Biosynthesis of rat preprosomatostatin.

The biologically active forms of somatostatin, somatostatin-14 (SS-14) and somatostatin-28 (SS-28) arise by post-translational cleavage of prosomatostatin. Prosomatostatin in turn is derived from a larger precursor, preprosomatostatin. We have previously reported the structure of a complementary DNA molecule encoding rat preprosomatostatin. The nucleotide sequence of this cDNA indicated that SS-14 and SS-28 are located at the carboxy-terminus of a 116 amino acid precursor. At the amino-terminus of the precursor is a hydrophobic region characteristic of a leader or pre-sequence. Sequential Edman degradations of cell-free translation products synthesized in the presence of microsomal membranes indicate that preprosomatostatin is cleaved within the endoplasmic reticulum to form prosomatostatin, a precursor of 92 amino acids. To begin to elucidate the factors which regulate the expression of the rat somatostatin gene, we have determined the sequence of the gene isolated from recombinant bacteriophage libraries. The gene spans 1.2 kilobases in length and is interrupted within the coding sequence of prosomatostatin by a single intron of 630 bases. A variant of the Goldberg-Hogness promotor, TTTAAA, is located 31 bases upstream from the transcriptional start point. A repetitive sequence was identified in the 5' region of the gene within 650 bases of the promoter. The nucleotide sequence of this region reveals an alternating GT sequence 42 bases in length characteristic of DNA with Z-forming potential. Such sequences are thought to influence the expression of other eukaryotic genes.