m6A-seq on chromatin-assoicated RNA (ChrMeRIP-seq) in mouse embryonic stem cell [ChrMeRIP-seq]

N6-methyladenosines (m6A) are stoichiometrically deposited on exons of nearly one third of RNA Pol II transcriptome, mainly catalysed by METTL3/14 complex. However, neither the intronic methylation pattern and its functional relevance nor the immediate response upon m6A loss have been fully understood. Here we applied MeRIP-seq on mESC nascent transcriptome and thus revealed that approximately 6-10% of m6A peaks occurred at intronic regions, preferentially the conserved and alternative exon/intron part of longer introns, proximately to 5'-splice sites. These intronic m6A deposition correlates with both Rbm15 binding and H3K36me3. Moreover, coupled 4sU-seq with METTL3 dTAG system in mESC, we found that m6A mediates alternative exon/intron inclusive in nascent transcriptome. Intriguingly, m6A self-regulation including writers and readers are early response and partially contributed by alternative splicing changes. Collectively, our study presents a unified model that m6A mediates alternative splicing, and dTAG METTL3 opens an avenue to interrogate the direct response of functional m6A disruption. Overall design: ChrRNA enriches substantial intronic fraction. Isolation of ChrRNA from E14 mESC and then subject to the standard MeRIP-seq to identify the m6A peaks, then classification.