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 (nucleolin and fibrillarin), and pre-rRNAs throughout mitosis and postmitotic nucleologenesis in HeLa cells. The results demonstrate that nucleolin, fibrillarin, and pre-rRNAs synthesized at G2/M phase of the previous cell cycle are directly recruited to UBF-associated nucleolar organizer regions (NORs) early in telophase before chromosome decondensation. Unlike the fusion of prenucleolar bodies to the nucleoli, this early recruitment of processing factors and pre-rRNAs is independent of RNA polymerase I transcription. In the absence of polymerase I transcription, the initial localization of nucleolin, fibrillarin, and pre-rRNAs to UBF-associated NORs generates segregated mininucleoli 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 electron microscopy. These findings lead to a revised model of nucleologenesis. We propose that nucleolar formation at the end of mitosis results from direct recruitment of processing factors and pre-rRNAs to UBF-associated NORs before or at the onset of rDNA transcription. This is followed by fusion of prepackaged prenucleolar bodies into the nucleolus. Pre-ribosomal ribonucleoproteins synthesized in the previous cell cycle may contribute to postmitotic nucleologenesis.