Reciprocal regulation of m A modification and miRNA production machineries via phase separation-dependent and -independent mechanisms.

Methyltransferase complex (MTC) deposits 6-adenosine (m A) onto RNA, whereas microprocessor produces miRNA. Whether and how these two distinct complexes cross-regulate each other has been poorly studied. Here we report that the MTC subunit B (MTB) tends to form insoluble condensates with poor activity, with its level monitored by 20S proteasome. Conversely, the microprocessor component SERRATE (SE) forms liquid-like condensates, which in turn promotes solubility and stability of MTB, leading to increased MTC activity. Consistently, the hypomorphic lines expressing SE variants, defective in MTC interaction or liquid-like phase behavior, exhibit reduced m A level. Reciprocally, MTC can recruit microprocessor to loci, prompting co-transcriptional cleavage of primary miRNA (pri-miRNAs) substrates. Additionally, pri-miRNAs carrying m A modifications at their single-stranded basal regions are enriched by m A readers, which retain 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.