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1.
2.
RNase II and RNase R are the two E. coli exoribonucleases that belong to the RNase II super family of enzymes. They degrade RNA hydrolytically in the 3' to 5' direction in a processive and sequence independent manner. However, while RNase R is capable of degrading structured RNAs, the RNase II activity is impaired by dsRNAs. The final end-product of these two enzymes is also different, being 4 nt for RNase II and 2 nt for RNase R. RNase II and RNase R share structural properties, including 60% of amino acid sequence similarity and have a similar modular domain organization: two N-terminal cold shock domains (CSD1 and CSD2), one central RNB catalytic domain, and one C-terminal S1 domain. We have constructed hybrid proteins by swapping the domains between RNase II and RNase R to determine which are the responsible for the differences observed between RNase R and RNase II. The results obtained show that the S1 and RNB domains from RNase R in an RNase II context allow the degradation of double-stranded substrates and the appearance of the 2 nt long end-product. Moreover, the degradation of structured RNAs becomes tail-independent when the RNB domain from RNase R is no longer associated with the RNA binding domains (CSD and S1) of the genuine protein. Finally, we show that the RNase R C-terminal Lysine-rich region is involved in the degradation of double-stranded substrates in an RNase II context, probably by unwinding the substrate before it enters into the catalytic cavity. 相似文献
3.
Polynucleotide phosphorylase (PNPase) is an exoribonuclease that cleaves single-stranded RNA substrates with 3'-5' directionality and processive behaviour. Its ring-like, trimeric architecture creates a central channel where phosphorolytic active sites reside. One face of the ring is decorated with RNA-binding K-homology (KH) and S1 domains, but exactly how these domains help to direct the 3' end of single-stranded RNA substrates towards the active sites is an unsolved puzzle. Insight into this process is provided by our crystal structures of RNA-bound and apo Caulobacter crescentus PNPase. In the RNA-free form, the S1 domains adopt a 'splayed' conformation that may facilitate capture of RNA substrates. In the RNA-bound structure, the three KH domains collectively close upon the RNA and direct the 3' end towards a constricted aperture at the entrance of the central channel. The KH domains make non-equivalent interactions with the RNA, and there is a marked asymmetry within the catalytic core of the enzyme. On the basis of these data, we propose that structural non-equivalence, induced upon RNA binding, helps to channel substrate to the active sites through mechanical ratcheting. Structural and biochemical analyses also reveal the basis for PNPase association with RNase E in the multi-enzyme RNA degradosome assembly of the α-proteobacteria. 相似文献
4.
The multifunctional ribonuclease RNase E and the 3'-exonuclease polynucleotide phosphorylase (PNPase) are major components of an Escherichia coli ribonucleolytic "machine" that has been termed the RNA degradosome. Previous work has shown that poly(A) additions to the 3' ends of RNA substrates affect RNA degradation by both of these enzymes. To better understand the mechanism(s) by which poly(A) tails can modulate ribonuclease action, we used selective binding in 1 m salt to identify E. coli proteins that interact at high affinity with poly(A) tracts. We report here that CspE, a member of a family of RNA-binding "cold shock" proteins, and S1, an essential component of the 30 S ribosomal subunit, are poly(A)-binding proteins that interact functionally and physically, respectively, with degradosome ribonucleases. We show that purified CspE impedes poly(A)-mediated 3' to 5' exonucleolytic decay by PNPase by interfering with its digestion through the poly(A) tail and also inhibits both internal cleavage and poly(A) tail removal by RNase E. The ribosomal protein S1, which is known to interact with sequences at the 5' ends of mRNA molecules during the initiation of translation, can bind to both RNase E and PNPase, but in contrast to CspE, did not affect the ribonucleolytic actions of these enzymes. Our findings raise the prospect that E. coli proteins that bind to poly(A) tails may link the functions of degradosomes and ribosomes. 相似文献
5.
Different specificities of ribonuclease II and polynucleotide phosphorylase in 3'mRNA decay 总被引:4,自引:0,他引:4
We review recent evidence on the in vivo and in vitro mRNA degradation properties of 2 3'-exonucleases, ribonuclease II and polynucleotide phosphorylase. Although secondary structures in the RNA can act as protective barriers against 3' exonucleolytic degradation, it appears that this effect depends on the stability of these structures. The fact that RNase II is more sensitive to RNA secondary structure than PNPase, could account for some differences observed in messenger degradation by the 2 enzymes in vivo. Terminator stem-loop structures are often very stable and 3' exonucleolytic degradation proceeds only after they have been eliminated by an endonucleolytic cleavage. Other secondary structures preceding terminator stem-loop seem to contribute to mRNA stability against exonucleolytic decay. 相似文献
6.
