Conclusion: In pre-mRNA processing, a poly(A) tail is added through cleavage and polyadenylation.

14.7 Most eukaryotic mRNAs have a 3' poly(A) tail.

The pre-mRNA is cleaved, and CstF and the cleavage factors leave the complex; the cleaved 3' end of the pre-mRNA is then degraded (IFigure 14.8c). CFSF and PAP remain bound to the pre-mRNA and carry out polyadenylation (I Figure 14.8d). After the addition of approximately 10 adenine nucleotides, a poly(A)-binding protein (PABII) attaches to the poly(A) tail and increases the rate of polyadenylation (I Figure 14.8e). As more of the tail is synthesized, additional molecules of PABII attach to it( < Figure 14.8f).

The poly(A) tail confers stability on many mRNAs, increasing the time during which the mRNA remains intact and available for translation before it is degraded by cellular enzymes. The stability conferred by the poly(A) tail is dependent on the proteins that attached to the tail.

Eukaryotic mRNAs that lack a poly(A) tail depend on a different mechanism for 3' cleavage that requires the formation of a hairpin structure in the pre-mRNA and a small ribonucleoprotein particle (snRNP) called U7 (iFigure 14.9). U7 contains an snRNA with nucleotides that are complementary to a sequence on the pre-mRNA just downstream of the cleavage site, and U7 most likely binds to this sequence. A hairpin-binding protein binds to the hairpin structure and stabilizes the binding of U7 to the complementary sequence on the pre-mRNA.

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