, Voi. 81, 179-188, April 21, 1995, Copyright © 1995 by Cell Press Degradation of mRNA in Review

Clare A. Beelman* and Roy Parker*t decay independently of deadenylation. This diversity of *Department of Molecular and Cellular Biology decay pathways, in addition to different rates of decay for tHoward Hughes Medical Institute individual mRNAs within one pathway, allows for a wide University of Arizona spectrum of mRNA half-lives and for their differential regu- Tucson, Arizona 85721 lation.

Deadenylation Can Be the First Step mRNA turnover is important in determining the levels and in mRNA Decay regulation of gene expression. Recent results have de- Several observations suggest that shortening of poly(A) fined several different, yet somewhat related, mechanisms tails is required for the decay of many eukaryotic mRNAs by which eukaryotic mRNAs are degraded (Figure 1). One (for reviews see Decker and Parker, 1994; Bernstein and mRNA decay pathway is initiated by shortening of the Ross, 1989; Peltz et al., 1991). Some of the key observa- poly(A) tail followed by decapping and 5' to 3' exonucleo- tions are as follows. First, in transcriptional pulse-chase lytic degradation of the transcript. A variation of this path- experiments (wherein a regulatable promoter is used to way has been observed in which transcripts undergo 3' to produce a burst of synchronous ), the mam- 5' decay after poly(A) shortening. Decay of mRNAs can also malian c-los mRNA and several mRNAs do not de- initiate prior to shortening of the poly(A) tail. For example, cay until their poly(A) tails have been shortened (Shyu et some specific transcripts can be degraded via deadeny- al., 1991; Decker and Parker, 1993), This temporal correla- lation-independent decapping followed by 5' to 3' degrada- tion is significant because within unstable tion. In addition, mRNA decay can also begin by endonu- mRNAs that decrease the rate of deadenylation or transfer cleolytic cleavage in the transcript body. These pathways of sequences capable of stimulating rapid deadenylation suggest a model of mRNA turnover in which all polyadeny- to a stable reporter mRNA correspondingly alter the time lated mRNAs are degraded by a"default" pathway initiated at which the transcript begins to decay (Shyu et al., 1991; by poly(A) shortening. In addition, pathways limited to sub- Wilson and Triesman, 1988; Muhlrad and Parker, 1992; sets of mRNAs exist in which specific sequences trigger Decker and Parker, 1993; Chen et al., 1994). In addition,

DEADENYLATION-DEPENDENT DECAY DEADENYLATION-INDEPENDENT DECAY

m7GI AUG UM I/gkA u~a

DEADENLYATION- / ENDONUCLEOL'gTIC CLEAVAGE POLY (A) SHORTENING T/

m7GI AUG 1UAA I AAA ou~ JAAA LOt~, m7G[ AUG i ? 5' TO 3' DECAY / \ DECAPPING 3' TO 5' DECAY

~AOG li~; u. IAAA~ / 5' TO 3' DECAY 5' TO 3' DECAY

~;,oG o" I~ ou~o

Figure 1. mRNA Decay Pathways in Eukaryotes The left pathway depicts deadenylation-dependentdecay, which can lead to decapping and 5' to 3' exonucleolyticdecay or to 3' to 5' decay mediated by endonucleases,exonucleases, or both. All eukaryotic mRNAs may undergo decay via this deadenylation-dependentpathway unless they are targeted for rapid deadenlyation-independentdecapping, by a nonsensecodon, or for endonucleolyticcleavage, by presence of a cleavage site (right). After deadenylation-independentdecapping, mRNA is degraded in a 5' to 3' manner. Endonucleolyticcleavage of mRNA may serve as a one-step deadenylationthat leads to decapping and 5' to 3' decay. Cell 180