We review recent evidence on the in vivo and in vitro mRNA degradation properties of 2 3'-exonucleases, ribonuclease II and polynucleotide phosphorylase. Although secondary structures in the RNA can act as protective barriers against 3' exonucleolytic degradation, it appears that this effect depends on the stability of these structures. The fact that RNase II is more sensitive to RNA secondary structure than PNPase, could account for some differences observed in messenger degradation by the 2 enzymes in vivo. Terminator stem-loop structures are often very stable and 3' exonucleolytic degradation proceeds only after they have been eliminated by an endonucleolytic cleavage. Other secondary structures preceding terminator stem-loop seem to contribute to mRNA stability against exonucleolytic decay. 相似文献
7.
Dorléans A Li de la Sierra-Gallay I Piton J Zig L Gilet L Putzer H Condon C 《Structure (London, England : 1993)》2011,19(9):1252-1261
RNase J is a key member of the β-CASP family of metallo-β-lactamases involved in the maturation and turnover of RNAs in prokaryotes. The B.?subtilis enzyme possesses both 5'-3' exoribonucleolytic and endonucleolytic activity, an unusual property for a ribonuclease. Here, we present the crystal structure of T.?thermophilus RNase J bound to a 4 nucleotide RNA. The structure reveals an RNA-binding channel that illustrates how the enzyme functions in 5'-3' exoribonucleolytic mode and how it can function as an endonuclease. A second, negatively charged tunnel leads from the active site, and is ideally located to evacuate the cleaved nucleotide in 5'-3' exonucleolytic mode. We show that B.?subtilis RNase J1, which shows processive behavior on long RNAs, behaves distributively for substrates less than 5 nucleotides in length. We propose a model involving the binding of the RNA to the surface of the β-CASP domain to explain the enzyme's processive action. 相似文献
8.
Domain analysis of the chloroplast polynucleotide phosphorylase reveals discrete functions in RNA degradation,polyadenylation, and sequence homology with exosome proteins 总被引:4,自引:0,他引:4
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The molecular mechanism of mRNA degradation in the chloroplast consists of sequential events, including endonucleolytic cleavage, the addition of poly(A)-rich sequences to the endonucleolytic cleavage products, and exonucleolytic degradation. In spinach chloroplasts, the latter two steps of polyadenylation and exonucleolytic degradation are performed by the same phosphorolytic and processive enzyme, polynucleotide phosphorylase (PNPase). An analysis of its amino acid sequence shows that the protein is composed of two core domains related to RNase PH, two RNA binding domains (KH and S1), and an alpha-helical domain. The amino acid sequence and domain structure is largely conserved between bacteria and organelles. To define the molecular mechanism that controls the two opposite activities of this protein in the chloroplast, the ribonuclease, polymerase, and RNA binding properties of each domain were analyzed. The first core domain, which was predicted to be inactive in the bacterial enzymes, was active in RNA degradation but not in polymerization. Surprisingly, the second core domain was found to be active in degrading polyadenylated RNA only, suggesting that nonpolyadenylated molecules can be degraded only if tails are added, apparently by the same protein. The poly(A) high-binding-affinity site was localized to the S1 domain. The complete spinach chloroplast PNPase, as well as versions containing the core domains, complemented the cold sensitivity of an Escherichia coli PNPase-less mutant. Phylogenetic analyses of the two core domains showed that the two domains separated very early, resulting in the evolution of the bacterial and organelle PNPases and the exosome proteins found in eukaryotes and some archaea. 相似文献
9.
