RNA capping occurs at
Five prime capping occurs at five prime end of nascent pre-mRNA soon after emergence of twenty to thirty nucleotides from exit channel of RNA polymerase II. Mammalian capping enzyme triphosphatase removes gamma phosphate, guanylyltransferase attaches GMP via unusual five prime to five prime triphosphate bridge, and N7-methyltransferase methylates guanine using SAM. Reaction requires serine five phosphorylated CTD that recruits capping enzyme to promoter proximal complex. Cap protects transcript from exonucleolytic degradation, promotes nuclear export via cap binding complex, and enhances trans
Ref: Alberts et al., Molecular Biology of the Cell, 7th ed., Chapter 6: 5' capping occurs cotranscriptionally; Lodish 9th ed., Capping enzymes and CTD recruitment
