Nucleolar localization of upstream binding factor in HeLa cells depends on rRNA synthesis activities

During Pol I-mediated rRNA synthesis, UBF plays a key role in transcription. Numerous localization studies provided, however, rather divergent results with regard to its presence in either of the two structural nucleolar subcompartments, FC and DFC. These observations suggest different roles of UBF within nucleoli. In order to expand our knowledge in this respect, we have performed the ultrastructural localization of UBF and DNA in HeLa cell nucleoli and complemented it with mapping DNA/RNA hybrids, which testify for the presence of newly synthesized rRNA. With immunogold UBF mapping, the results fell, depending on the type of nucleoli, basically into two categories. In nucleoli exhibiting one, or at most a few, large FC, UBF molecules were mapped mainly into FC. Such nucleoli are known to possess lower activity in rRNA synthesis. In the second category of reticulated nucleoli with many tiny FCs, which are characterized by high rRNA gene transcription, the gold particles were enriched mainly in DFC/FC borders and DFC. The established differential localization of UBF in nucleoli of HeLa cells thus has a functional meaning. It reflects both the level of rRNA synthesis activities and the architectural role of UBF in nucleoli of these cells.