Bidirectional activity of the rat insulin II 5'-flanking region in transgenic mice.

We have identified a new transcription initiation site in the 5'-flanking regulatory region of the rat insulin II gene. This site is located on the opposite strand with respect to the insulin gene promoter, upstream of the insulin gene transcriptional enhancer. The cell-specific activity of this reverse promoter element is demonstrated in two lineages of transgenic mice, in which it directs expression of simian virus 40 T antigen specifically to the beta cells of the endocrine pancreas, resulting in development of pancreatic tumors. Analysis of RNA from the tumor cells demonstrates bidirectional transcription from the insulin regulatory region of the transgene. These data raise the possibility that bidirectional activity is a quality of the regulatory region of the insulin gene in its natural genomic context.