intermediates in decay can be detected in yeast that accu- of the 5'cap structure (Muhlrad et al., 1994, 1995) suggests mulate after deadenylation and have oligo(A) tails, even an interaction between the 5' and 3' ends of an mRNA. at times when the population consists of a mixture of Interactions between the transcript termini have been pro- poly(A) and oligo(A) tails (Decker and Parker, 1993; Muhl- posed to explain the effects that the poly(A) tail and 3' tad et al., 1994). untranslated region (3'UTR) sequences can have on trans- The length to which a poly(A) tail must be shortened lation initiation (Munroe and Jacobson, 1990; for review for subsequent decay may differ between mammals and see Jackson and Standart, 1990). A unifying hypothesis yeast. A poly(A) tail of - 25-60 adenosine residues is suffi- is that the same 5' to 3' interaction may mediate the rates cient to allow decay of some mammalian transcripts (Shyu of initiation and decapping. Although there is et al., 1991; Chen et al., 1994), whereas yeast transcripts no direct biochemical evidence for such a 5' to 3' interac- decay when the poly(A) is -10-12 adenosine residues tion, the abundance of circular polysomes seen in some (Decker and Parker, 1993). Deadenylation to - 10-12 resi- electron micrographs suggests that the transcript termini dues as seen in yeast might lead to the loss of the last can be in close proximity (e.g., Christensen et al., 1987). poly(A)-binding associated with the transcript, and The nature of the 5' to 3' interaction may allow the poly(A) the loss of poly(A)-binding protein may trigger further de- tail to inhibit decapping indirectly or directly. For example, cay events. It is possible that the disassociation of poly(A)- because mRNAs may interact with the cytoskeleton binding protein may occur at a longer poly(A) tail length through their poly(A) tails (e.g., Taneja et al., 1992), the in mammalian cells. putative 5' to 3' interaction may cause deadenyiation to alter the subcellular localization of an mRNA, thereby Deadenylation Can Trigger Decapping exposing it to a localized decapping activity. Alternatively, and 5' to 3' Decay of mRNA the poly(A) tail might inhibit decapping directly by forming One important question is how deadenylation leads to or stabilizing an mRNP structure involving the 5' and 3' mRNA degradation. In yeast, observations indicate that termini of the mRNA. deadenylation to an oligo(A) tail allows the mRNA to be- come a substrate for a decapping reaction within the first few of the transcript, thereby exposing the Deadenylation Can Also Lead to 3' to 5' Decay transcript body to 5' to 3' exonucleolytic decay. The critical of mRNA observations are that when 5' to 3' decay is blocked in Eukaryotic mRNAs can also be degraded in a 3' to 5' direc- yeast, either by deletion of the XRN1 gene, which encodes tion following deadenylation. For example, fragments of a major 5' to 3' exonuclease (Larimer and Stevens, 1990), the yeast PGK1 mRNA shortened at the 3' end accumulate or by the insertion of strong RNA secondary structures, when the 5' to 3' decay pathway is blocked (Muhlrad and mRNA fragments lacking the cap structure accumulate Parker, 1994; Muhlrad et al., 1995). Decay intermediates after deadenylation (Muhlrad et al., 1994, 1995). This is that are consistent with 3' to 5' decay following deadenyla- true for both unstable and stable mRNAs. In addition, sev- tion have also been observed for the oat phytochrome A eral other mRNAs also accumulate in xrnl/t strains as mRNA in vivo (Higgs and Colbert, 1994). It is not known transcripts that have, at most, short (A) tails and that lack whether exonucleases, endonucleases, or both are in- the cap structure (Hsu and Stevens, 1993). These results volved in 3'to 5'decay or how prevalent this decay pathway suggest this pathway of degradation is a general mecha- may be. However, it should not be expected that deadeny- nism of decay that acts on many yeast transcripts. lation necessarily exposes transcripts to 3' to 5' degrada- Since poly(A) tails and cap structures are common fea- tion since several mRNAs can be quite stable with essen- tures of eukaryotic transcripts, an appealing model is that tially no poly(A) tail (e.g., Decker and Parker, 1993; Chert mRNA decay by deadenylation-dependent decapping and et al., 1994). 