An exoribonuclease from Saccharomyces cerevisiae: effect of modifications of 5' end groups on the hydrolysis of substrates to 5' mononucleotides

Eukaryotic mRNA decapping factors: molecular mechanisms and activity

Decapping is the enzymatic removal of 5' cap structures from mRNAs in eukaryotic cells. Cap structures normally enhance mRNA translation and stability, and their excision commits an mRNA to complete 5'-3' exoribonucleolytic digestion and generally ends the physical and functional cellular presence of the mRNA. Decapping plays a pivotal role in eukaryotic cytoplasmic mRNA turnover and is a critical and highly regulated event in multiple 5'-3' mRNA decay pathways, including general 5'-3' decay, nonsense-mediated mRNA decay (NMD), AU-rich element-mediated mRNA decay, microRNA-mediated gene silencing, and targeted transcript-specific mRNA decay. In the yeast Saccharomyces cerevisiae, mRNA decapping is carried out by a single Dcp1-Dcp2 decapping enzyme in concert with the accessory activities of specific regulators commonly known as decapping activators or enhancers. These regulatory proteins include the general decapping activators Edc1, 2, and 3, Dhh1, Scd6, Pat1, and the Lsm1-7 complex, as well as the NMD-specific factors, Upf1, 2, and 3. Here, we focus on in vivo mRNA decapping regulation in yeast. We summarize recently uncovered molecular mechanisms that control selective targeting of the yeast decapping enzyme and discuss new roles for specific decapping activators in controlling decapping enzyme targeting, assembly of target-specific decapping complexes, and the monitoring of mRNA translation. Further, we discuss the kinetic contribution of mRNA decapping for overall decay of different substrate mRNAs and highlight experimental evidence pointing to the functional coordination and physical coupling between events in mRNA deadenylation, decapping, and 5'-3' exoribonucleolytic decay.

A guard protein mediated quality control mechanism monitors 5'-capping of pre-mRNAs

Efficient gene expression requires properly matured mRNAs for functional transcript translation. Several factors including the guard proteins monitor maturation and act as nuclear retention factors for unprocessed pre-mRNAs. Here we show that the guard protein Npl3 monitors 5'-capping. In its absence, uncapped transcripts resist degradation, because the Rat1-Rai1 5'-end degradation factors are not efficiently recruited to these faulty transcripts. Importantly, in npl3Δ, these improperly capped transcripts escape this quality control checkpoint and leak into the cytoplasm. Our data suggest a model in which Npl3 associates with the Rai1 bound pre-mRNAs. In case the transcript was properly capped and is thus CBC (cap binding complex) bound, Rai1 dissociates from Npl3 allowing the export factor Mex67 to interact with this guard protein and support nuclear export. In case Npl3 does not detect proper capping through CBC attachment, Rai1 binding persists and Rat1 can join this 5'-complex to degrade the faulty transcript.

Transcription feedback dynamics in the wake of cytoplasmic mRNA degradation shutdown

In the last decade, multiple studies demonstrated that cells maintain a balance of mRNA production and degradation, but the mechanisms by which cells implement this balance remain unknown. Here, we monitored cells' total and recently-transcribed mRNA profiles immediately following an acute depletion of Xrn1-the main 5'-3' mRNA exonuclease-which was previously implicated in balancing mRNA levels. We captured the detailed dynamics of the adaptation to rapid degradation of Xrn1 and observed a significant accumulation of mRNA, followed by a delayed global reduction in transcription and a gradual return to baseline mRNA levels. We found that this transcriptional response is not unique to Xrn1 depletion; rather, it is induced earlier when upstream factors in the 5'-3' degradation pathway are perturbed. Our data suggest that the mRNA feedback mechanism monitors the accumulation of inputs to the 5'-3' exonucleolytic pathway rather than its outputs.

Unusual SMG suspects recruit degradation enzymes in nonsense-mediated mRNA decay

Degradation of eukaryotic RNAs that contain premature termination codons (PTC) during nonsense-mediated mRNA decay (NMD) is initiated by RNA decapping or endonucleolytic cleavage driven by conserved factors. Models for NMD mechanisms, including recognition of PTCs or the timing and role of protein phosphorylation for RNA degradation are challenged by new results. For example, the depletion of the SMG5/7 heterodimer, thought to activate RNA degradation by decapping, leads to a phenotype showing a defect of endonucleolytic activity of NMD complexes. This phenotype is not correlated to a decreased binding of the endonuclease SMG6 with the core NMD factor UPF1, suggesting that it is the result of an imbalance between active (e.g., in polysomes) and inactive (e.g., in RNA-protein condensates) states of NMD complexes. Such imbalance between multiple complexes is not restricted to NMD and should be taken into account when establishing causal links between gene function perturbation and observed phenotypes.