RNase II is a single-stranded-specific 3'-exoribonuclease that degrades RNA generating 5'-mononucleotides. This enzyme is the prototype of an ubiquitous family of enzymes that are crucial in RNA metabolism and share a similar domain organization. By sequence prediction, three different domains have been assigned to the Escherichia coli RNase II: two RNA-binding domains at each end of the protein (CSD and S1), and a central RNB catalytic domain. In this work we have performed a functional characterization of these domains in order to address their role in the activity of RNase II. We have constructed a large set of RNase II truncated proteins and compared them to the wild-type regarding their exoribonucleolytic activity and RNA-binding ability. The dissociation constants were determined using different single- or double-stranded substrates. The results obtained revealed that S1 is the most important domain in the establishment of stable RNA-protein complexes, and its elimination results in a drastic reduction on RNA-binding ability. In addition, we also demonstrate that the N-terminal CSD plays a very specific role in RNase II, preventing a tight binding of the enzyme to single-stranded poly(A) chains. Moreover, the biochemical results obtained with RNB mutant that lacks both putative RNA-binding domains, revealed the presence of an additional region involved in RNA binding. Such region, was identified by sequence analysis and secondary structure prediction as a third putative RNA-binding domain located at the N-terminal part of RNB catalytic domain. 相似文献
10.
RNase R and RNase II are the two representatives from the RNR family of
processive, 3′ to 5′ exoribonucleases in Escherichia
coli. Although RNase II is specific for single-stranded RNA, RNase R
readily degrades through structured RNA. Furthermore, RNase R appears to be
the only known 3′ to 5′ exoribonuclease that is able to degrade
through double-stranded RNA without the aid of a helicase activity.
Consequently, its functional domains and mechanism of action are of great
interest. Using a series of truncated RNase R proteins we show that the
cold-shock and S1 domains contribute to substrate binding. The cold-shock
domains appear to play a role in substrate recruitment, whereas the S1 domain
is most likely required to position substrates for efficient catalysis. Most
importantly, the nuclease domain alone, devoid of the cold-shock and S1
domains, is sufficient for RNase R to bind and degrade structured RNAs.
Moreover, this is a unique property of the nuclease domain of RNase R because
this domain in RNase II stalls as it approaches a duplex. We also show that
the nuclease domain of RNase R binds RNA more tightly than the nuclease domain
of RNase II. This tighter binding may help to explain the difference in
catalytic properties between RNase R and RNase II.Ribonucleases (RNases) play important roles in RNA metabolism. They are
responsible for the maturation of stable RNA and the degradation of RNA
molecules that are defective or no longer required by the cell. Both
maturation and degradation are initiated by endoribonucleolytic cleavage(s)
and completed by the action of exoribonucleases
(1). In Escherichia
coli, three, relatively nonspecific, 3′ to 5′ processive
exoribonucleases are responsible for degradation of RNA: RNase II, RNase R,
and polynucleotide phosphorylase
(PNPase).3 RNase II
and PNPase appear to be primarily responsible for mRNA decay
(2), although their precise
functions may differ (3).
However, mRNAs containing extensive secondary structure, such as repetitive
extragenic palindromic sequences, are degraded by PNPase
(4,
5) or RNase R
(5). Likewise, degradation of
highly structured regions of rRNA
(6) and tRNA
(7),4
is carried out by PNPase and/or RNase R. These findings suggest that PNPase
and RNase R are the universal degraders of structured RNAs in vivo,
leaving RNase II to act on relatively unstructured RNAs.Whether or not an RNase acts upon a particular RNA appears to depend upon
the specificity of the RNase and the accessibility of the RNA to that RNase
(1). Purified RNase R readily
degrades both single- and double-stranded RNA molecules
(5,
8), and it is the only known
3′ to 5′ exoribonuclease able to degrade through double-stranded
RNA without the aid of helicase activity. To degrade RNA molecules containing
double-stranded regions, RNase R requires a 3′ single-stranded overhang
at least 5 nucleotides long to serve as a binding site from which degradation
can be initiated (5,
8,
9).5
How RNase R then proceeds through the RNA duplex is of great interest. An
important step toward elucidating the mechanism of action of RNase R is to
determine the contribution that each of its domains makes to substrate binding
and exoribonuclease activity.Despite differences in their physiological roles and intrinsic substrate
specificities, RNase R and RNase II both belong to the widely distributed RNR
family of exoribonucleases
(10–12).