5' to 3' digestion is a conserved mechanism of m R NA tu rn- An important point is that an individual transcript can over. Although there is to date no direct data for such a simultaneously be a substrate for more than one mecha- pathway in more complex eukaryotes, mRNAs lacking the nism of decay. For example, both the oat phytochrome A cap structure are rapidly degraded in many eukaryotic and yeast PGK1 transcripts undergo deadenylation-depen- cells (e.g., Drummond et al., 1985). In addition, enzymes dent 3' to 5' decay in addition to a 5' to 3' decay mechanism that could catalyze the removal of the cap structure and (Higgs and Colbert, 1994; Muhlrad et al., 1995). The pres- subsequent 5' to 3' degradation of the transcript have been ence of multiple overlapping pathways has several conse- described in mammalian cells (e.g., Coutts and Brawer- quences. First, the observed half-life for an mRNA will be man, 1993). Finally, the oat phytochrome A mRNA under- a summation of the rates of decay through each individual goes a cleavage near the 5' terminus of the mRNA and is pathway. In addition, the mechanism by which an mRNA degraded, in part, in a 5' to 3' direction (Higgs and Colbert, is degraded may change under different conditions, even 1994). Taken together, these data raise the possibility that without a significant alteration of decay rate (e.g., Muhlrad deadenylation-dependent decapping followed by 5' to 3' et al., 1995). The existence of multiple decay pathways exonucleolytic decay may be a conserved eukaryotic also suggests that mutations that inactivate one pathway mRNA decay mechanism. may not always have significant effects on mRNA stability, yet combinations of mutations inactivating different path- Interactions between the 5' and 3' Termini ways may exhibit synergistic effects on stability and possi- of mRNAs bly viability. A system of redundant overlapping mecha- The observation that deadenylation can lead to removal nisms of mRNA turnover is highly analogous to that Review: rnRNA Degradation 181

observed in Escherichia coil (for review see Belasco and (Muhlrad and Parker, 1994; Hagan et al., 1995). The deg- Brawerman, 1993). radation of mRNAs with nonsense codons is part of a pro- cess, termed mRNA surveillance, that ensures the rapid Determination of mRNA Decay Rates via degradation of aberrant transcripts. These aberrant Deadenylation-Dependent Decay mRNAs contain early nonsense codons (Losson and La- In deadenylation-dependent mRNA decay, differences in croute, 1979; Leeds et al., 1991; Peltz et al., 1993; Pulak mRNA stability result from differences in the rates of and Anderson, 1993), unspliced introns (He et al., 1993), poly(A) shortening and steps required for the subsequent or extended 3'UTRs (Pulak and Anderson, 1993). mRNA decay of the deadenylated transcript. Thus, features of surveillance occurs in many organisms and thus appears mRNAs that influence the rates of poly(A) shortening, de- to be a conserved process. Mutations in related genes capping, and possibly 3' to 5' decay will affect mRNA stabil- that inactivate this surveillance pathway have been identi- ity. The rate of 5' to 3' digestion of the transcript body fied in yeast and nematodes, named upland smg mutants, appears to be relativelyfast and therefore unlikely to con- respectively (Hodgkin et al., 1989; Leeds et al., 1991, tribute to differences in decay rates. This is based on the 1992). observation that intermediates of 5' to 3' decay are not mRNA surveillance may exist, in part, to increase the observed unless the XRN1 nuclease has been deleted or fidelity of gene expression by degrading aberrant mRNAs a secondary structure blocks the nuclease (Decker and that, if translated, would produce truncated . This Parker, 1993; Hsu and Stevens, 1993; Muhlrad et al., process would be relevant since truncated proteins