Compact chromatin packaging of inactive X Chromosome involves the activelytranscribed Xist gene

The Xist gene responsible for X inactivation may take a unique chromatin st ructure because of exceptional expression from inactive X Chromosome, (Chr) . We have examined differential chromatin packaging of the Xist gene region between active and inactive X Chr with a novel method consisting of the ch romatin fractionation and allele-specific detection. Analysis of F-1 hetero zygous female mice from T(X;16)16H x MSM crosses and two cell clones derive d from inter-subspecific F-1 female mice demonstrated that the packaging le vel of the transcribed Xist region on inactive X Chr was as tight as that o f the repressed Pgk-1 allele on the same chromosome. On the other hand, res triction endonuclease sensitivity assay of chromatin showed that the promot er region, but not transcribed regions, of the transcribed Xist allele reta ined accessibility to nucleases. These results may suggest a cis-element(s) in a regulatory region of the Xist gene to prevent the transcriptionally i nhibitory effect of the chromatin packaging.