Antagonistic interactions between the cAMP-dependent protein kinase and Tor signaling pathways modulate cell growth in Saccharomyces cerevisiae

In mammals, X-chromosome inactivation (XCI) equalizes X-linked gene expression between XY males and XX females and is controlled by a specialized region known as the X-inactivation center (Xic). The Xic harbors two chromatin interaction domains, one centered around the noncoding Xist gene and the other around the antisense Tsix counterpart. Previous work demonstrated the existence of a chromatin transitional zone between the two domains. Here, we investigate the region and discover a conserved element, RS14, that presents a strong binding site for Ctcf protein. RS14 possesses an insulatory function suggestive of a boundary element and is crucial for cell differentiation and growth. Knocking out RS14 results in compromised Xist induction and aberrant XCI in female cells. These data demonstrate that a junction element between Tsix and Xist contributes to the initiation of XCI.     

Comparative analysis of the DXPas34 regulatory region in rodents

Mouse X chromosome inactivation center contains the DXPas34 minisatellite locus which plays an important role in expression regulation of the Tsix and Xist genes, involved into female dosage compensation. Comparative analysis of the DXPas34 locus from mouse, rat, and four common vole species revealed similar organization of this region in the form of tandem repeat blocks. A search for functionally important elements in this locus showed that all the species examined carried the conservative motif monomers, which could be involved in regulation of X inactivation.     

Identification of a Ctcf cofactor, Yy1, for the X chromosome binary switch

In mammals, inactivation of one X chromosome in the female equalizes gene dosages between XX females and XY males. Two noncoding loci, Tsix and Xite, together regulate X chromosome fate by controlling homologous chromosome pairing, counting, and mutually exclusive choice. Following choice, the asymmetry of Xite and Tsix expression drives divergent chromosome fates, but how this pattern becomes established is currently unknown. Although no proven trans-acting factors have been identified, a likely candidate is Ctcf, a chromatin insulator with essential function in autosomal imprinting. Here, we search for trans-factors and identify Yy1 as a required cofactor for Ctcf. Paired Ctcf-Yy1 elements are highly clustered within the counting/choice and imprinting domain of Tsix. A deficiency of Yy1 leads to aberrant Tsix and Xist expression, resulting in a deficit of male and female embryos. Yy1 and Ctcf associate through specific protein-protein interactions and together transactivate Tsix. We propose that the Ctcf-Yy1-Tsix complex functions as a key component of the X chromosome binary switch.     

X inactivation counting and choice is a stochastic process: evidence for involvement of an X-linked activator

Female mammalian cells achieve dosage compensation of X-encoded genes by X chromosome inactivation (XCI). This process is thought to involve X chromosome counting and choice. To explore how this process is initiated, we analyzed XCI in tetraploid XXXX, XXXY, and XXYY embryonic stem cells and found that every X chromosome within a single nucleus has an independent probability to initiate XCI. This finding suggests a stochastic mechanism directing XCI counting and choice. The probability is directly proportional to the X chromosome:ploidy ratio, indicating the presence of an X-encoded activator of XCI, that itself is inactivated by the XCI process. Deletion of a region including Xist, Tsix, and Xite still results in XCI on the remaining wild-type X chromosome in female cells. This result supports a stochastic model in which each X chromosome in a nucleus initiates XCI independently and positions an X-encoded trans-acting XCI-activator outside the deleted region.