A study in 1949 revealed that the inactive X (Xi) appeared as a nucleolar satellite in cat neurons. Despite this finding, the overwhelming impression in more recent decades has been that, in most cell types, the Xi is associated with the periphery of the nucleus, not nucleolus. The nuclear periphery, however, is also the preferred location for the active X (Xa). Using mouse embryonic stem cells, Zhang and colleagues now show that, although both Xs are often seen at the nuclear periphery, Xi takes a trip to the nucleolus at S phase, while Xa stays put.
Positioning of Xi at the nucleolus required Xist RNA, which is transcribed from within the X inactivation center (Xic). The Xist gene is only activated on Xi, and its noncoding product spreads along Xi, coating and silencing the entire chromosome. Chopping out the Xic led to an immediate loss of this Xist coating and of Xi's association with the nucleolus.
Previous studies implied that the Xic and Xist were necessary for the initiation of X inactivation but dispensable for its maintenance. Zhang et al. found that, immediately after the removal of the XIC, heterochromatin marks on Xi did indeed remain even after the Xist RNA had left. After several cell cycles, however, these marks began to be erased and Xi genes began to reactivate. Previous studies might have missed this maintenance role for Xist because they did not examine silencing long enough.
The authors believe that the Xist-dependent nucleolar recruitment of Xi is necessary for long-term maintenance of its inactivation. In support of this hypothesis, the team found that a ring around the nucleolus contained a chromatin-remodeling protein that is required to reestablish heterochromatin on newly replicated DNA.