Figure 3.

Synthetic NORs’ contribution to the understanding of the nucleolar cycle. The left panel summarizes the findings that resulted from the construction of synthetic NORs. Pseudo-NORs, UBF-binding site arrays, demonstrate that UBF seeds the formation of 2° constrictions during mitosis and FCs in interphase (top). Neo-NORs, arrays of UBF-binding sites interspersed with rDNA transcription units, indicate that pre-rRNAs are the only architectural requirement for DFC and GC formation (bottom). While UBF depletion has revealed that mitotic bookmarking is necessary for nucleolar formation, pseudo-NORs establish that it is not sufficient. Thus, the cell cycle inheritance of nucleoli is a staged process (represented in the right panel). Endogenous NORs that were active in the previous interphase are bookmarked by UBF, forming mitotic 2° constrictions that retain most of the FC components. This UBF-dependent bookmarking ensures the reactivation of rDNA transcription in late telophase, with pre-rRNAs seeding the recruitment of DFC and GC factors. Hence, staging establishes a temporal order to nucleolar compartmentalization and ensures the rapid re-formation of nucleoli in early G1. For diagrammatic simplicity, individual chromosomes are merged.