An atlas of single-base-resolution N6-methyl-adenine methylomes redefines RNA demethylase function as suppressors of disruptive RNA methylation

Various methylases and demethylases catalyze methylation and demethylation of N6-methyladenosine (m6A) and N6,2?-O-dimethyladenosine (m6Am) but precise methylomes uniquely mediated by each methylase/demethylase are still lacking. Here, we developed m6A-Crosslinking-Exonuclease-sequencing (m6ACE-seq) to map m6A and m6Am at transcriptome-wide single-base-resolution. m6ACE-seq's ability to quantify relative differences in methylation levels across samples enabled the generation of a comprehensive atlas of distinct methylomes uniquely mediated by every individual known methylase/demethylase. We determined METTL16 to indirectly impact manifold methylation targets beyond its consensus target motif, and highlighted the importance of precision in mapping PCIF1-dependent m6Am. Rather than reverse RNA methylation, we found that both ALKBH5 and FTO demethylases instead maintain their regulated sites in an unmethylated steady-state. In FTO's absence, anomalous m6Am disrupts snRNA interaction with nuclear export machinery, potentially causing aberrant pre-mRNA splicing events. We propose a model whereby RNA demethylases ensure normal RNA metabolism by suppressing disruptive RNA methylation in the nucleus. UV crosslinking, immunoprecipitation and exonuclease treatment with anti-m6A antibodies was used to map m6A/m6Am in transcriptome-wide at single-base-resolution.