Genomic organization and promoter analysis of the murine Id3 gene

Overexpression of the helix-loop-helix motif-containing transcription inhibitor Id3 has been shown to repress muscle-specific gene expression. Consistent with its putative negative regulatory role in the myogenic process, Id3 is highly expressed in proliferating myoblasts but down regulated when myoblasts are induced to differentiate. To investigate how Id3 expression may be transcriptionally regulated, we isolated a mouse Id3 genomic DNA fragment and characterized its organization and promoter activity. Comparison of the Id3 gene from human and mouse demonstrated a conserved exon-intron organization in which the first intron interrupts the C-terminal protein coding region and the second intron interrupts the 3' untranslated region at analogous positions in the two species. Sequence analysis of the 5'-flanking region revealed an unexpected mouse strain-specific genetic polymorphism due to a single base substitution. Deletion analysis revealed that as little as 180 base pairs of the mouse Id3 promoter upstream of the transcription start site is sufficient for a high level of gene expression in proliferating C2C12 myoblasts. In particular, the region between the nucleotide position -180 and -34 appeared to be crucial for maximal reporter gene activity and interacted specifically with C2C12 nuclear proteins. Finally, we showed that, despite the creation of a putative transcription factor-binding site, the genetic polymorphism observed did not affect Id3 promoter activity in proliferating C2C12 cells.