Characterization of the genomic corticotropin-releasing factor (CRF) gene from Xenopus laevis: two members of the CRF family exist in amphibians.

In mammals, the release of pituitary ACTH is stimulated by CRF. Two related peptides exist in nonmammalian vertebrates, sauvagine from frog skin and urotensin-I from the urophysis of teleost fish. Their related structures (approximately 50%) and capacity to stimulate the release of ACTH from mammalian and fish pituitaries has led to the proposal that sauvagine and urotensin-I are homologs of mammalian CRF. However, sauvagine does not appear to stimulate ACTH release in amphibians, although mammalian CRF (ovine) induces a potent response from amphibian pituitaries. This could indicate that the main function of sauvagine does not involve ACTH regulation and suggests that an additional CRF-like peptide exists in Amphibia. We report here the isolation of two highly homologous CRF-like genes from the frog, Xenopus laevis. Analysis of the expression pattern of these CRF-like genes revealed mRNA in splenic tissue and in the preoptic nucleus and paraventricular organ of the brain. The amino acid sequence of the mature peptide regions (1-41) of both X. laevis genes is strikingly conserved, sharing more than 93% homology with mammalian CRFs, yet only 50% homology with sauvagine. In view of the fact that these new amphibian CRF-like genes share far greater homology with mammalian CRF than that exhibited by sauvagine, we propose that the new Xenopus CRF-like genes are the amphibian counterparts to mammalian CRF. Thus, two members of the CRF family have now been identified in the Amphibia, namely CRF and sauvagine.