Initiation of nucleolar assembly is independent of RNA polymerase I transcription

This report examines the distribution of an RNA polymerase I transcription factor (upstream binding factor; UBF), pre-rRNA processing factors (nucleol in and fibrillarin), and pre-rRNAs throughout mitosis and postmitotic nucle ologenesis in HeLa cells. The results demonstrate that nucleolin, fibrillar in, and pre-rRNAs synthesized at G2/M phase of the previous cell cycle are directly recruited to UBF-associated nucleolar organizer regions (NORs) ear ly in telophase before chromosome decondensation. Unlike the fusion of pren ucleolar bodies to the nucleoli, this early recruitment of processing facto rs and pre-rRNAs is independent of RNA polymerase I transcription. In the a bsence of polymerase I transcription, the initial localization of nucleolin , fibrillarin, and pre-rRNAs to UBF-associated NORs generates segregated mi ninucleoli that are similar to the larger ones observed in interphase cells grown under the same conditions. Pre-rRNAs are juxtaposed to UBF-nucleolin -fibrillarin caps that may represent the segregated nucleoli observed by el ectron microscopy. These findings lead to a revised model of nucleologenesi s. We propose that nucleolar formation at the end of mitosis results from d irect recruitment of processing factors and pre-rRNAs to UBF-associated NOR s before or at the onset of rDNA transcription. This is followed by fusion of prepackaged prenucleolar bodies into the nucleolus. Pre-ribosomal ribonu cleoproteins synthesized in the previous cell cycle may contribute to postm itotic nucleologenesis.