An upstream promoter element blocks the reverse transcription of the mouse insulin-degrading enzyme gene

Despite the prevalence of bidirectional promoters among the mammalian genomes, the majority of promoters are unidirectional. The mechanism through which unidirectional promoters are prevented from reverse transcription remains to be clarified. Here we investigate the transcriptional directionality of the mouse insulin-degrading enzyme (IDE) promoter, which contains a CpG island and has dispersed transcription initiation sites. Although IDE is unidirectionally transcribed according to its genomic context, the basic promoter region of mouse IDE has bidirectional transcriptional properties. The region between ?219 and +133 of mouse IDE relative to its first transcription initiation site has bidirectional transcriptional activities, but the region between ?350 and +133 can only be transcribed from the normal direction, implying that an upstream promoter element locating between ?350 and ?219 blocks the reverse transcription of mouse IDE. We further mapped this upstream promoter element to the region between ?243 and ?287. Promoter mutation analysis showed that the upstream promoter element contains two functional sub-regions. In conclusion, we identified an upstream promoter element which blocks the reverse transcription of mouse IDE. Our studies are suggestive for the transcriptional mechanism of bidirectional promoters in mammalian genomes.