The dual role of helix-loop--helix-zipper protein USF in ribosomal RNA gene transcription in vivo.
We have previously demonstrated that the core promoter of rat ribosomal RNA gene (rDNA) contains an E-box-like sequence to which the core promoter binding factor CPBF binds and that the 44 kDa subunit of this protein is immunologically related to USF1, the helix--loop--helix-zipper DNA binding protein. Further, we showed that RNA polymerase I (pol I) transcription in vitro is competed by oligonucleotides containing USF-binding site, which suggested a key role for USF in rDNA transcription. To prove the potential role of USF in pol I transcription in vivo, USF1 and USF2 homodimers and USF1/USF2 heterodimer were overexpressed in CHO cells by transfection of the respective cDNAs. Co-transfection of a plasmid containing rDNA followed by primer extension analysis showed that overexpression of USF1 and USF2 as homodimers resulted in inhibition of rDNA transcription by as much as 85-90% whereas overexpression of USF1/USF2 in the heterodimeric form activated transcription approximately 3.5-fold. Transfection of mutant USF2 cDNA that is devoid of the basic DNA-binding domain produced only minimal inhibition of rDNA transcription. These data show that USF can modulate transcription of rRNA gene in vivo by functioning as a repressor (homodimer) or activator (heterodimer) of pol I transcription in vivo and suggest that inhibition of rDNA transcription may be responsible for the antiproliferative action of USF homodimers.
Ghosh
A K
AK
Department of Pharmacology and Molecular Biology, The Chicago Medical School, North Chicago, Illinois 60064, USA.
Datta
P K
PK
Jacob
S T
ST
D000818:Animals; D016466:CHO Cells; D006224:Cricetinae; D004275:DNA, Ribosomal / Q000096:biosynthesis* / Q000378:metabolism*; D004268:DNA-Binding Proteins; D019281:Dimerization; D017930:Genes, Reporter; D018257:Helix-Loop-Helix Motifs; D051381:Rats; D011993:Recombinant Fusion Proteins / Q000096:biosynthesis / Q000378:metabolism; D014157:Transcription Factors / Q000096:biosynthesis / Q000737:chemistry / Q000378:metabolism*; D014158:Transcription, Genetic; D014162:Transfection; D051720:Upstream Stimulatory Factors; D001616:beta-Galactosidase / Q000096:biosynthesis
D004275:DNA, Ribosomal; D004268:DNA-Binding Proteins; D011993:Recombinant Fusion Proteins; D014157:Transcription Factors; D051720:Upstream Stimulatory Factors; C497182:Usf1 protein, rat; C497187:Usf2 protein, rat; D001616:beta-Galactosidase