RNR family members are all large multidomain proteins with processive 3′
to 5′ hydrolytic exoribonuclease activity that share a common linear
domain organization. RNase R contains two cold-shock domains (CSD1 and CSD2)
near its N terminus, a central nuclease, or RNB domain, an S1 domain near the
C terminus, and a low complexity, highly basic region at the C terminus
(Fig. 1A). The
nuclease domain contains four highly conserved sequence motifs
(10,
11). Motif I contains four
conserved aspartate residues that are thought to coordinate two divalent metal
ions that facilitate a two-metal ion mechanism similar to that of DEDD family
exoribonucleases and the proofreading domains of many polymerases
(13,
14). CSDs
(15–17)
and S1 domains (18,
19) are well known examples of
RNA-binding domains. Interestingly, there are reports that both of these
domains can act as nucleic acid chaperones and unwind RNA
(20–29),
providing a possible explanation for the ability of RNase R to degrade
structured RNAs. The role of the basic region at the C terminus of RNase R is
unknown, but it may act as an RNA-binding domain and/or a mediator of
protein-protein interactions.Open in a separate windowFIGURE 1.Linear domain organization of RNase R and RNase II proteins. The
CSDs are colored in cyan and blue for CSD1 and CSD2,
respectively, the nuclease domains are in green, the S1 domains are
red, and the low complexity, highly basic region, found in RNase R
only, is in magenta. A, RNase R. RNase R full-length is the
full-length wild-type RNase R protein. RNase RΔCSDs
lacks both CSD1 and CSD2. RNase RΔBasic is missing the
low complexity, highly basic region. RNase RΔS1 is
missing both the S1 domain and the low complexity, highly basic region.
RNase RΔCSDsΔS1 consists of the
nuclease domain alone. B, RNase II. RNase II full-length is
the full-length wild-type RNase II protein. RNase
IIΔCSDsΔS1 contains the nuclease domain
alone.Crystal structures of E. coli wild-type RNase II and a D209N
catalytic site mutant in complex with single-stranded RNA have recently been
solved (14,
30). In these structures the
two CSDs and the S1 domain come together to form an RNA-binding clamp that
directs RNA to the catalytic center at the base of a narrow, basic channel
within the nuclease domain
(14,
30). Only single-stranded RNA
can be accommodated by the RNA-binding clamp and the nuclease domain channel,
which explains the single strand specificity of RNase II. It is expected that
RNase R will adopt a similar structure.In this study, we determine the contribution that each of the domains of
RNase R makes to RNA-binding and exoribonuclease activity. We show that the
CSDs and the S1 domain are important for substrate binding, although their
roles differ. Of most interest, we show that the nuclease domain alone of
RNase R is sufficient to degrade through double-stranded RNA, whereas the
nuclease domain of RNase II is unable to carry out this reaction. The nuclease
domain of RNase R also binds RNA more tightly, which may explain the
difference in catalytic properties between RNase R and RNase II. 相似文献
11.
12.
Sok antisense RNA from plasmid R1 is functionally inactivated by RNase E and polyadenylated by poly(A) polymerase I 总被引:5,自引:3,他引:2
The hok/sok system of plasmid R1, which mediates plasmid stabilization by the killing of plasmid-free cells, codes for two RNA species, Sok antisense RNA and hok mRNA. Sok RNA, which is unstable, inhibits translation of the stable hok mRNA. The 64 nt Sok RNA folds into a single stem-loop domain with an 11 nt unstructured 5' domain. The initial recognition reaction between Sok RNA and hok mRNA takes place between the 5' domain and the complementary region in hok mRNA. In this communication we examine the metabolism of Sok antisense RNA. We find that RNase E cleaves the RNA 6 nt from its 5' end and that this cleavage initiates Sok RNA decay. The RNase E cleavage occurs in the part of Sok RNA that is responsible for the initial recognition of the target loop in hok mRNA and thus leads to functional inactivation of the antisense. The major RNase E cleavage product (denoted pSok-6 ) is rapidly degraded by polynucleotide phosphorylase (PNPase). Thus, the RNase E cleavage tags pSok−6 for further rapid degradation by PNPase from its 3' end. We also show that Sok RNA is polyadenylated by poly(A) polymerase I (PAP I), and that the poly(A)-tailing is prerequisite for the rapid 3'-exonucleolytic degradation by PNPase. 相似文献
13.
