RESUMEN
BMP-4 signaling is mediated through Smad proteins which may translocate to the nucleus to activate transcription. Little is known about how BMP-4 signaling regulates the transcription of its target genes, e.g., Xvent genes. Therefore, we isolated the genomic clone of a BMP-4 responsive homeobox gene, Xbr-1a/Xvent-2. This clone contains a promoter and three exons for the entire coding region. Using the primer extension, we identified the transcription initiation site corresponding to position -64 bp upstream to the ATG codon of the Xvent-2 gene. The promoter was linked to the luciferase reporter gene, and promoter activity determined by luciferase assay. The temporal promoter activity peaked between embryonic stages 13~17, in agreement with its temporal mRNA expression in the whole embryo. Through the serial deletion mutation, the upstream -235 bp of the promoter retains the full transcriptional activity, and is regulated by BMP-4 signaling. The present results suggest that the BMP-4 responsive element is located on the upstream 235 bp of the promoter.