WTTA
A- repeat
A-repeat Xist
∆AXist cloud
の作るヘテロクロマチン領域
hnRNP U
によるX
染色体への集積A
リピートによるクロマチン結合5.
(XX) (XY) X
2 X
X Xist RNA X
X
Xist RNA RNA II
mRNA X
Xist RNA X
Xist RNA X
hnRNP U hnRNP U
X Xist RNA
Xist RNA X
X
SmcHD1 HBiX1 X
Xist RNA Xist RNA X
X
hnRNP U
Xist RNA hnRNP U
Xist RNA X
Xist RNA
hnRNP U Xist RNA X
Xist RNA Xist RNA
NIH3T3 Xist RNA
6-kb 3 Xist RNA hnRNP U
53
hnRNP U CLIP-seq hnRNP U
hnRNP U XIST RNA 5' A
A XIST RNA
(950-nt) hnRNP
U NIH3T3
950-nt RNA 950-nt
hnRNP U Xist RNA
Xist RNA A X
950-nt A
A X
950-nt RNA A
(950-nt mut) 950-nt (950-nt as) NHI3T3 950-nt mut 950-as RNA-FISH
950-nt 950-nt mut 950-nt as
Col1a1 1 ES
NIH3T3
Xist RNA hnRNP U X
Xist RNA X
A
A 950-nt RNA
Xist RNA X
Xist RNA Xist RNA X
A X PRC2
H3K27me3 X
A Xist RNA X H3K27me3
X
A
Xist RNA X
Xist RNA 5' hnRNP U
Xist RNA hnRNP U X A
Xist RNA
X X
55 6. SUMMARY
X chromosome inactivation (XCI) is a mechanism to compensate for the dose difference of X-linked genes between XX females and XY males.
X-linked X-inactive specific transcript (Xist) is a long noncoding RNA that plays an essential role in XCI in the mouse. Although Xist RNA, like common protein-coding mRNAs, is transcribed by RNA polymerase II, spliced and polyadenylated, it is retained in the nucleus and associates with the X chromosome it originates from. One of the major proteins constituting the nuclear matrix, hnRNP U, has been shown to be required for the association of Xist RNA with the inactive X chromosome (Xi). It has also been suggested that in human cultured cells, structural maintenance of chromosomes hinges domain containing 1 (SMCHD1), and its binding partner, HBiX1, are involved in compaction of Xi. However, the sequences in Xist RNA that are involved in compaction of Xi have not yet been identified.
In this study, I found that the first 950-nt sequence of Xist RNA had the potential to associate with chromatin in a manner independent of hnRNP U.
Furthermore, its chromatin association is apparently dependent on the presence of an intact A-repeat sequence, which is one of the repeats in Xist/XIST RNA conserved among many mammalian species, and has been shown to be essential for Xist RNA-mediated chromosome silencing. This raises an interesting possibility that the ability of chromatin association of the 950-nt RNA might be functionally involved in the mechanisms of XCI.
It has been proposed that the role of the A-repeat in XCI is to mediate the interaction between Xist RNA coating the X chromosome and polycomb repressive complex 2 (PRC2) which catalyzes trimethylation of the lysine residue located at the twenty-seventh position from the N-terminus of histone H3 (H3K27me3), which is one of histone modifications typical for facultative heterochromatin such as Xi. However previous studies showed that H3K27me3 was not sufficient for chromosome silencing as it still accumulated on the X chromosome coated with the mutated Xist RNA
lacking the A-repeat defective in chromosome silencing. Subsequent study further demonstrated that X-linked genes, which failed to be silenced by the mutated Xist RNA were located outside or at the periphery of the domain formed by the mutated Xist RNA in the nucleus. Taking all these findings and those in this study, I suggest a new hypothesis that the A-repeat captures chromatin near the initial loading site of Xist RNA and relocates it into the core of the Xist RNA domain consisting of heterochromatin.
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