Harpers

394 / CHAPTER 39mouse amylase and myosin light chain, rat glucokinase,and drosophila alcohol dehydrogenase and actin. Alternativepolyadenylation sites in the µ immunoglobulinheavy chain primary transcript result in mRNAs thatare either 2700 bases long (µ m ) or 2400 bases long (µ s ).This results in a different carboxyl terminal region ofthe encoded proteins such that the µ m protein remainsattached to the membrane of the B lymphocyte and theµ s immunoglobulin is secreted. Alternative splicingand processing results in the formation of sevenunique α-tropomyosin mRNAs in seven different tissues.It is not clear how these processing-splicing decisionsare made or whether these steps can be regulated.Regulation of Messenger RNA StabilityProvides Another Control MechanismAlthough most mRNAs in mammalian cells are verystable (half-lives measured in hours), some turn oververy rapidly (half-lives of 10–30 minutes). In certain instances,mRNA stability is subject to regulation. Thishas important implications since there is usually a directrelationship between mRNA amount and thetranslation of that mRNA into its cognate protein.Changes in the stability of a specific mRNA can thereforehave major effects on biologic processes.Messenger RNAs exist in the cytoplasm as ribonucleoproteinparticles (RNPs). Some of these proteinsprotect the mRNA from digestion by nucleases, whileothers may under certain conditions promote nucleaseattack. It is thought that mRNAs are stabilized or destabilizedby the interaction of proteins with these variousstructures or sequences. Certain effectors, such as hormones,may regulate mRNA stability by increasing ordecreasing the amount of these proteins.It appears that the ends of mRNA molecules areinvolved in mRNA stability (Figure 39–19). The 5′cap structure in eukaryotic mRNA prevents attack by 5′exonucleases, and the poly(A) tail prohibits the actionof 3′ exonucleases. In mRNA molecules with thosestructures, it is presumed that a single endonucleolyticcut allows exonucleases to attack and digest the entiremolecule. Other structures (sequences) in the 5′ noncodingsequence (5′ NCS), the coding region, and the3′ NCS are thought to promote or prevent this initialendonucleolytic action (Figure 39–19). A few illustrativeexamples will be cited.Deletion of the 5′ NCS results in a threefold to fivefoldprolongation of the half-life of c-myc mRNA. Shorteningthe coding region of histone mRNA results in aprolonged half-life. A form of autoregulation of mRNAstability indirectly involves the coding region. Free tubulinbinds to the first four amino acids of a nascent chainof tubulin as it emerges from the ribosome. This appearsto activate an RNase associated with the ribosome (RNP)which then digests the tubulin mRNA.Structures at the 3′ end, including the poly(A) tail,enhance or diminish the stability of specific mRNAs.The absence of a poly(A) tail is associated with rapiddegradation of mRNA, and the removal of poly(A)from some RNAs results in their destabilization. HistonemRNAs lack a poly(A) tail but have a sequencenear the 3′ terminal that can form a stem-loop structure,and this appears to provide resistance to exonucleolyticattack. Histone H4 mRNA, for example, is degradedin the 3′ to 5′ direction but only after a singleendonucleolytic cut occurs about nine nucleotides fromthe 3′ end in the region of the putative stem-loop structure.Stem-loop structures in the 3′ noncoding sequenceare also critical for the regulation, by iron, ofthe mRNA encoding the transferrin receptor. Stemloopstructures are also associated with mRNA stabilityin bacteria, suggesting that this mechanism may becommonly employed.Cap 5′ NCS Coding 3′ NCSA–A–A–A–A nAUUUAFigure 39–19. Structure of a typical eukaryotic mRNA showingelements that are involved in regulating mRNA stability. The typicaleukaryotic mRNA has a 5′ noncoding sequence (5′ NCS), a codingregion, and a 3′ NCS. All are capped at the 5′ end, and most have apolyadenylate sequence at the 3′ end. The 5′ cap and 3′ poly(A) tailprotect the mRNA against exonuclease attack. Stem-loop structuresin the 5′ and 3′ NCS, features in the coding sequence, and the AUrichregion in the 3′ NCS are thought to play roles in mRNA stability.