rRNA intermediates associate with nucleolar reshaping in C. elegans

The nucleolus is a multilayered organelle essential for ribosome biogenesis, yet how ribosomal RNA (rRNA) events coordinate its organization remains unclear. We previously reported that the accumulation of 27SA2 pre-rRNA upon class I RPLs (large subunit ribosomal proteins) depletion accompanies the formation of nucleolar rings and vacuoles in Caenorhabditis elegans. Here, we systematically investigated the effects of pre-18S rRNA processing and rRNA transcription on nucleolar structure and reported that defects in these molecular events result in reorganization of the nucleolar architecture. Depleting class I RPSs (small subunit ribosomal proteins) induced a concentric three-layered nucleolar architecture, while inhibiting rRNA transcription induced another three-layered sandwich-like nucleolus. During nucleolar reshaping, nucleolar proteins redistributed into distinct subnucleolar compartments. Tracking the subcellular localization of ribosomal DNA (rDNA) via a lacI::tagRFP/lacO system revealed that rDNA also redistributed upon nucleolar reorganization. We identified Ce.22SB pre-rRNA accumulation upon class I RPSs depletion as a key intermediate associated with concentric-spherical nucleoli. Moreover, the conserved nucleolar protein NUCL-1/nucleolin, FIB-1/fibrillarin, and LPD-6/PPAN are essential for nucleolar architecture reorganization and regulates the development of nematodes during impaired rRNA production. Together, these findings demonstrate that nucleolar architecture is associated with the status of rRNA biogenesis and that the accumulation of specific rRNA intermediates correlates with the emergence of distinct nucleolar architectures.