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. Author manuscript; available in PMC: 2015 Jan 23.
Published in final edited form as: Nat Rev Genet. 2014 Feb 11;15(3):163–175. doi: 10.1038/nrg3662

Figure 1. The major co-transcriptional mRNA processing steps.

Figure 1

Human protein names are given throughout. a | The RNA is shown in green; both GTP and the added guanosine cap (Gp) are shown in blue. The mRNA-capping enzyme in metazoans is bifunctional and has both triphosphatase and guanylyl-transferase activities that remove the γ-phosphate of the nascent transcript and transfer GMP from the GTP donor, respectively. The methyl donor S-adenosyl-l-methionine (SAM) is converted to S-adenosyl-l-homocysteine (SAH), which results in the 7-methylguanosine cap (shown in pink). b | Splicing clips out an intron or intervening sequences as a lariat and ligates the flanking exons together through two transesterification reactions. Conserved intronic splicing elements are indicated in red. Spliceosomal U1, U2, U4, U5 and U6 small nuclear ribonucleoprotein particles (snRNPs) and U2 auxiliary factor (U2AF) are shown, but numerous spliceosomal proteins are omitted79. c | 3′ ends of mRNAs are formed by coupled cleavage and polyadenylation. Cleavage of mammalian pre-mRNAs occurs ∼25 bases downstream of a consensus sequence (AAUAAA) and is carried out by the multisubunit complex (shown in purple), which comprises cleavage stimulation factor (CstF), cleavage and polyadenylation specificity factor (CPSF) that bears the endonuclease, and cleavage factors I and II (CFIm and CFIIm). Poly(A) polymerase (PAP) adds the poly(A) tail. 3′ ends of non-polyadenylated histone mRNAs (not shown) are also made co-transcriptionally by a cleavage complex that has many subunits in common with CstF and CPSF. The 5′-to-3′ RNA exonuclease 2 (XRN2) degrades RNA downstream of the cleavage site and facilitates transcription termination. d | Adenosine-to-inosine (A-to-I) editing is carried out by adenosine deaminases acting on RNA (ADAR), which deaminate adenosines into inosines. The folded GLUR-2 pre-mRNA substrate is shown with the exon in blue and intron in grey. Pi, inorganic phosphate. Part b is modified with permission from Chen & Cheng (2012) Biosci. Rep. 32, 345-359. © Biochemical Society.187