mRNA Cap Formation

UB Department of Biological Sciences: Kiong Ho: "Cap formation is mediated by three enzymatic reactions in which the 5' triphosphate end of pre-mRNA is hydrolyzed to a 5' diphosphate by RNA triphosphatase, then capped with GMP by RNA guanylyltransferase, and methylated by RNA (guanine-N7) methyltransferase. The sequential steps in the capping reaction are universal to all eukaryotes, yet there is a significant divergence in the catalytic mechanism of the triphosphatase component. Metazoan triphosphatases belong to a superfamily of phosphatases that act via the formation and hydrolysis of a cysteinyl-phosphate intermediate. Fungal and viral triphosphatases comprise a novel family of metal-dependent phosphohydrolases with a unique tertiary structure."