Cap-specific terminalN6-methylation of RNA by an RNA polymerase II-associated methyltransferase.

N 6 -methyladenosine (m 6 A), a major modification of mRNAs, plays critical roles in RNA metabolism and function. In addition to the internal m 6 A, N 6 , 2′- O -dimethyladenosine (m 6 Am) is present at the transcription start nucleotide of capped mRNAs in vertebrates. However, its biogenesis and functional role remain elusive. Using a reverse genetics approach, we identified PCIF1, a factor that interacts with the Ser5-phosphorylated C-terminal domain of RNA polymerase II, as cap-specific adenosine methyltransferase (CSAM) responsible for N 6 -methylation of m 6 Am. Crystal structure of CSAM in complex with substrates revealed the molecular basis of cap-specific m 6 A formation. A transcriptome-wide analysis revealed that N 6 -methylation of m 6 Am promotes the translation of capped mRNAs. Thus, a cap-specific m 6 A writer promotes translation of mRNAs starting from m 6 Am.