Bacterial polynucleotide phosphorylase (PNPase) plays a major role in mRNA turnover by the degradation of RNA from the 3′- to 5′-ends. Here, we determined the crystal structures of the wild-type and a C-terminal KH/S1 domain-truncated mutant (ΔKH/S1) of Escherichia coli PNPase at resolutions of 2.6 Å and 2.8 Å, respectively. The six RNase PH domains of the trimeric PNPase assemble into a ring-like structure containing a central channel. The truncated mutant ΔKH/S1 bound and cleaved RNA less efficiently with an eightfold reduced binding affinity. Thermal melting and acid-induced trimer dissociation studies, analyzed by circular dichroism and dynamic light scattering, further showed that ΔKH/S1 formed a less stable trimer than the full-length PNPase. The crystal structure of ΔKH/S1 is more expanded, containing a slightly wider central channel than that of the wild-type PNPase, suggesting that the KH/S1 domain helps PNPase to assemble into a more compact trimer, and it regulates the channel size allosterically. Moreover, site-directed mutagenesis of several arginine residues in the channel neck regions produced defective PNPases that either bound and cleaved RNA less efficiently or generated longer cleaved oligonucleotide products, indicating that these arginines were involved in RNA binding and processive degradation. Taking these results together, we conclude that the constricted central channel and the basic-charged residues in the channel necks of PNPase play crucial roles in trapping RNA for processive exonucleolytic degradation. 相似文献
14.
Rott R Zipor G Portnoy V Liveanu V Schuster G 《The Journal of biological chemistry》2003,278(18):15771-15777
The mechanism of RNA degradation in Escherichia coli involves endonucleolytic cleavage, polyadenylation of the cleavage product by poly(A) polymerase, and exonucleolytic degradation by the exoribonucleases, polynucleotide phosphorylase (PNPase) and RNase II. The poly(A) tails are homogenous, containing only adenosines in most of the growth conditions. In the chloroplast, however, the same enzyme, PNPase, polyadenylates and degrades the RNA molecule; there is no equivalent for the E. coli poly(A) polymerase enzyme. Because cyanobacteria is a prokaryote believed to be related to the evolutionary ancestor of the chloroplast, we asked whether the molecular mechanism of RNA polyadenylation in the Synechocystis PCC6803 cyanobacteria is similar to that in E. coli or the chloroplast. We found that RNA polyadenylation in Synechocystis is similar to that in the chloroplast but different from E. coli. No poly(A) polymerase enzyme exists, and polyadenylation is performed by PNPase, resulting in heterogeneous poly(A)-rich tails. These heterogeneous tails were found in the amino acid coding region, the 5' and 3' untranslated regions of mRNAs, as well as in rRNA and the single intron located at the tRNA(fmet). Furthermore, unlike E. coli, the inactivation of PNPase or RNase II genes caused lethality. Together, our results show that the RNA polyadenylation and degradation mechanisms in cyanobacteria and chloroplast are very similar to each other but different from E. coli. 相似文献
15.
Action of RNase II and polynucleotide phosphorylase against RNAs containing stem-loops of defined structure
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The 3'-->5' exoribonucleases, RNase II and polynucleotide phosphorylase (PNPase), play an essential role in degrading fragments of mRNA generated by prior cleavages by endonucleases. We have assessed the ability of small RNA substrates containing defined stem-loop structures and variable 3' extensions to impede the exonucleolytic activity of these enzymes. We find that stem-loops containing five G-C base pairs do not block either enzyme; in contrast, more stable stem-loops of 7, 9, or 11 bp block the processive action of both enzymes. Under conditions where enzyme activity is limiting, both enzymes stall and dissociate from their substrates six to nine residues, on average, from the base of a stable stem-loop structure. Our data provide a clear mechanistic explanation for the previous observation that RNase II and PNPase behave as functionally redundant. 相似文献
16.
17.
Rosenzweig JA Weltman G Plano GV Schesser K 《The Journal of biological chemistry》2005,280(1):156-163
Both low temperatures and encounters with host phagocytes are two stresses that have been relatively well studied in many species of bacteria. Previous work has shown that the exoribonuclease polynucleotide phosphorylase (PNPase) is required for Yersiniae to grow at low temperatures. Here, we show that PNPase also enhances the ability of Yersinia pseudotuberculosis and Yersinia pestis to withstand the killing activities of murine macrophages. PNPase is required for the optimal functioning of the Yersinia type three secretion system (TTSS), an organelle that injects effector proteins directly into host cells. Unexpectedly, the effect of PNPase on the TTSS is independent of its ribonuclease activity and instead requires its S1 RNA binding domain. In contrast, catalytically inactive enzyme does not enhance the low temperature growth effect of PNPase. Surprisingly, wild-type-like TTSS functioning was restored to the pnp mutant strain by expressing just the approximately 70 amino acid S1 domains from either PNPase, RNase R, RNase II, or RpsA. Our findings suggest that PNPase plays multifaceted roles in enhancing Yersinia survival in response to stressful conditions. 相似文献
18.
