Abstract
Ribosomal transcription constitutes the major energy consuming process in cells and is regulated in response to proliferation, differentiation and metabolic conditions by several signalling pathways. These act on the transcription machinery but also on chromatin factors and ncRNA. The many ribosomal gene repeats are organised in a number of different chromatin states; active, poised, pseudosilent and repressed gene repeats. Some of these chromatin states are unique to the 47rRNA gene repeat and do not occur at other locations in the genome, such as the active state organised with the HMG protein UBF whereas other chromatin state are nucleosomal, harbouring both active and inactive histone marks. The number of repeats in a certain state varies on developmental stage and cell type; embryonic cells have more rRNA gene repeats organised in an open chromatin state, which is replaced by heterochromatin during differentiation, establishing different states depending on cell type. The 47S rRNA gene transcription is regulated in different ways depending on stimulus and chromatin state of individual gene repeats. This review will discuss the present knowledge about factors involved, such as chromatin remodelling factors NuRD, NoRC, CSB, B-WICH, histone modifying enzymes and histone chaperones, in altering gene expression and switching chromatin states in proliferation, differentiation, metabolic changes and stress responses.
Highlights
Ribosomal genes, both the nucleolar 47S genes and the 5S rRNA genes, are highly transcribed and contribute to the major part of all transcripts
Gene repeats organised with upstream binding factor (UBF) [53]. This suggest that the decondenced chromatin state organised with UBF forms the basis for transcription, as the competent state [53,55] (Figure 1B, panel 1), which is activated by specific transcription factors and by post-translational modification (PMT) of general factors
In addition to eNOSC, NuRD establishes a repressed chromatin state in glucose starved cells, and we propose that
Summary
Both the nucleolar 47S genes and the 5S rRNA genes, are highly transcribed and contribute to the major part of all transcripts. The nucleolus constitutes a hub for responses, the 47S rRNA transcription is regulated by environmental states and stress on several levels; modification of the RNA pol I machinery and the chromatin landscape as well as altering nucleolar morphology and structure to alter the expression level. UBF establishes an open chromatin state specific to the active 47S gene repeats [42] and is associated with GC-rich DNA at approximately every 170 bp in the gene body [37]. Upstream of the spacer promoter is the boundary element, binding CTCF and cohesin, separating the nucleosomal IGS from the active UBF associated gene repeat [35,37,47,48,49]. The organisation of gene repeats with UBF is not sufficient for transcription, some of the active repeats are not engaged in transcription [35,37], and other factors, such as transcription factors, may be necessary for proper PIC formation to induce transcription
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Free) 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
