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. Author manuscript; available in PMC: 2014 Mar 24.
Published in final edited form as: Wiley Interdiscip Rev RNA. 2010 May 25;1(1):152–172. doi: 10.1002/wrna.19

FIGURE 1.

FIGURE 1

Structures and synthesis of RNA caps. Cap 0 or m7G caps (GMP colored blue) are formed in sequential steps by three enzymatic activities that act on the 5′ triphosphate end (colored red) of nascent transcripts. Transfer of a methyl group (colored green) from S-adenosylmethionine (AdoMet) completes the synthesis of cap 0. Transfer of two methyl groups (colored magenta) from S-adenosylmethionine (AdoMet) is required to form the TMG cap. Caps 1 and 2 structures require methylation (colored magenta) of cap 0 at the ribose 2′-O at the first and second nucleosides, respectively. Cap 4 structure is generated by six rounds of methylation (colored magenta). The first three rounds of methylation (colored magenta) occur at two positions on the base and ribose of the first nucleoside of the primary transcript. The next three rounds of methylation (colored magenta) occur on ribose 2′-O positions of the next three nucleosides. The γ-methyl phosphate cap is formed by transfer of a methyl group (colored magenta) from AdoMet to the γ-phosphate of the primary transcript.