can 1994). Several sequence elements within mRNAs that pro- have dominant negative phenotypes. A striking example mote rapid poly(A) shortening have been identified. These of this phenomenon is seen in , include sequences within the c-fos coding region and where mutations in smg genes convert recessive non- 3'UTR, which contains a prototypical AU-rich element sense mutations in the myosin gene uric-54 into dominant (ARE) (Wilson and Triesman, 1988; Shyu et al., 1989; Wel- negatives (Pulak and Anderson, 1993). Since mRNA sur- lington et al., 1993; Chen et al., 1994; Schiavi et al., 1994), veillance degrades unspliced and aberrantly processed and in the yeast MFA2 3'UTR (Muhlrad and Parker, 1992; transcripts, this decay mechanism might be most im- Decker and Parker, 1993; Muhlrad et al., 1994). Interest- portant in organisms with a large number of introns where ingly, mutations in the c-los ARE and the MFA2 3'UTR processing errors might lead to truncated proteins. can also slow degradation following deadenylation (Shyu How a nonsense-containing transcript is recognized as et al., 1991; Muhlrad and Parker, 1992). Thus, at least for aberrant and how mRNA decay is triggered are unclear. the MFA2 mRNA, 3'UTR sequences can also specify the The simplest model is that premature translation termina- rate of decapping. tion sends a signal to expose the 5' end immediately to There is little information about the nucleases involved decapping by the same nucleases that degrade normal in deadenylation-dependent decay and how mRNA se- mRNAs. This hypothesis is supported by the observations quences modulate their rate of activity. The product of the that the sites of decapping are the same in deadenylation- XRN1 gene appears to be the primary nuclease responsi- independent and deadenylation-dependent decapping (cf. ble for 5'to 3' degradation of yeast m RNAs after decapping Muhlrad and Parker, 1994 with Muhlrad et al., 1995), and (Hsu and Stevens, 1993; Muhlrad et al., 1994). Decapping the XRN1 exonuclease performs 5' to 3' decay in both activities (Nuss et al., 1975; Kumagai et al., 1992; Coutts cases. The generation of the destabilizing signal requires and Brawerman, 1993) and poly(A) nucleases (,~str0m et both the failure to translate a significant part of the coding al., 1991, 1992; Sachs and Deardorff, 1992; Lowell et al., region and the presence of specific sequences down- 1992) have been biochemically defined in cell extracts of stream of the nonsense codon (Cheng et al., 1990; Peltz yeast and mammals, although the in vivo roles of these et al., 1993; Zhang et al., 1995). In a similar mechanism, particular activities remain to be established. Several pro- transcripts with extended 3'UTRs may also be recognized teins have been identified that interact with the ARE and degraded because destabilizing sequences that are (Malter, 1989; Bohjanen et al., 1991; Vakalopoulou, 1991; not normally in the mRNA are now included within the Chen et al., 1992; Zhang et al., 1993; Katz et al., 1994) extended 3'UTR. and therefore might modulate deadenylation rate. How- A related and surprising observation is that early non- ever, only one of these proteins, called the ARE/poly(U)- sense mRNAs can reduce the levels of fully spliced but binding factor, has been shown to stimulate decay in an nuclear-associated transcripts (Belgrader et al., 1994; in vitro system (Brewer, 1991). Progress in this area may Baserga et at., 1992). One possibility is that the first round be aided by the identification of proteins that bind to a of translation occurs while the transcripts are still nuclear small sequence, UUAUUUA(U/A)(U/A), determined to be associated and that premature termination during this important for a functional ARE to stimulate mRNA decay round of translation can generate the signal to expose the (Lagnado et al., 1994; Zubiaga et al., 1995). ~ 5' end to decapping.

