SERRATE drives phase separation behaviours to regulate m6A modification and miRNA biogenesis.

The methyltransferase complex (MTC) deposits N6-adenosine (mA) onto RNA, whereas the microprocessor produces microRNA. Whether and how these two distinct complexes cross-regulate each other has been poorly studied. Here we report that the MTC subunit B tends to form insoluble condensates with poor activity, with its level monitored by the 20S proteasome. Conversely, the microprocessor component SERRATE (SE) forms liquid-like condensates, which in turn promote the solubility and stability of the MTC subunit B, leading to increased MTC activity. Consistently, the hypomorphic lines expressing SE variants, defective in MTC interaction or liquid-like phase behaviour, exhibit reduced mA levels. Reciprocally, MTC can recruit the microprocessor to the MIRNA loci, prompting co-transcriptional cleavage of primary miRNA substrates. Additionally, primary miRNA substrates carrying mA modifications at their single-stranded basal regions are enriched by mA readers, which retain the microprocessor in the nucleoplasm for continuing processing. This reveals an unappreciated mechanism of phase separation in RNA modification and processing through MTC and microprocessor coordination.