Polymorphism and absence of Leu-enkephalin sequences in proenkephalin genes in Xenopus laevis

The structures of the genes coding for the opioid peptide precursors proopiomelanocortin, proenkephalin (proenkephalin A) and prodynorphin (proenkephalin B), are known for some mammalian species. To gain insight into the evolutionary history of these precursors, we have examined the proenkephalin gene in the South African clawed toad, Xenopus laevis, which diverged from the principal line of vertebrate evolution some 350 Myr ago. The human proenkephalin gene consists of four exons, of which the main exon (exon IV) contains all known biologically active peptides--six Met-enkephalin sequences and one Leu-enkephalin sequence. We report here the primary structures of the putative main exons of two proenkephalin genes in X. laevis, each of which codes for seven Met-enkephalin sequences but no Leu-enkephalin, indicating that Met-enkephalin preceded Leu-enkephalin in the evolution of the proenkephalin gene. The organization of the main exons of the toad genes is remarkably similar to that of the human gene and conserved regions provide evidence for functionally significant structures. We also detect a polymorphism in one of the toad proenkephalin genes, mapping 1.5 kilobases (kb) 5' of the main exon; it is caused by an insertion/deletion of a 1-kb repetitive sequence which has the characteristics of a transposable element.