page_title: XIST
section_title: A region
hierarchical_section_title: XIST / Transcript organization / A region
caption_attribution_description: English: Structure model of the Xist lncRNA repA region adapted from Fang R, Moss WN, Rutenberg-Schoenberg M, Simon MD (2015) Probing Xist RNA Structure in Cells Using Targeted Structure-Seq. PLoS Genet 11(12): e1005668. doi:10.1371/journal.pgen.1005668
context_page_description: Xist is a non-coding RNA on the X chromosome of the placental mammals that acts as a major effector of the X-inactivation process. It is a component of the Xic – X-chromosome inactivation centre – along with two other RNA genes and two protein genes.
  The Xist RNA, a large 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.
  The human XIST gene was discovered by Carolyn J. Brown in the laboratory of Hunt Willard.
context_section_description: The Xist RNA contains a region of conservation called the repeat A (repA) region that contains up to nine repeated elements. It was initially suggested that repA repeats could fold back on themselves to form local intra-repeat stem-loop structures. Later work using in vitro biochemical structure probing proposed several inter-repeat stem-loop structures. A recent study using in vivo biochemical probing and comparative sequence analysis proposed a revision of the repA structure model that includes both intra-repeat and inter-repeat folding found in previous models as well as novel features (see Figure). In addition to its agreement with the in vivo data, this revised model is highly conserved in rodents and mammals (including humans) suggesting functional importance for repA structure. Although the exact function of the repA region is uncertain, it was shown that the entire region is needed for efficient binding to the Suz12 protein.
caption: Structure model of the repeat A (repA) region of Xist based on in vivo biochemical structure probing and comparative sequence analysis. Repeats 1 to 8(1/2) are numbered and boxed - they are shown in red on the cartoon of repA in the upper left hand panel. Reactive nucleotides are colored red, where open and closed circles are medium and strongly reactive, respectively (reactivity suggests that a nucleotide is unpaired or loosely structured). Consistent and compensatory mutations (single and double point mutations that preserve pairing) are annotated in blue and purple, respectively. Base pairs that are 100% conserved in rodents are bold and black, while those conserved in rodents and mammals are in green. The data and model are taken from Fang R, Moss WN, Rutenberg-Schoenberg M, Simon MD (December 2015). "Probing Xist RNA Structure in Cells Using Targeted Structure-Seq". PLoS Genetics. 11 (12): e1005668. doi:10.1371/journal.pgen.1005668. PMC 4672913. PMID 26646615..
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