Splice site m6A methylation prevents binding of U2AF35 to inhibit RNA splicing

The N 6 -methyladenosine (m 6 A) RNA modification is used widely to alter the fate of mRNAs. Here we demonstrate that the C. elegans writer METT-10 (the ortholog of mouse METTL16) deposits an m 6 A mark on the 3′ splice site (AG) of the S-adenosylmethionine (SAM) synthetase pre-mRNA, which inhibits its proper splicing and protein production. The mechanism is triggered by a rich diet and acts as an m 6 A-mediated switch to stop SAM production and regulate its homeostasis. Although the mammalian SAM synthetase pre-mRNA is not regulated via this mechanism, we show that splicing inhibition by 3′ splice site m 6 A is conserved in mammals. The modification functions by physically preventing the essential splicing factor U2AF35 from recognizing the 3′ splice site. We propose that use of splice-site m 6 A is an ancient mechanism for splicing regulation.