Understanding the Relationship Between the Size and Transcriptional Output of the Nucleolus

Abstract

Cellular organization can be derived from organelles, including cell walls, membranes and nuclei. It was long believed that cellular organization could only be established by a membrane and that the cytoplasm was completely homogeneous. However, both the cytoplasm and the nucleoplasm of cells contain regions with higher local concentrations of biomolecules that are in constant flux with their environment. These regions are referred to as non-membrane bound organelles, and typically contain both RNA and protein. The nucleolus, site of rRNA transcription and processing in the nucleus, is the most studied non-membrane bound organelle. Both the size of the nucleolus and the rate at which the nucleolus transcribes rRNA have been positively correlated with an increase in cell size. However, the relationship between these two markers for growth has not been quantified for individual nucleoli. Here, we use a fluorescence in situ hybridization approach (oligoFISH) to target the internal transcribed spacer (ITS) regions of rRNA in C. elegans to quantify the steady state amount of nascent rRNA. We compare steady state rRNA levels to the amount of fibrillarin1, a small nucleolar ribonucleoprotein (snoRNP) used as a marker for nucleolar size. Our results suggest that, within a given worm, the relationship between the amount of nascent rRNA and the size of the nucleolus is linear. However, we find that under conditions that perturb cell growth and nucleolar size the ratio of nascent rRNA to nucleolar size is not universal. Unexpectedly, under both enhanced and diminished growth conditions, the ratio of rRNA is significantly different. We analyze these results in the context of a simple steady-state model, and suggest future experiments.