mRNA cap methyltransferase, RNMT-RAM, promotes RNA pol II transcription

mRNA cap addition occurs early during RNA pol II transcription, facilitating pre-mRNA processing and translation. We report that the mammalian mRNA cap methyltransferase, RNMT-RAM, promotes RNA pol II transcription, independently of mRNA capping and translation. In cells, sub-lethal suppression of RNMT-RAM reduces RNA pol II occupancy, net mRNA synthesis and pre-mRNA levels. Conversely, expression of RNMT-RAM increases transcription independently of cap methyltransferase activity. In isolated nuclei, recombinant RNMT-RAM stimulates transcriptional output; this requires the RAM RNA-binding domain. RNMT-RAM interacts with nascent transcripts along their entire length and with transcription associated factors including RNA pol II subunits, SPT4, SPT6 and PAFc. Suppression of RNMT-RAM inhibits transcriptional markers including histone H2B K120 ubiquitination, H3 K4 and K36 methylation, RNA pol II S5 and S2 phosphorylation and PAFc recruitment. These findings suggest that multiple interactions between RNMT-RAM, RNA pol II factors and RNA along the transcription unit stimulate transcription. Overall design: CLIP experiment performed on HA-RNMT expressing HeLa cells. RNASeq and ChIP-Seq performed on HeLa cells treated with siRNA for RAM and scrambled control.