We conducted these experiments in the presence of HDAC1/2 inhibition, which increases induction but was previously shown not to impact silencing during XCI (?ylicz et al., 2019). similarity to the A-repeat of RNA and A-repeat bind the RRM domains of Spen in a competitive manner. Insertion of an ERV into an A-repeat deficient Xist rescues binding of RNA to Spen and results in strictly local gene silencing in may coopt transposable element RNA-protein interactions to repurpose powerful antiviral chromatin silencing machinery for sex chromosome dosage compensation. is responsible for switching off the extra X genes in female cells. It does this by covering the entirety of the second X chromosome. Normally, RNA molecules transmit the coded instructions in genes to the cellular machinery that produces proteins. Noncoding RNAs like might have acquired its ability to switch genes off. Initial experiments used mouse cells produced in the laboratory, in which a protein called Spen was deleted. Spen is known to help silence the X chromosome. In female cells lacking Spen, the second X chromosome remained active. Other chromosomes in male and female cells also experienced stretches of DNA that became active upon Spens removal. These DNA sequences, termed endogenous retroviruses, were remnants of ancestral viral infections. In other words, Spen normally acted as an antiviral defense. Analysis of genetic sequences showed that Spen acknowledged endogenous retrovirus sequences resembling a key region in to work properly. Inserting fragments of endogenous retroviruses into a defective version of lacking this region also partially restored its ability to inactivate genes, suggesting that X chromosome silencing might work by hijacking cellular defenses against viruses. That is, female cells essentially pretend there is a viral contamination on the second X chromosome by covering it with (which mimics endogenous retroviruses), thus directing Spen to shut it down. This research is an important step towards understanding how female cells carry out dosage compensation in mammals. More broadly, it sheds new light on how ancient viruses may have shaped the development of noncoding RNAs in the human genome. Introduction is usually a 17 kb long noncoding RNA that functions through specific interactions between its unique RNA domains and nuclear effector proteins. The RNA-associated protein complex was recognized in 2015 using both genetic and affinity-based methods, and consists of multiple pleiotropic proteins, many of which are highly conserved throughout development and take action on chromatin structure and gene regulation in myriad systems (Augui et al., 2011; Chu et al., 2015; McHugh et al., 2015; Minajigi et al., 2015; Monfort et al., 2015; Moindrot et al., 2015). This suggests that evolved the ability to bind these proteins in the eutherian mammals, coopting those which developed in the beginning to perform other epigenetic functions. developed in the eutherian clade through exaptation of a combination of coding genes that were pseudogenized, as well as transposable elements (TEs) that inserted into this locus. contains Darifenacin six tandem repeat regions (A-F), all of which show sequence similarity to TEs, suggesting they arose from eutheria-specific TE insertions (Elisaphenko et al., 2008). One of these is the A-repeat, which is essential for gene silencing. When this?~500 bp region is deleted, RNA coats the X chromosome, but silencing and reorganization of the X does not follow (Wutz et al., 2002; Giorgetti et al., 2016). The A-repeat sequence is thought to derive from the insertion and duplication of an endogenous retrovirus (ERV), a class of TEs present in many copies throughout the genome (Elisaphenko et al., 2008). In general, lncRNAs are not well-conserved compared to protein-coding genes but are enriched for TE content, suggesting they may be able to rapidly evolve functional domains by exapting protein- and nucleic acid-binding activity from entire TEs that colonize their loci (Johnson and Guig, 2014; Kelley and Rinn, 2012). Understanding how the RNA sequence was evolutionarily stitched together from these existing building blocks to gain protein-binding potential is usually of great interest towards understanding dosage compensation and lncRNA-mediated gene regulation genome-wide. Spen (also known as SHARP, MINT) is usually a?~?400 kDa RNA binding protein (RBP) that contains four canonical RNA binding domains, as well as a SPOC domain name to facilitate protein-protein interactions. Spen Darifenacin is usually a co-repressor that binds to PPIA several chromatin remodeling complexes, including histone deacetylases (HDACs), and the NuRD complex (McHugh et al., 2015; Shi et Darifenacin al., 2001). Though now acknowledged for its central role in the eutherian-specific XCI process, Spen is an ancient protein that plays functions in gene repression during development in species including?RNA and inactivation abrogates silencing of.