mRNA processing mRNA capping GO:0006370 PMID:11051760 messenger RNA capping UPa:UPA00922 PMID:18298088 pathway The 5' ends of most eukaryotic mRNAs and many viral mRNAs harbor a m7GpppN cap structure that plays a critical role in the translation and stability of mRNAs. Biosynthesis of this cap structure involves three distinct enzymatic activities: (i) the 5' end of pre-mRNAs is initially hydrolyzed to a diphosphate end by an RNA triphosphatase; (ii) the diphosphate end of the mRNA is then capped with GMP by an RNA guanylyltransferase, and finally, (iii) the cap is methylated by an RNA (guanine-7) methyltransferase. Addition of the 7-methylguanosine cap to the 5' end of a eukaryotic mRNA transcript.