Cloning and localization of the murine Xpct gene: evidence for complex rearrangements during the evolution of the region around the Xist gene.

The overall organization of the X-inactivation center (XIC/Xic) candidate region seems poorly conserved between human and mouse. The orientation of a region containing the X-inactive-specific transcript (Xist/ XIST) gene and three genes located 3' of Xist/XIST has been shown to be inverted between the two species, although the actual extent of this rearrangement is unknown. We have cloned and mapped the mouse homolog of the human XPCT (X-linked PEST-containing transporter) gene, which encodes a putative transmembrane transporter. Human XPCT is located about 200 kb outside of the XIC candidate region and 600 kb 5' of or telomeric to the XIST gene. The mouse Xpct gene, which lies approximately 300 kb 5' of and centromeric to Xist, displays 85% identity at the nucleotide level with the human gene, and the overall protein structure is conserved. The transcriptional orientation of mouse Xpct with respect to Xist is the opposite of that in human. Consequently, the evolution of the region between human and mouse appears to be highly complex, with structural rearrangements involving a region of up to 600 kb or more around the Xist gene.