N 6 -methyladenosine in poly(A) tails stabilize VSG transcripts.

RNA modifications are important regulators of gene expression 1 . In Trypanosoma brucei, transcription is polycistronic and thus most regulation happens post-transcriptionally 2 . N 6 -methyladenosine (m 6 A) has been detected in this parasite, but its function remains unknown 3 . Here we found that m 6 A is enriched in 342 transcripts using RNA immunoprecipitation, with an enrichment in transcripts encoding variant surface glycoproteins (VSGs). Approximately 50% of the m 6 A is located in the poly(A) tail of the actively expressed VSG transcripts. m 6 A residues are removed from the VSG poly(A) tail before deadenylation and mRNA degradation. Computational analysis revealed an association between m 6 A in the poly(A) tail and a 16-mer motif in the 3' untranslated region of VSG genes. Using genetic tools, we show that the 16-mer motif acts as a cis-acting motif that is required for inclusion of m 6 A in the poly(A) tail. Removal of this motif from the 3' untranslated region of VSG genes results in poly(A) tails lacking m 6 A, rapid deadenylation and mRNA degradation. To our knowledge, this is the first identification of an RNA modification in the poly(A) tail of any eukaryote, uncovering a post-transcriptional mechanism of gene regulation.