Role of use a helix-loop-helk-zipper dna binding protein, in rRNA gene regulation

Earlier studies from this laboratory demonstrated that the rat ribosomal RNA gene (rDNA) core promoter contains an E box-like sequence to which the core promoter-binding factor (CPBF) binds and that the 44 kDa subunit of this factor is immunologically related to USF. The competition by oligonucleotides containing USF-binding site further confirmed the role of USF, a helix-loop-helix-zipper DNA binding protein in rDNA transcription [Datta, P.K., Ghosh, A.K. and Jacob, S.T. (1995) J. Biol. Chem. 270:86378641J. To prove further the involvement of USF in rDNA regulation in vivo, the effect of USF1 and USF2 overexpression as homodimers and heterodimers was assayed by co-transfection of respective cDNAs and rDNA reporter construct into CHO cells. Primer extension analysis of the RNA synthesized from the reporter construct showed that overexpression of USF1 and USF2 individually resulted in inhibition of rDNA transcription by 8590% whereas overexpression of both the subunits of USF (USF1 and USF2) together activated pol I mediated transcription 3.5 fold. Transfection of a mutant USF2 cDNA devoid of the basic DNA binding domain produced minimal inhibition of rDNA transcription. This effect is E-box specific as overexpression of USF1 and USF2 individually or together did not influence the activity of a non-E box containing promoter such as S V40 promoter. This is the first report that USF can modulate pol I transcription and that it functions as a repressor or activator of pol I transcription in vivo depending upon the dimeric status of USF1 and USF2. This work was supported by an NIH grant CA31894. 'Equal contribution.