Observations on the radiation of lobe-finned fishes, ray-finned fishes, and cartilaginous fishes: phylogeny of the opioid/orphanin gene family and the 2R hypothesis.
A large cDNA fragment that codes for proenkephalin (PENK) was cloned from the rough-skinned newt, Taricha granulosa (GenBank Accession: AY817670). This 1299-bp PENK cDNA extends from the poly(A) sequence on the 3' end into the 5'-UTR (221bp) upstream of an open reading frame that codes for 264 amino acids and a stop codon. Within the precursor are five Met-enkephalin sequences and two C-terminally extended forms of Met-enkephalin (YGGFMRGV and YGGFMRY). The organization of the opioid core sequences within the newt PENK closely resembles that reported for other vertebrates. In this urodele amphibian, as in anurans, PENK does not contain the penultimate Leu-enkephalin opioid sequence found in mammals, and instead has in this position Met-enkephalin. PENK cDNA was amplified from newt brain in a RACE PCR targeting the 3' end of the newt delta opioid receptor (DOR). It remains to be determined whether generating the cDNA for the newt PENK while cloning its receptor was serendipitous or the result of a meaningful coincidence between the DOR and PENK sequences.