Cloning and characterization of the mouse ortholog of mi-er1.

Mi-er1 is a fibroblast growth factor immediate-early gene whose expression is differentially regulated in breast tumours. MI-ER1 functions as a transcriptional repressor of a number of genes, including Sp1 target genes. The Xenopus and human orthologs have been described and here we report the characterization of the mouse gene and its products. Mouse mi-er1 is a single copy gene located on chromosome 4. It has the same intron-exon structure as the human gene with the exception of exon 3A, in which an upstream 3' splice acceptor is utilized. As described in humans, multiple transcripts are produced in the mouse, and we have isolated the mouse orthologs of the human N1-beta, N2-beta and N3-beta, all of, which differ in their N-terminal sequence. Furthermore, we have isolated a novel isoform, N4-beta, containing sequence from an additional exon located between exon 4 and exon 5 that produces an fourth alternate N-terminus. The human mi-er1 transcripts also include isoforms encoding two alternate C-termini, alpha and beta. Like Xenopus, only isoforms containing the beta C-terminus have been detected in the mouse. Expression analysis, using a panel of mouse tissues and embryos, revealed that the N1-beta and N3-beta are ubiquitously expressed while N4-beta is only expressed in testis. N2-beta is expressed in most tissues but was not detected in heart, brain, eye or skeletal muscle. Sequence comparison revealed 95% identity between mouse and human MI-ER1 isoforms and 72% identity between mouse and Xenopus. The most conserved region in the MI-ER1 protein is the SANT domain, which is identical in all three species. Further analysis of the SANT domain using sequences retrieved from the genome databases for rat, cow, chicken, zebrafish, trout and Xenopustropicalis revealed that this domain is highly conserved, with 88% identity among the 9 species. Moreover, an additional 10 residues C-terminal to the published end of this domain are 100% conserved, suggesting that in MI-ER1, the functional domain includes this extended sequence.