mRNA (2'-O-methyladenosine-N(6)-)-methyltransferase
RNA polymerase II C-terminal domain phosphoserine binding
The Mammalian Cap-Specific m<sup>6</sup>Am RNA Methyltransferase PCIF1 Regulates Transcript Levels in Mouse Tissues.
The 5' end of eukaryotic mRNAs is protected by the m<sup>7</sup>G-cap structure. The transcription start site nucleotide is ribose methylated (Nm) in many eukaryotes, whereas an adenosine at this position is further methylated at the N<sup>6</sup> position (m<sup>6</sup>A) by the mammalian Phosphorylated C-terminal domain (CTD)-interacting Factor 1 (PCIF1) to generate m<sup>6</sup>Am. Here, we show that although the loss of cap-specific m<sup>6</sup>Am 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 m<sup>6</sup>Am 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 polymerase II (RNA Pol II). Finally, we show that the Trypanosoma Pcif1 is an m<sup>6</sup>Am methylase that contributes to the N<sup>6</sup>,N<sup>6</sup>,2'-O-trimethyladenosine (m<sup>6</sup><sub>2</sub>Am) in the hypermethylated cap4 structure of trypanosomatids. Thus, PCIF1 has evolved to function in catalytic and non-catalytic roles.