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. 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.