Promoter region and alternatively spliced exons of the rat mu-opioid receptor gene.

The actions of exogenous and endogenous opioids are mediated by at least three different opioid receptors, called mu, kappa, and delta. Recently, we have detected a new variant of the rat mu-opioid receptor, which we termed rMOR1B and which differs from rMOR1 (now also called rMOR1A) in the amino acid sequence at the C-terminus. Both isoforms were proposed to be splicing variants of the same gene. To elucidate the molecular mechanism leading to the formation of the new variant, the exon/intron structure of the rat mu-opioid receptor gene in the respective area has been determined by analyzing a genomic P1 phage clone. In addition, we have investigated the putative promoter region of this gene. The present study revealed that rMOR1B is generated by an alternative splicing event whereby a previously unknown exon will be placed behind exon 3 to form rMOR1B mRNA, which is separated from the latter by an intron. Therefore, this new exon has to be called exon 4, whereas the former exon 4, which encodes the C-terminus of MOR1A, now becomes exon 5. Examination of the putative rat promoter region revealed a high degree of nucleotide sequence homology to the mouse gene. Using an RNase protection approach, one single transcription initiation site could be located at 230 bp upstream of the translation start. This is similar to the situation in the mouse, where four major transcription start sites were reported to lie close together around 270 bp upstream of the protein coding region.