The nucleolar factor UBF Is phosphorylated by casein kinase II (CKII) at serlne residues within the C-terminal acidic domain which is required for transcriptionactivation. To investigate the biological significance of UBF modification, we have compared the trans-activating properties of cellular UBF and recombinant UBF expressed in Escherichla coll. 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 CKIImedlated phosphorylatlons of UBF contribute to, but are not sufficient for, transcriptlonal 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.
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