A mammalian transcription factor, which, along with other factors, is essential for accurate initiation of transcription from promoters by RNA polymerase II, has been found to regulate the interaction of polymerase and DNA. This factor, designated beta gamma, drastically reduces the affinity of RNA polymerase II for free DNA containing either promoter or nonpromoter sequences. In this respect, beta gamma functions as does the bacterial transcription initiation factor sigma 70, which expedites the binding of Escherichia coli RNA polymerase to promoters in part by accelerating dissociation of the polymerase from nonpromoter sites in DNA.