Transcriptional activity of domain A of the rat glucagon G3 element conferred by an islet-specific nuclear protein that also binds to similar pancreatic islet cell-specific enhancer sequences (PISCES).

A pancreatic islet cell-specific enhancer element in the rat glucagon gene, Glu-G3, contains two domains, one of which, domain A, has been shown to be necessary for Glu-G3 activity. In the present study, the functions of the isolated domain A of Glu-G3 were investigated by using transient reporter fusion gene expression and DNA binding assays. A single copy of domain A was transcriptionally inactive in glucagon-producing islet cell lines, whereas it did confer activity when combined with domain B, suggesting that Glu-G3 may be a bipartite element. Multiple copies of domain A did function independently as transcriptional enhancer in phenotypically distinct islet cell lines but not in several nonislet cell lines. Sequences (PISCES, pancreatic islet cell-specific enhancer sequences), similar to that of domain A of Glu-G3 and present in cell-specific control elements of the rat insulin I (Ins-E1) and rat somatostatin genes (SMS-UE), are shown to be required for transcriptional activity of these elements. In addition, a protein was detected in islet cell lines that bound to the PISCES motifs within Glu-G3, Ins-E1, and SMS-UE. These results support the view that cell-specific control elements of the glucagon, insulin, and somatostatin genes share a functional regulatory sequence, PISCES, and provide direct evidence for the existence of an islet-specific, PISCES-binding transcription factor or closely related proteins being involved in the coordinate expression of islet hormone genes.