The enzymatic reaction is catalyzed specifically by the phosphorylated carboxyl-terminal domain (CTD) of RNA polymerase II.

Three enzymes, RNA triphosphatase, guanylyltransferase (or CE), and methyltransferase are involved in the addition of the methylated 5' cap to the mRNA.

Capping is a three-step process that utilizes the enzymes RNA triphosphatase, guanylyltransferase, and methyltransferase.

[1][2] Through a series of three steps, the cap is added to the first nucleotide's 5' hydroxyl group of the growing mRNA strand while transcription is still occurring.

[1][7][8] After successful capping, an additional phosphorylation event initiates the recruitment of machinery necessary for RNA splicing, a process by which introns are removed to produce a mature mRNA.

[4][13][14] The human capping enzyme is an example of a bifunctional polypeptide, which has both triphosphatase (N-terminal) and guanylyltransferase (C-terminal) domains.

A defect in splicing can occur as a result of mutation(s) in the guanylyltransferase, which can inhibit enzyme activity, preventing the formation of the cap.