Biosynthesis of caerulein in the skin of Xenopus laevis: partial sequences of precursors as deduced from cDNA clones.

The decapeptide caerulein represents one of the main constituents of the skin secretion of Xenopus laevis. Total mRNA was isolated from skin, transcribed into cDNA and inserted via GC-tailing into the plasmid pUC8. Among the transformants, 300 clones were selected at random and screened with a cDNA primed with the synthetic deoxynucleotide d(AGTCCATCCA), which is complementary to the mRNA region coding for the fragment Trp-Met-Asp-Phe of cerulein. Of nine strongly hybridizing clones, three were sequenced and these were found to contain inserts with very similar nucleotide sequences. The cloned cDNAs code for parts of two different caerulein precursors. These contain one or two copies of caerulein and five additional amino acids located between pairs of arginine residues. The extra glycine at the carboxy terminus is considered to serve as the signal for amidation, while the tetrapeptide Phe-Ala-Asp-Gly, linked to the amino end of caerulein in these precursors, must be cleaved by an unusual processing mechanism.