Differential phosphorylation and localization of the transcription factor UBF in vivo in response to serum deprivation. In vitro dephosphorylation of UBF reduces its transactivation properties

We have analyzed the expression, phosphorylation, and localization of the ribosomal DNA transcription factors UBF1 and UBF2 in Chinese hamster ovary cells in response to serum deprivation. In vivo labeling experiments demonstrate that UBF1 and UBF2 are phosphoproteins. Phosphoamino acid analysis of the in vivo labeled proteins demonstrate that UBF is phosphorylated on serine residues. Following serum deprivation there is no alteration in the cellular levels of UBF1 and UBF2 as determined by Western blotting, but there is an 80% reduction in the level of phosphorylation of UBF compared with logarithmically growing cells. Following serum deprivation there is a redistribution of UBF between the nucleolus, the nucleus, and the cytoplasm. Phosphatase-treated UBF demonstrated a reduced ability to rescue transcription by RNA polymerase I from the rDNA spacer promoter in vitro. These findings suggest that phosphorylation of UBF is a prerequisite for transactivation of RNA polymerase I.