Deadenylation-lndependent Decapping: Decay of EukaryoUc mRNA via mRNA Surveillance Endonucleolytic Cleavage mRNA decay can also be initiated by decapping and 5' Eukaryotic mRNAs can be degraded via endonucleolytic to 3'decay of the transcript independent of poly(A) shorten- cleavage prior to deadenylation. Evidence for this mecha- ing. An example of this process is the degradation of the nism comes from the analysis of transcripts such as mam- yeast PGK1 mRNA containing an early nonsense codon malian 9E3, IGF2, transferrin receptor (TfR), and Xenopus Cell 182

X/hbox2B mRNA where mRNA fragments are detected in as regulatory inputs that control mRNA turnover, are likely vivo that correspond to the 5' and 3' portion of the transcript to affect all the steps of these decay pathways. and are consistent with internal cleavage within the 3'UTR One important goal in future work will be to identify the (Stoekle and Hanafusa, 1989; Nielson and Christiansen, gene products that are responsible for the nucleolytic 1992; Binder et al., 1994; Brown et al., 1993). It appears events in these pathways and to delineate how specific that deadenylation to an oligo(A) tail length is not required mRNA features act to affect the function of these degrada- for endonucleolytic cleavage since the 3' fragment of 9E3 tive activities. The identification of distinct mRNA decay and TfR mRNAs is polyadenylated(Stoekle and Hanafusa, pathways should allow genetic and biochemical ap- 1989; Binder et al., 1994) and since cleavage of the proaches that can be designed to identify these gene prod- Xlhbox2B mRNA is not affected by the adenylation state ucts. A second important goal is to understand the nature of the mRNA (Brown et al., 1993). However, it is possible of the interaction between the 5' and 3' termini, which may that endonucleolytic cleavage of some mRNAs could be also be critical for efficient translation. dependent on the length of the poly(A) tail. Endonucleo- lytic cleavages have also been defined in vitro for the albu- References min mRNA (Dompenciel et al., 1995) and in the coding /~str~m, J., ,~str~m, A., and Virtanen,A. (1991). In vitro deadenylation region of the c-myc mRNA (Bernstein et al., 1992). Since of mammalianmRNA by a HeLa cell 3' exonuclease.EMBO J. 10, there does not appear to be any similarity between the 3067-3071. cleavage sites in these mRNAs, there may be a wide /~str6m, J.,/~.strGm, A., and Virtanen, A. (1992). Propertiesof a HeLa variety of endonucleases with different cleavage specific- cell 3' exonucleasespecific for degradingpoly(A) tails of mammalian mRNA. J. Biol. Chem. 267, 18154-18159. ities. Baserga,S. J., and Benz,E. J., Jr. (1992). I~-Globinnonsense : Since sequence-specific endonuclease target sites are deficient accumulationof mRNA occurs despite normal cytoplasmic likely to be limited to individual mRNAs or classes of stability. Proc~ Natl. Acad. Sci. USA 89, 2935--2939. mRNAs, their presence allows for specific control of the Belasco, J., and Brawerman,G. (1993). Controlof MessengerRNA decay rate of these transcripts. In some cases, the rate Stability (New York: Academic Press). of endonucleolytic cleavage is modulated by the activity Belgrader,P., Cheng,J., Zhou,X., Stephenson,S., and Maquat, L. E. of protective factors that bind at or near the cleavage site (1994). Mammaliannonsense codons can be cis effectors of nuclear and compete with the endonuclease (Brown et al., 1993; mRNA half-life.MoL Cell. Biol. 14, 8219-8228. Binder et al., 1994). For example, the binding of the iron Bernstein, P., and Ross, J. (1989). Poly(A), poly(A) binding protein and the regulationof mRNA stability.Trends Biochem.Sci. 14, 373- response element-binding protein in the TfR 3'UTR in re- 377. sponse to low intracellular iron