The RNA polymerase I promoter-activating factor CPBF is functionally and immunologically related to the basic helix-loop-helix-zipper DNA-binding protein USF.

Analysis of the core promoter sequence of the mammalian ribosomal RNA genes revealed an E-box-like sequence (CACGCTG) to which the upstream stimulatory factor (USF) binds. Because the core promoter binding factor (CPBF) binds specifically to the core promoter sequence of the ribosomal RNA gene (Liu, Z., and Jacob, S. T. (1994) J. Biol. Chem. 269, 16618-16626) and resembles USF in some respects, we explored the potential relationship between USF and CPBF. Competition electrophoretic mobility shift assay using labeled core promoter probe and several unlabeled competitor oligonucleotides showed that USF can indeed bind to the core promoter and that only those oligonucleotides with an E-box sequence could compete in the promoter-protein complex formation characteristic of CPBF. This complex formation was thermostable, a unique property of USF. Furthermore, antibodies raised against USF cross-reacted with the 44-kDa component of rat CPBF. To prove further the relationship between CPBF and USF, we examined the effects of the unlabeled USF and ribosomal gene core promoter oligonucleotides on polymerase (pol) I and pol II transcription. Both oligonucleotides inhibited rDNA transcription as well as transcription from the adenovirus major late promoter. Only the oligonucleotides that contain the E-box sequence competed in the transcription. These data indicate that the promoter sequence of mammalian ribosomal RNA gene contains an USF-binding site, that the 44-kDa polypeptide of CPBF is related to the 44/43-kDa polypeptide of human USF, and that USF or USF-related protein can transactivate both pol I and pol II promoters.