Capping enzyme

A capping enzyme is a guanylyl transferase enzyme that catalyzes the attachment of the 5' cap to messenger RNA molecules that are in the process of being synthesized in the cell nucleus during the first stages of gene expression. The addition of the cap occurs co-transcriptionally, after the growing RNA molecule contains about 30 nucleotides. The enzyme can only catalyze its reaction when bound to the phosphorylated carboxyl-terminal domain of RNA polymerase II; therefore it is specific to RNAs synthesized by this polymerase rather than those synthesized by RNA polymerase I or RNA polymerase III.