The METHYLTRANSFERASE B-SERRATE interaction mediates the reciprocal regulation of microRNA biogenesis and RNA mA modification.

In eukaryotes, RNA N-methyladenosine (mA) modification and microRNA (miRNA)-mediated RNA silencing represent two critical epigenetic regulatory mechanisms. The mA methyltransferase complex (MTC) and the microprocessor complex both undergo liquid-liquid phase separation to form nuclear membraneless organelles. Although mA methyltransferase has been shown to positively regulate miRNA biogenesis, a mechanism of reciprocal regulation between the MTC and the microprocessor complex has remained elusive. Here, we demonstrate that the MTC and the microprocessor complex associate with each other through the METHYLTRANSFERASE B (MTB)-SERRATE (SE) interacting module. Knockdown of MTB impaired miRNA biogenesis by diminishing microprocessor complex binding to primary miRNAs (pri-miRNAs) and their respective MIRNA loci. Additionally, loss of SE function led to disruptions in transcriptome-wide mA modification. Further biochemical assays and fluorescence recovery after photobleaching (FRAP) assay indicated that SE enhances the liquid-liquid phase separation and solubility of the MTC. Moreover, the MTC exhibited enhanced retention on chromatin and diminished binding to its RNA substrates in the se mutant background. Collectively, our results reveal the substantial regulatory interplay between RNA mA modification and miRNA biogenesis.