Role of ribosomal RNA processing in the multiphase nucleolus

Abstract

The nucleolus is a multiphase nuclear condensate and the site of ribosome production in the cell. After ribosomal RNA (rRNA) is transcribed in the innermost phase of the nucleolus, it moves to the outer phases where it undergoes cleavage, modification, and folding events. This directional rRNA movement is critical to the export of correctly processed ribosomes to the cytoplasm. However, it is unclear how this directionality is established. Here, I provide evidence for my hypothesis that specific rRNA processing events (cleavage, modification, and folding) are required for directional rRNA movement through the nucleolar phases. Testing this hypothesis has previously been challenging due to a lack of tools capable of resolving where and when rRNA processing events occur in each nucleolar phase. Therefore, I co-develop* a genomic-imaging platform to map the cleavage, modification, and folding state of nascent rRNAs in time and space with base-pair resolution. Using this novel method, I* create a map of the rRNA processing steps occurring in each nucleolar phase. I* then systematically perturb specific rRNA processing steps and show their necessity to directional rRNA movement. Based on these findings, I propose a biological model for how rRNA processing enables directional rRNA movement and export of only correctly processed ribosomes to the cytoplasm. This work 1) provides novel mechanistic insight into how directional rRNA movement is maintained in the nucleolus and 2) establishes a novel method for studying RNA kinetics in the nucleolus and other cellular condensates in normal, perturbed, and diseased systems. *In collaboration with Sofia Quinodoz, Lifei Jiang, and Troy Comi.