Mosaic evolution of prepropancreatic polypeptide. II. Structural conservation and divergence in pancreatic polypeptide gene

We demonstrated that nucleotide and amino acid sequences in the carboxyl-terminal regions of rat, mouse, and human prepropancreatic polypeptide exhibit a high degree of divergence, whereas the amino-terminal domains are highly conserved. To understand the molecular basis of this divergence and conservation, we determined the nucleotide sequence of the rat pancreatic polypeptide gene from an islet genomic library and compared it with that of the human gene. Exon 2 of the rat gene encodes the signal peptide and pancreatic polypeptide, exon 3 encodes the carboxyl-terminal region, and exons 1 and 4 encode the 5'- and 3'-untranslated regions of the mRNA, respectively. Exons 1 and 2 of rat and human genes are well conserved. The rat and human genes, however, have exons 3 and 4 of different lengths and heterologous nucleotide sequences. Mutational accumulation in exons 3 and 4 and intron 3 of the rat gene appears to have caused splice junction sliding and translational frameshift, resulting in a structural divergence in the carboxyl-terminal region. Available evidence indicates that the mosaicism of structural conservation and divergence in pancreatic polypeptide genes may have been caused by a difference in the evolutionary rates of the genomic regions.