5′ RNA cap structures. (A) Structure of m7G cap linked to the 5′-5′ triphosphate bridge (or cap 0) and m6A methylation if the first transcribed nucleotide is an adenosine. Additionally, both the first (N1, cap 1) and the second (N2, cap 2) can be 2′-O-methylated. Enzymes involved in each of these modifications are illustrated in blue circles and methyl groups are marked in orange. (RNTM) RNA guanine-7 methyltransferase, (CAPAM) cap-specific adenosine N6-methyltransferase, and (CMTR) cap 2′-O-methyltransferase 1 and 2. (B) Tri-methylguanosine (TMG) cap structure. Methyl groups at the N2 position on the cap 0 structure are marked in orange. (C) Structure of the noncanonical RNA cap, diadenosine tetraphosphate (Ap4 A). (D) Metabolite-derived cap structures. (Left to right) Nicotinamide adenine dinucleotide (NAD) caps can exist in the oxidized form, as NAD+ (shown), or in the reduced form, NADH. Flavin adenine dinucleotide (FAD) caps structures. UDP-glucose-derived caps include uridine diphosphate-glucose (UDP-Glc) and UDP-N-acetylglucosamine (UDP-GlcNAc).
