To establish the complete coding region of MOR-1K isoforms, we performed RT–PCR reactions with the forward primer situated at exon 13 and reverse primers located at exons 4, 7, 8 or X. Only the form containing exons 13, 2, 3 and 4 was amplified (Fig. 2D), consistent with the mouse MOR-1K form (43). Again, consistent with the mouse MOR-1K, translation of these new isoforms seems to be initiated at the first AUG in exon 2, as all upstream reading frames code for short peptides. Consequently, both human MOR-1K1 and MOR-1K2 isoforms encode a truncated version of OPRM1 that lacks an extracellular N-terminal domain and transmembrane domain I. To identify the transcriptional start site of MOR-1K isoforms, we performed 5′RACE PCR. The 5′ start site has been mapped to a novel OPMR1 promoter region that is located upstream of exon 13 (Supplementary Material, Fig. S5), unlike the mouse MOR-1K isoform where transcription is initiated from an alternative promoter upstream to exon 11 (43).
