FIG. 4.

Translocation of genic loci into the silent repeat compartment during silencing. (A) Active X^P genes reside outside of the Xist⁺ Cot-1⁻ repeat compartment before silencing. Three representative blastomeres from morulae are shown expressing three X-linked genes. Arrows indicate linear distances between the gene and the outer edge of the Xist⁺ Cot-1⁻ compartment. (B) During the process of silencing (deduced by diminished X^P expression), genes are translocated into the Xist⁺ Cot-1⁻ compartment in the morula. The boxed region is magnified in the right panel. Arrows indicate linear distances between the gene and the outer edge of the Xist⁺ Cot-1⁻ compartment. (C) Summary of linear distances between the gene and the silent compartment during silencing. The normalized distance is the linear distance from the center of the nascent RNA signal to the edge of the Xist⁺ Cot-1⁻ compartment, each normalized to the nuclear diameter. Negative distances imply genic movement into the Xist⁺ Cot-1⁻ compartment, whereas a zero distance implies localization at the edge of the Xist⁺ Cot-1⁻ compartment. P values were calculated using an unpaired t test. (D) X^P territory contracts over time. X^P and X^M territories were measured by Volocity software (Improvision) and normalized to total nuclear volume to yield the chromosome condensation index. P values were calculated using an unpaired t test. (E) Pictorial representation of genic localization into the preformed Xist⁺ Cot-1⁻ compartment during silencing. The silent compartment is present by the two-cell stage, and it enlarges as genic loci are translocated into it as they are gradually inactivated, beginning at the morula stage. X^P silencing is not complete until the blastocyst stage or later.