ACTIVATION OF MAMMALIAN RIBOSOMAL GENE-TRANSCRIPTION REQUIRES PHOSPHORYLATION OF THE NUCLEOLAR TRANSCRIPTION FACTOR UBF

The nucleolar factor UBF is phosphorylated by casein kinase II (CKII) at serine residues within the C-terminal acidic domain which is requir ed for transcription activation. To investigate the biological signifi cance of UBF modification, we have compared the trans-activating prope rties of cellular UBF and recombinant UBF expressed in Escherichia col i, Using a variety of assays we demonstrate that unphosphorylated UBF is transcriptionally inactive and has to be phosphorylated at multiple sites to stimulate transcription. Examination of cDNA mutants in whic h the serine residues within the C-terminal domain were altered by sit e-directed mutagenesis demonstrates that CKII-mediated phosphorylation s of UBF contribute to, but are not sufficient for, transcriptional ac tivation. Besides CKII, other cellular protein kinases phosphorylate U BF at distinct sites in a growth-dependent manner. The marked differen ces in the tryptic peptide maps of UBF from growing and serum-starved cells suggest that alterations in the degree of UBF phosphorylation ma y modulate rRNA synthetic activity in response to extracellular signal s.