The helix-loop-helix transcription factor USF (upstream stimulating factor) binds to a regulatory sequence of the human insulin gene enhancer.

Two important sequence elements, designated insulin enhancer binding site 1 (IEB1) or NIR and IEB2 or FAR, are involved in regulating expression of the rat insulin I gene. These elements bind a helix-loop-helix transcription factor, insulin enhancer factor 1 (IEF1). The IEB1 site is highly conserved among insulin genes but the IEB2 site is not conserved. To investigate the factors binding at the equivalent IEB1 and IEB2 sites in the human insulin gene enhancer, electrophoretic mobility shift assays were performed using a variety of cell extracts and probes specific for the homologous IEB1 and IEB2 sites. The results indicate that a factor with similar tissue distribution and binding characteristics to those of IEF1 binds to the IEB1 site in the human insulin gene, but that a separate factor, identified as the adenovirus major late transcription factor [MLTF, or upstream stimulating factor (USF)] binds to the IEB2 site.