tRNA (guanine-N(7)-)-methyltransferase
internal mRNA (guanine-N7-)-methyltransferase activity
Transcriptome-wide Mapping of Internal N<sup>7</sup>-Methylguanosine Methylome in Mammalian mRNA.
N<sup>7</sup>-methylguanosine (m<sup>7</sup>G) is a positively charged, essential modification at the 5' cap of eukaryotic mRNA, regulating mRNA export, translation, and splicing. m<sup>7</sup>G also occurs internally within tRNA and rRNA, but its existence and distribution within eukaryotic mRNA remain to be investigated. Here, we show the presence of internal m<sup>7</sup>G sites within mammalian mRNA. We then performed transcriptome-wide profiling of internal m<sup>7</sup>G methylome using m<sup>7</sup>G-MeRIP sequencing (MeRIP-seq). To map this modification at base resolution, we developed a chemical-assisted sequencing approach that selectively converts internal m<sup>7</sup>G sites into abasic sites, inducing misincorporation at these sites during reverse transcription. This base-resolution m<sup>7</sup>G-seq enabled transcriptome-wide mapping of m<sup>7</sup>G in human tRNA and mRNA, revealing distribution features of the internal m<sup>7</sup>G methylome in human cells. We also identified METTL1 as a methyltransferase that installs a subset of m<sup>7</sup>G within mRNA and showed that internal m<sup>7</sup>G methylation could affect mRNA translation.