the XIC of the inactive X chromosome. The transcript is spliced but apparently does not encode a protein. The transcript remains in the nucleus where it coats the inactive X chromosome. Alternatively spliced transcript variants have been identified, but their full length sequences have not been determined.〔
The functional role of the Xist transcript was first definitively demonstrated in mouse female ES cells using a novel antisense technology, called peptide nucleic acid (PNA) interference mapping. In the reported experiments, a single 19-bp antisense cell-permeating PNA targeted against a particular region of Xist RNA prevented the formation of Xi and inhibited cis-silencing of X-linked genes. The association of the Xi with macro-histone H2A is also disturbed by PNA interference mapping.
X-inactivation process occurs in mice even in the absence of this gene via epigenetic regulation, but Xist is required to stabilize this silencing.
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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