Five-prime cap

This process, known as mRNA capping, is highly regulated and vital in the creation of stable and mature messenger RNA able to undergo translation during protein synthesis.

This is more common in higher eukaryotes and thought to be part of the innate immune system to recognize mRNAs from other organisms.

[7] In multicellular eukaryotes and some viruses,[8] further modifications can be made, including the methylation of the 2′ hydroxy-groups of the first 2 ribose sugars of the 5′ end of the mRNA.

The starting point for capping with 7-methylguanylate is the unaltered 5′ end of an RNA molecule, which terminates at a triphosphate group.

[3] The capping process is initiated before the completion of transcription, as the nascent pre-mRNA is being synthesized.

Capping with NAD+, NADH, or 3′-dephospho-coenzyme A is accomplished through an "ab initio capping mechanism," in which NAD+, NADH, or 3′-desphospho-coenzyme A serves as a "non-canonical initiating nucleotide" (NCIN) for transcription initiation by RNA polymerase and thereby directly is incorporated into the RNA product.

[12][13][14][15] The enzymes for capping can only bind to RNA polymerase II, ensuring specificity to only these transcripts, which are almost entirely mRNA.

This increases the half-life of the mRNA, essential in eukaryotes as the export and translation processes take significant time.