Differential binding of the chicken ovalbumin upstream promoter (COUP) transcription factor to two different promoters.

The COUP (chicken ovalbumin upstream promoter) transcription factor binds to the upstream promoters of both the chicken ovalbumin and the rat insulin II genes, even though the two binding sites have no obvious sequence similarity (Hwung, Y.-P., Crowe, D. T., Wang, L.-H., Tsai, S. Y., and Tsai, M.-J. (1988) Mol. Cell. Biol. 8, 2070-2077). Comparison of the contact points and important nucleotides in the two binding sites suggests that the COUP transcription factor binds to them in different ways. The purine contacts in the two promoters are limited to one face of the DNA helix; however, studies on phosphate contacts suggest that the COUP transcription factor wraps around the ovalbumin promoter, while it binds to only one face of the DNA helix in the insulin promoter. In addition, the binding factor makes a more extended contact on the insulin promoter as compared to the ovalbumin promoter. The potential significance of this novel finding is discussed.