Ribosomal RNAs are generally stable in growing Escherichia coli cells. However, their degradation increases dramatically under conditions that lead to slow cell growth. In addition, incomplete RNA molecules and molecules with defects in processing, folding, or assembly are also eliminated in growing cells in a process termed quality control. Here, we show that there are significant differences between the pathways of ribosomal RNA degradation during glucose starvation and quality control during steady-state growth. In both processes, endonucleolytic cleavage of rRNA in ribosome subunits is an early step, resulting in accumulation of large rRNA fragments when the processive exoribonucleases, RNase II, RNase R, and PNPase are absent. For 23S rRNA, cleavage is in the region of helix 71, but the exact position can differ in the two degradative processes. For 16S rRNA, degradation during starvation begins with shortening of its 3' end in a reaction catalyzed by RNase PH. In the absence of this RNase, there is no 3' end trimming of 16S rRNA and no accumulation of rRNA fragments, and total RNA degradation is greatly reduced. In contrast, the degradation pattern in quality control remains unchanged when RNase PH is absent. During starvation, the exoribonucleases RNase II and RNase R are important for fragment removal, whereas for quality control, RNase R and PNPase are more important. These data highlight the similarities and differences between rRNA degradation during starvation and quality control during steady-state growth and describe a role for RNase PH in the starvation degradative pathway. 相似文献
19.
Schmier BJ Seetharaman J Deutscher MP Hunt JF Malhotra A 《Journal of molecular biology》2012,415(3):547-559
Exoribonucleases are vital in nearly all aspects of RNA metabolism, including RNA maturation, end-turnover, and degradation. RNase II and RNase R are paralogous members of the RNR superfamily of nonspecific, 3'→5', processive exoribonucleases. In Escherichia coli, RNase II plays a primary role in mRNA decay and has a preference for unstructured RNA. RNase R, in contrast, is capable of digesting structured RNA and plays a role in the degradation of both mRNA and stable RNA. Deinococcus radiodurans, a radiation-resistant bacterium, contains two RNR family members. The shorter of these, DrR63, includes a sequence signature typical of RNase R, but we show here that this enzyme is an RNase II-type exonuclease and cannot degrade structured RNA. We also report the crystal structure of this protein, now termed DrII. The DrII structure reveals a truncated RNA binding region in which the N-terminal cold shock domains, typical of most RNR family nucleases, are replaced by an unusual winged helix-turn-helix domain, where the "wing" is contributed by the C-terminal S1 domain. Consistent with its truncated RNA binding region, DrII is able to remove 3' overhangs from RNA molecules closer to duplexes than do other RNase II-type enzymes. DrII also displays distinct sensitivity to pyrimidine-rich regions of single-stranded RNA and is able to process tRNA precursors with adenosine-rich 3' extensions in vitro. These data indicate that DrII is the RNase II of D. radiodurans and that its structure and catalytic properties are distinct from those of other related enzymes. 相似文献
20.
In Escherichia coli, REP-stabilizers are structural elements in polycistronic messages that protect 5'-proximal cistrons from 3'-->5' exonucleolytic degradation. The stabilization of a protected cistron can be an important determinant in the level of gene expression. Our results suggest that RNase E, an endoribonuclease, initiates the degradation of REP-stabilized mRNA. However, subsequent degradation of mRNA fragments containing a REP-stabilizer poses a special challenge to the mRNA degradation machinery. Two enzymes, the DEAD-box RNA helicase, RhlB and poly(A) polymerase (PAP) are required to facilitate the degradation of REP-stabilizers by polynucleotide phosphorylase (PNPase). This is the first in vivo evidence that these enzymes are required for the degradation of REP-stabilizers. Furthermore, our results show that REP degradation by RhlB and PNPase requires their association with RNase E as components of the RNA degradosome, thus providing the first in vivo evidence that this ribonucleolytic multienzyme complex is involved in the degradation of structured mRNA fragments. 相似文献