Prior to X-inactivation, Xist is transcribed in unstable form. The initiation of X-inactivation is associated with the appearance of stable Xist transcripts which coat the X chromosome to be inactivated. Using strand specific RT-PCR analysis of the 5' region of Xist, we have detected antisense transcripts (Xist AS) in undifferentiated embryonic stem (ES) cells, but not in female somatic cells. Screening of a female ES cell cDNA library allowed us to isolate one poly(A)-tailed cDNA clone corresponding to this RNA. 5' RACE analysis showed that XistAS and the P1 sense product of Xist overlap by at least 707 bp. Expression of XistAS was also detected in early mouse embryos before random X-inactivation in the epiblast lineage. Although XistAS is low in abundance, it may be involved in destabilizing Xist mRNA in undifferentiated ES cells.
Copyright 1999 Academic Press.