Transcription of the mouse mu-opioid receptor gene is regulated by two promoters

Two transcription initiation sites have been identified in the mouse mu-opioid receptor (MOR) gene at approximately -793 and -268 upstream of the translation start site. To test if the MOR gene contains two functional promoters, chimeric constructs of mouse MOR deletion fragments, fused to a luciferase reporter gene, were transiently transfected into the neuroblastoma cell line SK-N-SH, a MOR-expressing cell line, and two MOR-non-expressing cell lines. Results from transient transfection assays confirmed the existence of two functional independent promoters in the mouse MOR gene, and also revealed that the region from -1337 to -93 does not contain all the elements necessary to confer tissue-specific expression of the MOR gene.