RNA Processing:- Eukaryotic mRNAs Have a Distinctive 3 End Structure
At their 3 end, most eukaryotic mRNAs have a string of 80 to 250 A residues, making up the poly(A) tail. This tail serves as a binding site for one or more specific proteins. The poly(A) tail and its associated proteins probably help protect mRNA from enzymatic destruction. Many prokaryotic mRNAs also acquire poly(A) tails, but these tails stimulate decay of mRNA rather than protecting it from degradation. The poly(A) tail is added in a multistep process. The transcript is extended beyond the site where the poly(A) tail is to be added, then is cleaved at the poly(A) addition site by an endonuclease component of a large enzyme complex, again associated with the CTD of RNA polymerase II (Fig. 26–17). The mRNA site where cleavage occurs is marked by two sequence elements: the highly conserved sequence (5) AAUAAA (3), 10 to 30 nucleotides on the 5 side (upstream) of the cleavage site, and a less well-defined sequence rich in G and U residues, 20 to 40 nucleotides downstream of the cleavage site. Cleavage generates the free 3-hydroxyl group that defines the end of the mRNA, to which A residues are immediately added by polyadenylate polymerase, which catalyzes the reaction
RNA+nATP88→RNA–(AMP)n+ PPi
where n=80 to 250. This enzyme does not require a template but does require the cleaved mRNA as a primer.
The overall processing of a typical eukaryotic mRNA is summarized in Figure 26–18. In some cases the polypep tide-coding region of the mRNA is also modified by RNA “editing” (see Box 27–1 for details). This editing includes processes that add or delete bases in the coding regions of primary transcripts or that change the sequence (by, for example, enzymatic deamination of a C residue to create a U residue). A particularly dramatic example occurs in trypanosomes, which are parasitic protozoa: large regions of an mRNA are synthesized without any uridylate, and the U residues are inserted later by RNA editing.
FIGURE 26–17 Addition of the poly(A) tail to the primary RNA transcript of eukaryotes. Pol II synthesizes RNA beyond the segment of the transcript containing the cleavage signal sequences, including the highly conserved upstream sequence (5) AAUAAA. 1 The cleavage signal sequence is bound by an enzyme complex that includes an en donuclease, a polyadenylate polymerase, and several other multisubunit proteins involved in sequence recognition, stimulation of cleavage, and regulation of the length of the poly(A) tail. 2 The RNA is cleaved by the endonuclease at a point 10 to 30 nucleotides 3 to (downstream of) the sequence AAUAAA. 3 The polyadenylate polymerase synthesizes a poly(A) tail 80 to 250 nucleotides long, beginning at the cleavage site.
FIGURE 26–18 Overview of the processing of a eukaryotic mRNA. The ovalbumin gene, shown here, has introns A to G and exons 1 to 7 and L . About three-quarters of the RNA is removed during processing. Pol II extends the primary transcript well beyond the cleavage and polyadenylation site (“extra RNA”) before terminating transcription. Termination signals for Pol II have not yet been defined.
