Comparison of Xist gene intron–exon structure in mouse, bovine, and human. Introns and the first exon are not drawn to scale. Conserved regions corresponding to known exons (in at least one species) are indicated. Consensus splice signals that align with boundaries of known exons are in capitals. In mouse and human, 8 exons have been identified (Brown et al. 1992; Sheardown et al. 1997). Note, however, that some exons in mouse have no counterpart in the human and vice versa. To avoid ambiguities in exon numbering, we used the prefix “h” to identify human exons, and “m” to identify mouse exons. Mouse exons m1, m3, m4, m6, m7, and m8 are conserved and correspond to human exons h1, h3, h4, h5, h6, and h8, respectively. In both species, we identified an alternative donor splice site in exon m7–h6, although it is not located exactly at the same position in human and mouse. With the exception of this splice site, splice signals (donor and acceptor) of these six exons, as well as the polyadenylation signal in the last exon, are perfectly conserved in mouse, man, and bovine (which suggests that they also correspond to true exons in the latter species). Human exon h2 is located in a region that is not conserved in mouse or bovine. Mouse exon m2 is conserved in the three species, with correct splice signals, except in human where the splice acceptor is mutated (AT instead of AG), which suggests that this exon is no longer functional in human. Mouse exon m5 is conserved in the three species with correct splice signals and might correspond to an unidentified alternative exon in humans. Human exon h7 is located in a region that is conserved in the three species, but consensus splice signals are not found in mouse and bovine. a, mouse Xist cDNA (GenBank accession number L04961; Brockdorff et al. 1992); b, mouse ESTs (BE626785, BE632200, and R74734); c, mouse exon m7a reported by Hong et al. (1999); d, human XIST cDNA (M97168; Brown et al. 1992); e, human exon h6a reported by Hong et al. (2000); f, human ESTs (AV699347,AV700119, and AV700677). Exon positions in mouse (bp): m1, 354096…363614; m2, 366392…366482; m3, 366632…366763; m4, 367512…367722; m5, 367867…368013; m6, 368341…368495; m7a, 369277…376940; m7b, 369277…371600; and m8, 376603…376940. In human, exon positions are indicated relative to the XIST genomic sequence (U80460): h1, 18317…29688; h2, 33567…33630; h3, 37696…37832; h4, 39796…40004; h5, 41840…42003; h6a, 43087…50396; h6b, 43087…44956; h7, 48855…49000; and h8, 50042…50396.
