D. Warshawsky et al., Further examination of the Xist promoter-switch hypothesis in X inactivation: Evidence against the existence and function of a P-0 promoter, P NAS US, 96(25), 1999, pp. 14424-14429
Citations number
27
Categorie Soggetti
Multidisciplinary
Journal title
PROCEEDINGS OF THE NATIONAL ACADEMY OF SCIENCES OF THE UNITED STATES OF AMERICA
The onset of X inactivation coincides with accumulation of Xist RNA along t
he future inactive X chromosome. A recent hypothesis proposed that accumula
tion is initiated by a promoter switch within Xist. in this hypothesis, an
upstream promoter (P-0) produces an unstable transcript, while the known do
wnstream promoter (P-1) produces a stable RNA. To test this hypothesis, we
examined expression and half-life of Xist RNA produced from an Xist transge
ne lacking P-0 but retaining P-1. We confirm the previous finding that P-0
is dispensable for Xist expression in undifferentiated cells and that P-1 c
an be used in both undifferentiated and differentiated cells. Herein, we sh
ow that Xist RNA initiated at P-1 is unstable and does not accumulate. Furt
her analysis indicates that the transcriptional boundary at P-0 does not re
present the 5' end of a distinct Xist isoform. Instead, P-0 is an artifact
of cross-amplification caused by a pseudogene of the highly expressed ribos
omal protein S12 gene Rps12. Using strand-specific techniques, we find that
transcription upstream of P-1 originates from the DNA strand opposite Xist
and represents the 3' end of the antisense Tsix RNA. Thus, these data do n
ot support the existence of a P-0 promoter and suggest that mechanisms othe
r than switching of functionally distinct promoters control the up-regulati
on of Xist.