Steroid-dependent interaction of transcription factors with the inducible promoter of mouse mammary tumor virus in vivo.

We have used exonuclease protection in vivo as an assay to detect interaction of nuclear factors with the steroid-inducible promoter of mouse mammary tumor virus. Binding of two factors is detected uniquely at the steroid-activated promoter, and results in protection of sequences between -82 and approximately +12 One factor is identified as the murine homolog of nuclear factor 1. The second (designated factor i) binds downstream of nuclear factor 1 and protects sequences extending over the cap site. Binding activities associated with both factors can be detected in crude nuclear extracts; their apparent concentrations are unaffected by hormone treatment of the cells. These results demonstrate that glucocorticoid induction of transcription results from receptor-mediated establishment of a transcription factor complex at the promoter rather than activation of a preexisting complex.