Abstract
The nucleolus is the ribosome biogenesis center. The nucleolar structure is disrupted upon entry into mitosis and is formed in early G1 phase. To understand the molecular mechanisms of nucleolar assembly and disassembly, we have studied the mechanism of association between factors involved in pre-ribosome RNA (rRNA) processing and rRNA gene chromatin (r-chromatin). We found that the pre-rRNA transcription-processing linking factor Nopp140 and pre-rRNA processing factors such as DKC1 and fibrillarin (FBL) associate with r-chromatin during interphase, while Nopp140, DKC1, and FBL were released from r-chromatin in mitosis. The association of these factors with r-chromatin was found to be restored independent of pre-rRNA transcription in early G1 phase, but a mature nucleolar structure was not formed, suggesting that nucleolar assembly can be divided into at least two steps with respect to pre-rRNA transcription. Moreover, we found that the r-chromatin association of Nopp140, DKC1, and FBL was dependent on the transcription factor upstream binding factor (UBF). However, we demonstrated that UBF alone was not sufficient to recruit these pre-rRNA processing factors to r-chromatin. Thus, UBF is necessary but not sufficient for the associations between pre-rRNA processing factors and r-chromatin.