Sequence-specific interactions of nuclear factors with the insulin gene enhancer

Insulin 5'-flanking DNA contains two elements controlling cell-specific expression, one a cell-specific enhancer. We show that factors in nuclear extracts derived from an insulin-secreting cell line interact with three distinct regions within the insulin enhancer. The 5' border of the farthest upstream protected region coincides with the 5' border of the enhancer element, indicating a functional role for this interaction in insulin gene transcription. This protected region covers 46 bp, including an enhancer core sequence. This region was not protected in nuclear extracts prepared identically from two heterologous cell lines, indicating at least a 3-fold lower level of interacting factors in these cells. The size of this protected region suggests that more than one factor is binding, possibly facilitating cell-specific interaction of a binding protein with the core enhancer nucleotides. The other two protected regions were observed in extracts prepared from one or both of the heterologous cells.