Feedback control via H3K9 methylation and siRNA biogenesis autonomously maintains RNAi heterochromatin through continuous low-level transcription

In fission yeast (Schizosaccharomyces pombe), pericentromeric repeat transcription is rare, heterogeneous, and maintained by feedback where small RNA biogenesis and H3K9 methylation sustain RNAi heterochromatin.

• Why it matters: RNAi recruitment must balance antagonistic transcription and silencing, but existing models rely on cell-cycle confinement that may not explain how heterochromatin persists.
• What they did: Using strand-specific smFISH, the study measured forward and reverse pericentromeric transcripts at single-cell resolution, then combined experiments with mathematical modeling of feedback through H3K9 methylation and siRNA biogenesis.
• The result: Low levels of H3K9me or siRNAs prevent heterochromatin establishment, while high levels of H3K9me cause stochastic switching, implying feedback strength tunes transcriptional regimes without external inputs.

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