The mammalian cap-specific m6Am RNA methyltransferase PCIF1 regulates transcript levels in mouse tissues

The 5' end of eukaryotic mRNAs is protected by the m7G-cap structure. The transcription start site nucleotide is ribose methylated (Nm) in many eukaryotes, while an adenosine at this position is further methylated at the N6 position (m6A) by the mammalian Phosphorylated CTD-interacting Factor 1 (PCIF1) to generate m6Am. Here we show that while loss of cap-specific m6Am in mice does not affect viability or fertility, the Pcif1 mutants display reduced body-weight. Transcriptome analyses of mutant mouse tissues support a role for the cap-specific m6Am modification in stabilizing transcripts. In contrast, the Drosophila Pcif1 is catalytically-dead, but like its mammalian counterpart it retains the ability to associate with the Ser5-phosphorylated CTD of RNA pol II. Finally, we show that the Trypanosoma Pcif1 is an m6Am methylase that contributes to the N6,N6,2?-O-trimethyladenosine (m62Am) in the hypermethylated cap4 structure of Trypanosomatids. Thus, PCIF1 has evolved to function in catalytic and non-catalytic roles. RNA sequencing was used to identify gene expression differences between Pcif1-/- and Pcif1+/+ in mouse brain, spleen and testes. To correlate the gene expression changes with loss of m6Am we used m6Am-Exo-Seq method to identify the m6Am transcripts in mouse testes. Ribosome footprinting in mouse brain was used to search for effects on translation.