Transposon targeting non-coding RNA transcription targets G/C-rich tracts and is facilitated by an intrinsically disordered protein in Tetrahymena

Tetrahymena is a well-established ciliate model organism, known for its nuclear dualism and unique mechanism for transposon defense, wherein transposons are kept within the highly heterochromatinized micronucleus (MIC), which is transcriptionally inert except during meiosis. The discovery of preferential transcription in the meiotic MIC at regions enriched with transposons, and the identification of MIC transcription regulatory proteins, emphasizes the necessity to elucidate the underlying mechanisms driving this process. In this study, we demonstrate that G/C-rich tracts are overrepresented in meiotic non-coding RNA (ncRNA) transcription regions and that RNA polymerase II (Pol II) is highly enriched at and around these tracts. Further analysis revealed that Pol IIs association with G/C-rich tracts is not abolished in cells lacking Rib1, a Mediator complex-associated protein essential for MIC ncRNA biogenesis. Nevertheless, in the absence of Rib1, Pol II showed abnormal association with the meiotic MIC chromatin, suggesting that Rib1 is critical for maintaining Pol IIs binding specificity. Through Rib1 truncation analysis, we found that the intrinsically disordered region, which contains putative phase-separating peptides, is crucial for its function. Disruption of phase-separation, either by deleting these peptides or by treating cells with a phase-separation disruption reagent, leads to aberrant localization of both Rib1 and Pol II on the MIC chromatin, implicating that Rib1 likely facilitates the MIC ncRNA transcription via phase-separation.