Phosphorylation by G 1 ‐specific cdk–cyclin complexes activates the nucleolar transcription factor UBF

Transcription of rRNA genes by RNA polymerase I increases following serum stimulation of quiescent NIH 3T3 fibroblasts. To elucidate the mechanism underlying transcriptional activation during progression through the G 1 phase of the cell cycle, we have analyzed the activity and phosphorylation pattern of the nucleolar transcription factor upstream binding factor (UBF). Using a combination of tryptic phosphopeptide mapping and site‐directed mutagenesis, we have identified Ser484 as a direct target for cyclin‐dependent kinase 4 (cdk4)–cyclin D1‐ and cdk2–cyclin E‐directed phosphorylation. Mutation of Ser484 impairs rDNA transcription in vivo and in vitro . The data demonstrate that UBF is regulated in a cell cycle‐dependent manner and suggest a link between G 1 cdks–cyclins, UBF phosphorylation and rDNA transcription activation.