concentrations inhibits the Bernstein, P. L., Herrick, D. J., Prokipcak, R. D., and Ross,J. (1992). endonucleolytic cleavage of this mRNA (Binder et al., Controlof c-myc mRNAhalf-life in vitro by a proteincapable of binding 1994). Therefore, some endonucleases may be constitu- to a coding region stabilitydeterminant. Genes Dev. 6, 642-654. tively active, and the accessibility of the cleavage site is Binder, R., Horowitz,J. A., Basilion,J. P., Koeller, D. M., Klausner, regulated. R. D., and Harford, J. B. (1994). Evidencethat the pathwayof trans- ferrin receptormRNA degradation involves an endonucleolyticcleav- In other cases, the endonuclease activity may be directly age within the 3' UTR and does not involve poly(A) tail shortening. regulated. For example, the mammalian endonuclease EMBO J. 13, 1969-1980. RNase L is normally inactive and is only activated by oligo- Bohjanen, P. R., Bronislawa,P., June, C. H., Thompson, C. B., and mers of 2',5' phosphodiester-bonded adenylate residues, Lindsten,T. (1991). An induciblecytoplasmic factor (AU-B) binds selec- which are produced in response to the presence of double- tivelyto AUUUA multimersin the 3' untranslatedregion of lymphokine stranded RNA (for discussion see Silverman, 1994). Al- mRNA. Mol. Cell. Biol. 11, 3288-3295. though RNase L is important in mediating interferon re- Brewer, G. (1991). An A+U-rich elementRNA-binding factor regulates c-myc mRNA stability in vitro. Mol. Cell. Biol. 11, 2460-2466. sponses (e.g., Hassel et al., 1993), it has yet to be Brown, B. D., Zipkin, I. D., and Harland, R. M. (1993). Sequence- established whether this enzyme normally degrades any specific endonucleolyticcleavage and protectionof mRNAin Xenopus cellular mRNAs. and Drosophila. Genes Dev. 7, 1620-1631. Chen, C.-Y. A., You, Y., and Shyu, A.-B. (1992). Two cellularproteins bind specificallyto a purine-richsequence necessary for the destabili- Summary zation function of a c-fos protein coding region determinantof mRNA Based on the above mechanisms of mRNA degradation, instability. Mol. Cell. Biol. 12, 5748-5757. an integrated model of mRNA turnover can be proposed Chen, C.-Y. A., Chen, T.-M., and Shyu, A.-B. (1994). Interplayof two (Figure 1). In this model, all polyadenylated mRNAs would functionallyand structurallydistinct domainsof the c-fos AU-rich ele- be degraded by the deadenylation-dependentpathway at ment specifies its mRNA-destabilizingfunction. Mol. Cell. Biol. 14, 416-426. some rate. In addition to this default pathway, another layer of complexity would come from degradation mecha- Cheng, J., FogeI-Petrovic,M., and Maquat, L~ E. (1990). Translation to near the distal end of the penultimateexon is required for normal nisms specific to individual mRNAs or to classes of levels of spliced triosephosphateisomerase mRNA. Mol. Cell. Biol. mRNAs. Such mRNA-specific mechanisms would include 10, 5215-5225. sequence-specific endonuclease cleavage and deadeny- Christensen, A. K., Kahn, L. E., and Bourne, C. M. (1987). Circular lation-independent decapping. Thus, the overall decay polysomespredominate on the roughendoplasmic reticulum of soma- rate of an individual transcript will be a function of its sus- tropes and mammotropesin the rat anterior pituitary. Am. J. Anat. ceptibility to these turnover pathways. In addition, cis- 178, 1-10. acting sequences that specify mRNA decay rate, as well C0utts, M., and Brawerman, G. (1993). A 5' exoribonucleasefrom Review: mRNA Degradation 183

