Nucleotide sequence of a cloned structural gene coding for a precursor of pancreatic somatostatin.

We have constructed and cloned, in bacteria, recombinant plasmids containing DNA complementary to mRNA coding for a pancreatic pre-prosomatostatin, a product of the cell-free translation of pancreatic islet mRNAs shown previously by immunoprecipitation to be a precursor of somatostatin. A clone containing a nearly full-length cDNA insert of 550 base pairs was identified and appeared to contain the entire coding sequence for the somatostatin precursor in addition to portions of the 5' and 3' untranslated regions. mRNA coding for the pre-prosomatostatin is 600-630 bases long as determined by agarose gel electrophoresis and hybridization with labeled cDNA. Analyses of the nucleotide sequence of the cDNA revealed a protein of 119 amino acid beginning with methionine followed by a typical leader sequence containing 18 hydrophobic amino acids. The tetradecapeptide somatostatin, identical in sequence to mammalian hypothalamic somatostatin, is located at the carboxy terminus followed immediately by a stop codon. An ARg-Lys sequence immediately preceding the sequence of somatostatin is typical of a prohormone cleavage site. A sequence Ala-Pro-Arg-Glu preceding the Arg-Lys cleavage site is identical to that found in porcine prosomatostatin. The evolutionary conservation of the identical amino acid sequence of the somatostatin tetradecapeptide from fish to mammals is remarkable. In addition, similar conservation, in fish and mammals, of the sequence Ala-Pro-Arg-Glu-Arg-Lys preceding the coding region for somatostatin suggets that this particular sequence may have biologic importance in cellular processing of the somatostatin precursor.