Dynamic methylome of internal mRNA N7-methylguanosine and its regulatory role in translation.

Over 150 types of RNA modifications are identified in RNA molecules. Transcriptome profiling is one of the key steps in decoding the epitranscriptomic panorama of these chemical modifications and their potential functions. N7-methylguanosine (m7G) is one of the most abundant modifications present in tRNA, rRNA and mRNA 5'cap, and has critical roles in regulating RNA processing, metabolism and function. Besides its presence at the cap position in mRNAs, m7G is also identified in internal mRNA regions. However, its transcriptome-wide distribution and dynamic regulation within internal mRNA regions remain unknown. Here, we have established m7G individual-nucleotide-resolution cross-linking and immunoprecipitation with sequencing (m7G miCLIP-seq) to specifically detect internal mRNA m7G modification. Using this approach, we revealed that m7G is enriched at the 5'UTR region and AG-rich contexts, a feature that is well-conserved across different human/mouse cell lines and mouse tissues. Strikingly, the internal m7G modification is dynamically regulated under both H2O2 and heat shock treatments, with remarkable accumulations in the CDS and 3'UTR regions, and functions in promoting mRNA translation efficiency. Consistently, a PCNA 3'UTR minigene reporter harboring the native m7G modification site displays both enriched m7G modification and increased mRNA translation upon H2O2 treatment compared to the m7G site-mutated minigene reporter (G to A). Taken together, our findings unravel the dynamic profiles of internal mRNA m7G methylome and highlight m7G as a novel epitranscriptomic marker with regulatory roles in translation.