cytoplasmic extracts of mouse sarcoma 180 ascites cells. Biochim. Muhlrad, D., and Parker, R. (1994). Premature translational termina- Biophys. Acta 1173, 57-62. tion triggers mRNA decapping. Nature 340, 578-581. Decker, C. J., and Parker, R. (1993). A turnover pathway for both Muhlrad, D., Decker, C. J., and Parker, R. (1994). Deadenylation of the stable and unstable mRNAs in yeast: evidence for a requirement for unstable mRNA encoded by the yeast MFA2 gene leads to decapping deadenylation. Genes Dev. 7, 1632-1643. followed by 5' to 3' digestion of the transcript. Genes Dev. 8, 855- Decker, C. J., and Parker, R. (1994). Mechanisms of mRNA degrada- 866. tion in eukaryotes. Trends Biochem. Sci. lg, 336-340. Muhlrad, D., Decker, C. J., and Parker, R. (1995). Turnover mecha- Dompenciel, R. E., Garnepudi, V. R., and Schoenberg, D. R. (1995). nisms of the stable yeast PGK1 mRNA. Mol. Cell. Biol., 16, 2145-2156. Purification and characterization of an estrogen-regulated Xenopus Munroe, D., and Jacobson, A. (1990). mRNA poly(A) tail, a 3' enhancer liver polysomal nuclease involved in the selective destabilization of of translational initiation. Mol. Cell. Biol. 10, 3441-3455. albumin mRNA. J. Biol. Chem., 270, 6108-6118. Nielson, F. C., and Christiansen, J. (1992). Endonucleolysis in the Drummond, D. R., Armstrong, J., and Colman, A. (1985). The effect turnover of insulin-like growth factor II mRNA. J. Biol. Chem. 267, of capping and polyadenylation on the stability, movement and transla- 19404-19411. tion of synthetic messenger RNAs in Xenopus oocytes. Nucl. Acids Nuss, D. L., Furuichi, Y., Kock, G., and Shatkin, S. J. (1975). Detection Res. 13, 7375-7394. in HeLa cell extracts of a 7-methyl guanosinb specific enzyme activity Hagan, K. W., Ruiz-Echervarda, M. J., Quan, Y., and Peltz, S. W. that cleaves mTGpppN m. Cell 6, 21-27. (1995). Characterization of cis-acting sequences and decay intermedi- Peltz, S. W., Brewer, G., Bernstein, P., Hart, P., and Ross, J. (1991). ates involved in nonsense-mediated mRNA turnover. Mol. Cell. Biol. Regulation of mRNA turnover in eukaryotic cells. CRC Crit. Rev. Euk. 15, 809-823. Gene Exp. 1, 99-126. Hassel, B. A., Zhou, A., Sotomayor, C., Maran, A., and Silverman, Peltz, S. W., Brown, A. H., and Jacobson, A. (1993). mRNA destabiliza- R. H. (1993). A dominant negative mutant of 2-5A-dependent RNase tion triggered by premature translational termination depends on three suppresses antiproliferative and antiviral effects of interferon. EMBO mRNA sequence elements and at least one trans-acting factor. Genes J. 12, 3297-3304. Dev. 7, 1737-1754. He, F., Peltz, S. W., Donahue, J. L., Rosbash, M., and Jacobson, A. Pulak, R., and Anderson, P. (1993). mRNA surveillance by the Caeno- (1993). Stabilization and association of unspliced pre- rhabditis elegans smg genes. Genes Dev. 7, 1885-1897. mRNAs in a yeast upfl- mutant. Proc. Natl. Acad. Sci. USA 90, 7034- Sachs, A. B., and Deardorff, J. (1992). Translation initiation requires 7038. the PAB-dependent poly(A) ribonuclease in yeast. Cell 70, 961-973. Higgs, D. C., and Colbert, J. T. (1994). Oat phytochrome A mRNA Schiavi, S. C., Wellington, C. L., Shyu, A.-B., Chen, C.-Y. A., degradation appears to occur via two distinct pathways. Plant Cell 6, Greenberg, M. E., and Belasco, J. G. (1994). Multiple elements in the 1007-1019. c-fos protein coding region facilitate mRNA deadenylation and decay Hodgkin, J., Papp, A., Pulak, R., Am bros, V., and Anderson, P. (1989). by a mechanism coupled to translation. J. Biol. Chem. 269, 3441- A new kind of informational suppression in the nematode Caenorhab- 3448. ditis elegans. Genetics 123, 301-313. Shyu, A.-B., Greenberg, M. E., and Belasco, J. G. (1989). The c-fos Hsu, C. L., and Stevens, A. (1993). Yeast cells lacking 5' to 3' exoribo- transcript is targeted for rapid decay by two distinct mRNA degradation nuclease 1 contain mRNA species that are poly(A) deficient and par- pathways. Genes Dev. 3, 60-72. tially lack the 5' cap structure. Mol. Cell. Biol. 13, 4826-4835. Shyu, A.