Dual promoters of mouse μ-opioid receptor gene

Two transcriptional initiation sites, distal and proximal, located at approximately 500 bp apart in mouse μ-opioid receptor gene were identified recently. Using deletional and transfection analyses, the distal or proximal promoter alone displayed similar activity in neuronal cells constitutively expressing μ-opioid receptors (SH-SY-5Y). The presence of both promoters did not result in an increase in activity. However, when distal promoter linked with 3'-downstream sequences without the proximal promoter region, the distal promoter activity was abolished. Transfection analysis using various cell lines further suggested that only the proximal promoter preferentially directed the neuronal subtype expression. To verify the physiological importance of each promoter, the ratio of μ-receptor transcripts initiated by either promoter was examined by quantitative RT-PCR using mouse adult brain mRNA. We found that receptor mRNA was predominantly initiated by the proximal promoter. These studies suggested that the proximal promoter was the major functional promoter.