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 required for transcription activation. To investigate the biological significance of UBF modification, we have compared the trans-activating properties of cellular UBF and recombinant UBF expressed in Escherichia coli. 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 which the serine residues within the C-terminal domain were altered by site-directed mutagenesis demonstrates that CKII-mediated phosphorylations of UBF contribute to, but are not sufficient for, transcriptional activation. Besides CKII, other cellular protein kinases phosphorylate UBF at distinct sites in a growth-dependent manner. The marked differences in the tryptic peptide maps of UBF from growing and serum-starved cells suggest that alterations in the degree of UBF phosphorylation may modulate rRNA synthetic activity in response to extracellular signals.
Voit
R
R
Division of Molecular Biology of the Cell II, German Cancer Research Center, Heidelberg.
Kuhn
A
A
Sander
E E
EE
Grummt
I
I
D016475:3T3 Cells; D000595:Amino Acid Sequence; D000818:Animals; D001483:Base Sequence; D047390:Casein Kinase II; D002455:Cell Division; D002466:Cell Nucleolus / Q000378:metabolism; D017931:DNA Primers / Q000235:genetics; D018076:DNA, Complementary / Q000235:genetics; D004275:DNA, Ribosomal / Q000235:genetics*; D004268:DNA-Binding Proteins / Q000235:genetics / Q000378:metabolism*; D051379:Mice; D008969:Molecular Sequence Data; D016297:Mutagenesis, Site-Directed; D010449:Peptide Mapping; D010766:Phosphorylation; D035463:Pol1 Transcription Initiation Complex Proteins; D017346:Protein Serine-Threonine Kinases / Q000378:metabolism; D011994:Recombinant Proteins / Q000235:genetics / Q000378:metabolism; D014157:Transcription Factors / Q000235:genetics / Q000378:metabolism*; D015533:Transcriptional Activation
D017931:DNA Primers; D018076:DNA, Complementary; D004275:DNA, Ribosomal; D004268:DNA-Binding Proteins; D035463:Pol1 Transcription Initiation Complex Proteins; D011994:Recombinant Proteins; D014157:Transcription Factors; C063844:transcription factor UBF; D047390:Casein Kinase II; D017346:Protein Serine-Threonine Kinases