m 6 A RNA Degradation Products Are Catabolized by an Evolutionarily Conserved N 6 -Methyl-AMP Deaminase in Plant and Mammalian Cells.

N6 -methylated adenine (m6 A) is the most frequent posttranscriptional modification in eukaryotic mRNA. Turnover of RNA generates N6 -methylated AMP (N6 -mAMP), which has an unclear metabolic fate. We show that Arabidopsis thaliana and human cells require an N6 -mAMP deaminase (ADAL, renamed MAPDA) to catabolize N6 -mAMP to inosine monophosphate in vivo by hydrolytically removing the aminomethyl group. A phylogenetic, structural, and biochemical analysis revealed that many fungi partially or fully lack MAPDA, which coincides with a minor role of N6 A-RNA methylation in these organisms. MAPDA likely protects RNA from m6 A misincorporation. This is required because eukaryotic RNA polymerase can use N6 -mATP as a substrate. Upon abrogation of MAPDA , root growth is slightly reduced, and the N6 -methyladenosine, N6 -mAMP, and N6 -mATP concentrations are increased in Arabidopsis. Although this will potentially lead to m6 A misincorporation into RNA, we show that the frequency is too low to be reliably detected in vivo. Since N6 -mAMP was severalfold more abundant than N6 -mATP in MAPDA mutants, we speculate that additional molecular filters suppress the generation of N6 -mATP. Enzyme kinetic data indicate that adenylate kinases represent such filters being highly selective for AMP versus N6 -mAMP phosphorylation. We conclude that a multilayer molecular protection system is in place preventing N6 -mAMP accumulation and salvage.