-B., Belasco, J. G., and Greenberg, M. E. (1991). Two distinct Jackson, R. J., and Standart, N. (1990). Do the poly(A) tail and 3' destabilizing elements in the c-fos message trigger deadenylation as untranslated region control mRNA translation? Cell 62, 15-24. a first step in rapid mRNA decay. Genes Dev. 5, 221-234. Katz, D. A., Theodorakis, N. G., Cleveland, D. W., Lindsten, T., and Silverman, R. H. (1994). Fascination with 2-5A-dependent RNase: a Thompson, C. B. (1994). AU-A, an RNA-binding activity distinct from unique enzyme that functLons in interferon action. J. Interferon Res. hnRNP A1, is selective for AUUUA repeats and shuttles between the 14, 101-104. nucleus and the . Nucl. Acids Res. 22, 238-246. Stoekle, M. Y., and Hanafusa, H. (1989). Processing of 9E3 mRNA and Kumagai, H., Kon, R., Hoshino, T., Aramaki, T., Nishikawa, M., Hirose, regulation of its stability in normal and rous sarcoma virus-transformed S., and Igarashi, K. (1992). Purification and properties of a decapping cells. Mol. Cell. Biol. 9, 4738-4745. enzyme from rat liver cytosol. Biochim. Biophys. Acta 13, 45-51. Taneja, K. L, Lifshitz, L. M., Fay, F. S., and Singer, R. H. (1992). Lagnado, C. A., Brown, C. L., and Goodall, G. J. (1994). AUUUA is Poly(A) RNA codistribution with microfilaments: evaluation by in situ not sufficient to promote poly(A) shortening and degradation of an hybridization and quantitative digital imaging microscopy. J. Cell Biol. mRNA: the functional sequence within AU-rich elements may be UUA- 119, 1245-1260. UUUA(U/A)(U/A). Mol. Cell. Biol. 14, 7984-7995. Vakalopoulou, E., Schaack, J., and Shenk, T. (1991). A 32-kilodalton Larimer, F. W., and Stevens, A. (1990). Disruption of the gene XRN1, protein binds to AU-rich domains in the 3'untranslated region of rapidly coding for a 5'--3' exoribonuclease, restricts yeast cell growth. Gene degraded mRNAs. MoI. Cell. Biol. 11, 3355-3364. 95, 85-90. Wellington, C. L., Greenberg, M. E., and Belasco, J. G. (1993). The Leeds, P., Peltz, S. W., Jacobson, A., and Culbertson, M. R. (1991). destabilizing elements in the coding region of c-fos mRNA are recog- The product of the yeast UPF1 gene is required for rapid turnover of nized as RNA. Mol. Cell. Biol. 13, 5034-5042. mRNAs containing a premature translational termination codon. Wilson, T, and Triesman, R. (1988). Removal of poly(A) and conse- Genes Dev. 5, 2303-2314. quent degradation of the c-fos mRNA facilitated by 3' AU-rich se- Leeds, P., Wood, J. M., Lee, B.-S., and Culbertson, M. R. (1992). Gene quences. Nature 336, 396-399. products that promote mRNA turnover in . Zhang, A., Ruiz-Echevarria, M. J., Quan, Y., and Peltz, S. W. (1995). Mol. Cell. Biol. 12, 2165-2177. Identification and characterization of a sequence motif involved in non- Losson, R., and Lacroute, F. (1979). Interference of nonsense muta- sense-mediated mRNA decay. Mol. Cell. Biol., 15, 2231-2244. tions with eukaryotic messenger RNA stability. Proc. Natl. Acad. Sci. Zhang, W., Wagner, B. J., Ehrenman, K., Schaefer, A. W., DeMaria, USA 76, 5134-5137. C. T., Crater, D., DeHaven, K., Long, L., and Brewer, G. (1993). Purifi- Lowell, J., Rudner, D., and Sachs, A. (1992). 3'-UTR-dependent de- cation, characterization, and cDNA cloning of an AU-rich element adenylation by the yeast poly(A) nuclease. Genes Dev. 6, 2088-2099~ RNA-binding protein, AUFI. Mol. Cell. Biol. 13, 7652-7665. Malter, J. S. (1989). Identification of an AUUUA-specific messenger Zubiaga, A. M., Belasco, J. G., and Greenberg, M. E. (1995). The RNA binding protein. Science 246, 664-666. nonamer UUAUUUAUU is the key AU-rich sequence motif that medi- ates mRNA degradation. Mol. Cell. Biol. 15, 2219-2230. Muhlrad, D., and Parker, R. (1992). Mutations affecting stability and deadenylation of the yeastMFA2transcript. Genes Dev. 6, 2100-2111.