| Popis: |
Avian gastrin is a gastric acid secretagogue, but is structurally more closely related to mammalian cholecystokinin, which is functionally distinct from gastrin. This apparently anomalous structure/activity relationship raises important issues for understanding the evolution of regulatory peptides and the mechanisms that control their expression. To clarify the possible mechanisms, we have determined the sequence of a 6.5-kb BamHI genomic DNA fragment that includes the entire avian gastrin transcriptional unit. The complete cDNA sequence, determined by anchored PCR, encoded a precursor of 105 amino acids. Conserved primary amino acid structures were limited to a decapeptide determining biological activity, and essential sites for post-translational processing. Significantly, however, the genomic regulatory regions, and introns, were unlike those of any previously reported gastrin/cholecystokinin gene. The avian gastrin gene contained no recognizable TATA-box motif, a feature unique to this gene family, but had a G+C-rich region immediately upstream of the transcription initiation site, and a Z-DNA purine-pyrimidine repeat sequence. Moreover, physiological regulation of the avian gastrin gene differed markedly from that observed in mammals, in that the important local paracrine downregulation by antral somatostatin was absent; instead, evidence for a hormonal regulation was demonstrated. The data indicate that genomic regulatory elements within regulatory peptide families such as the gastrin/cholecystokinin family, and mechanisms of physiological control, are not conserved during evolution, even though biologically important amino acid sequence information is retained. |
