The relative rDNA content of a NOR determines its level of expression and its probability of becoming active. A sequential silver staining and in-situ hybridization study.

Silver staining was used to estimate the expression of nucleolar organizing regions (NORs), and in-situ hybridization (ISH) with rDNA probes was used to estimate the relative content of rDNA in each NOR in chromosome preparations of the dormouse, Eliomys quercinus, a species with two NOR-bearing chromosome pairs. Both types of signals were sequentially investigated on every NOR by using an Ag-ISH sequential staining method, which made it possible to demonstrate that the relative amount of rDNA in a NOR in comparison with the other chromosomes of the complement determines its level of expression and its likelihood of becoming active, regardless of whether the NORs are homologous or not. We suggest that the NORs in each cell are activated in accordance with an established hierarchy. We propose a two-stage model to relate NOR structure and function, which is consistent with these results and with current knowledge on the molecular regulation of NOR transcription.