In female mammals, up-regulation of Xist triggers X-chromosome inactivation in cis. Up-regulation is inhibited by sequences 3 to Xist contained within the antisense locus, Tsix. Inhibition could depend on transcription of Tsix and͞or on DNA elements therein. Here we test the role of Tsix transcription by augmenting the duration and strength of Tsix expression. We find that Tsix hypertranscription is sufficient to block Xist RNA accumulation in a cis-limited manner. We propose that Tsix transcription is necessary to restrict Xist activity on the future active X and, conversely, that Tsix repression is required for Xist RNA accumulation on the future inactive X. We also find that Tsix hypertranscription does not affect X-chromosome choice. Thus, choice is mediated by elements within Tsix that are independent of promoter activity.

Dosage compensation in mammals occurs by X inactivation, a silencing mechanism regulated in cis by the X inactivation center (Xic). In response to developmental cues, the Xic orchestrates events of X inactivation, including chromosome counting and choice, initiation, spread, and establishment of silencing. It remains unclear what elements make up the Xic. We previously showed that the Xic is contained within a 450-kb sequence that includes Xist, an RNA-encoding gene required for X inactivation. To characterize the Xic further, we performed deletional analysis across the 450-kb region by yeast-artificial-chromosome fragmentation and phage P1 cloning. We tested Xic deletions for cis inactivation potential by using a transgene (Tg)-based approach and found that an 80-kb subregion also enacted somatic X inactivation on autosomes. Xist RNA coated the autosome but skipped the Xic Tg, raising the possibility that X chromosome domains escape inactivation by excluding Xist RNA binding. The autosomes became late-replicating and hypoacetylated on histone H4. A deletion of the Xist 5' sequence resulted in the loss of somatic X inactivation without abolishing Xist expression in undifferentiated cells. Thus, Xist expression in undifferentiated cells can be separated genetically from somatic silencing. Analysis of multiple Xic constructs and insertion sites indicated that long-range Xic effects can be generalized to different autosomes, thereby supporting the feasibility of a Tg-based approach for studying X inactivation.