Menant, A. | bioRxiv Science

bioRxiv2024

A bifunctional snoRNA with separable activities in guiding rRNA 2'-O-methylation and scaffolding gametogenesis effectors

Small nucleolar RNAs are non-coding transcripts that guide chemical modifications of RNA substrates and modulate gene expression at the epigenetic and post-transcriptional levels. However, the extent of their regulatory potential and the underlying molecular mechanisms remain poorly understood. Here, we identify a conserved, previously unannotated intronic C/D-box snoRNA, termed snR107, hosted in the fission yeast long non-coding RNA mamRNA and carrying two independent cellular functions. On the one hand, snR107 guides site-specific 25S rRNA 2-O-methylation and promotes pre-rRNA processing and 60S subunit biogenesis. On the other hand, snR107 associates with the gametogenic RNA-binding proteins Mmi1 and Mei2, mediating their reciprocal inhibition and restricting meiotic gene expression during sexual differentiation. Both functions require distinct cis-motifs within snR107, including a conserved 2-O-methylation guiding sequence. Together, our results position snR107 as a dual regulator of rRNA modification and gametogenesis effectors, expanding our vision on the non-canonical functions exerted by snoRNAs in cell fate decisions.

molecular biology↗

bioRxiv2023

A dual, catalytic role for the fission yeast Ccr4-Not complex in gene silencing and heterochromatin spreading

AbstractHeterochromatic gene silencing relies on combinatorial control by specific histone modifications, the occurrence of transcription, and/or RNA degradation. Once nucleated, heterochromatin propagates within defined chromosomal regions and is maintained throughout cell divisions to warrant proper genome expression and integrity. The fission yeast Ccr4-Not complex has been involved in gene silencing, but its relative contribution to distinct heterochromatin domains and its role in nucleation versus spreading have remained elusive. Here, we unveil major functions for Ccr4-Not in silencing and heterochromatin spreading at the mating type locus and subtelomeres. Mutations of the catalytic subunits Caf1 or Mot2, involved in RNA deadenylation and protein ubiquitinylation respectively, result in impaired propagation of H3K9me3 and massive accumulation of nucleation-distal heterochromatic transcripts. Both silencing and spreading defects are suppressed upon disruption of the heterochromatin antagonizing factor Epe1. Overall, our results position the Ccr4-Not complex as a critical, dual regulator of heterochromatic gene silencing and spreading. Author SummaryEukaryotic genomes are partitioned into relaxed, gene-rich regions, and condensed, gene-poor domains called heterochromatin. The maintenance of heterochromatin is crucial for proper genome expression and integrity, and requires multiple factors regulating histone modifications and/or the levels of RNA molecules produced from these regions. Such effectors not only promote heterochromatin assembly but also ensure its propagation from specific nucleation sites to defined domain boundaries. However, while the mechanisms involved in initiation of heterochromatin formation have been well documented, the molecular and biochemical properties underlying its spreading remain largely elusive. By combining genetic and single-cell approaches, we report here that the fission yeast Ccr4-Not complex, a multisubunit complex conserved throughout eukaryotes, is essential for efficient heterochromatin spreading to repress expression of nucleation-distal RNAs. The two catalytic activities of the complex, RNA deadenylation and protein ubiquitinylation, are each critical, thereby defining a dual enzymatic requirement in the process.

genetics↗