rRNA intermediates coordinate the multilayered nucleolar phase transition in C. elegans

The nucleolus is the most prominent membraneless organelle within the nucleus and plays essential roles in rRNA transcription and processing and ribosome assembly. How the structure of the nucleolus is maintained and regulated is poorly understood. Here, we identified two types of nucleoli in C. elegans. Type I nucleoli are spherical, and rRNA transcription and processing factors are evenly distributed throughout the nucleolus. In type II nucleoli, rRNA transcription and processing factors exclusively accumulate in the periphery rim, which is named the nucleolar ring. The hollow vacuole inside the nucleolar ring contains proteins that usually localize in the nucleoplasm but are capable of exchanging contents across the ring. The high-order structure of the nucleolus is dynamically regulated in C. elegans. Faithful rRNA processing is important to maintain the spherical structure of the nucleoli. The depletion of a class of rRNA processing factors, for example, class I ribosomal proteins of the large subunit (RPL), which are involved in 27SA2 rRNA processing, reshaped spherical nucleoli to a ring-shaped nucleolar structure. The inhibition of RNAP I transcription and depletion of two conserved nucleolar factors, nucleolin and fibrillarin, prohibits the formation of the nucleolar ring. We concluded that the integrity of nucleoli is highly dependent on rRNA processing and maturation, which may provide a mechanism to coordinate structure maintenance and gene expression.