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 s timulation of quiescent NIH 3T3 fibroblasts. To elucidate the mechanism und erlying transcriptional activation during progression through the G(1) phas e of the cell cycle, we have analyzed the activity and phosphorylation patt ern of the nucleolar transcription factor upstream binding factor (UBF). Us ing a combination of tryptic phosphopeptide mapping and site-directed mutag enesis, we have identified Ser484 as a direct target for cyclin-dependent k inase 4 (cdk4)-cyclin D1- and cdk2-cyclin E-directed phosphorylation. Mutat ion of Ser484 impairs rDNA transcription in vivo and in vitro. The data dem onstrate that UBF is regulated in a cell cycle-dependent manner and suggest a link between G(1) cdks-cyclins, UBF phosphorylation and rDNA transcripti on activation.