THE RNA POLYMERASE-I TRANSCRIPTION FACTOR UBF IS THE PRODUCT OF A PRIMARY RESPONSE GENE

Transcription of the ribosomal RNA genes by RNA polymerase I is tightl y coordinated with the rate of cell growth. The RNA polymerase I trans cription factor, UBF, activates transcription by binding to elements w ithin the promoter and enhancer elements within the intergenic spacer but is not required for basal transcription. To assess the role of UBF in modulating ribosomal DNA transcription, we studied its expression in NIH3T6 fibroblasts when transcription was repressed in response to serum starvation and stimulated following refeeding. Our results demon strate a correlation be tween the amounts of UBF protein and the rates of ribosomal DNA transcription in quiescent and serum-stimulated cell s. Nuclear run-on assays and Northern blot analyses demonstrated that the UBF gene was a primary response gene, exhibiting characteristics s imilar to those of c-myc and SRF. These results suggest that the regul ation of transcription of the UBF gene by polymerase II represents a p athway by which cells modulate transcription by RNA polymerase I.