The Cap-Binding Complex CBC and the Eukaryotic Translation Factor eIF4E: Co-Conspirators in Cap-Dependent RNA Maturation and Translation

The translation of RNA into protein is a dynamic process which is heavily regulated during normal cell physiology and can be dysregulated in human malignancies. Its dysregulation can impact selected groups of RNAs, modifying protein levels independently of transcription. Integral to their suitability for translation, RNAs undergo a series of maturation steps including the addition of the m7G cap on the 5' end of RNAs, splicing, as well as cleavage and polyadenylation (CPA). Importantly, each of these steps can be coopted to modify the transcript signal. Factors that bind the m7G cap escort these RNAs through different steps of maturation and thus govern the physical nature of the final transcript product presented to the translation machinery. Here, we describe these steps and how the major m7G cap-binding factors in mammalian cells, the cap binding complex (CBC) and the eukaryotic translation initiation factor eIF4E, are positioned to chaperone transcripts through RNA maturation, nuclear export, and translation in a transcript-specific manner. To conceptualize a framework for the flow and integration of this genetic information, we discuss RNA maturation models and how these integrate with translation. Finally, we discuss how these processes can be coopted by cancer cells and means to target these in malignancy.

Processing of RNA Containing 8-Oxo-7,8-Dihydroguanosine (8-oxoG) by the Exoribonuclease Xrn-1

Understanding how oxidatively damaged RNA is handled intracellularly is of relevance due to the link between oxidized RNA and the progression/development of some diseases as well as aging. Among the ribonucleases responsible for the decay of modified (chemically or naturally) RNA is the exonuclease Xrn-1, a processive enzyme that catalyzes the hydrolysis of 5′-phosphorylated RNA in a 5′→3′ direction. We set out to explore the reactivity of this exonuclease towards oligonucleotides (ONs, 20-nt to 30-nt long) of RNA containing 8-oxo-7,8-dihydroguanosine (8-oxoG), obtained via solid-phase synthesis. The results show that Xrn-1 stalled at sites containing 8-oxoG, evidenced by the presence of a slower moving band (via electrophoretic analyses) than that observed for the canonical analogue. The observed fragment(s) were characterized via PAGE and MALDI-TOF to confirm that the oligonucleotide fragment(s) contained a 5′-phosphorylated 8-oxoG. Furthermore, the yields for this stalling varied from app. 5–30% with 8-oxoG located at different positions and in different sequences. To gain a better understanding of the decreased nuclease efficiency, we probed: 1) H-bonding and spatial constraints; 2) anti-syn conformational changes; 3) concentration of divalent cation; and 4) secondary structure. This was carried out by introducing methylated or brominated purines (m¹G, m^6,6A, or 8-BrG), probing varying [Mg²⁺], and using circular dichroism (CD) to explore the formation of structured RNA. It was determined that spatial constraints imposed by conformational changes around the glycosidic bond may be partially responsible for stalling, however, the results do not fully explain some of the observed higher stalling yields. We hypothesize that altered π-π stacking along with induced H-bonding interactions between 8-oxoG and residues within the binding site may also play a role in the decreased Xrn-1 efficiency. Overall, these observations suggest that other factors, yet to be discovered/established, are likely to contribute to the decay of oxidized RNA. In addition, Xrn-1 degraded RNA containing m¹G, and stalled mildly at sites where it encountered m^6,6A, or 8-BrG, which is of particular interest given that the former two are naturally occurring modifications.

Biochemical Characterization of Yeast Xrn1

Messenger RNA degradation is an important component of overall gene expression. During the final step of eukaryotic mRNA degradation, exoribonuclease 1 (Xrn1) carries out 5' → 3' processive, hydrolytic degradation of RNA molecules using divalent metal ion catalysis. To initiate studies of the 5' → 3' RNA decay machinery in our lab, we expressed a C-terminally truncated version of Saccharomyces cerevisiae Xrn1 and explored its enzymology using a second-generation, time-resolved fluorescence RNA degradation assay. Using this system, we quantitatively explored Xrn1's preference for 5'-monophosphorylated RNA substrates, its pH dependence, and the importance of active site mutations in the molecule's conserved catalytic core. Furthermore, we explore Xrn1's preference for RNAs containing a 5' single-stranded region both in an intermolecular hairpin structure and in an RNA-DNA hybrid duplex system. These results both expand and solidify our understanding of Xrn1, a centrally important enzyme whose biochemical properties have implications in numerous RNA degradation and processing pathways.

Functional RNA during Zika virus infection

Zika virus (ZIKV; family Flaviviridae; genus Flavivirus) is a pathogenic mosquito-borne RNA virus that currently threatens human health in the Americas, large parts of Asia and occasionally elsewhere in the world. ZIKV infection is often asymptomatic but can cause severe symptoms including congenital microcephaly and Guillain-Barré syndrome. The positive single-stranded RNA genome of the mosquito-borne ZIKV requires effective replication in two evolutionary distinct hosts - mosquitoes and primates. In addition to some of the viral proteins, the ZIKV genomic RNA and functional RNAs produced thereof aid in the establishment of productive infection and the evasion of host cell antiviral responses. ZIKV has evolved to contain a nucleotide composition and RNA modifications, such as methylation and the formation of G-quadruplexes that allow effective replication in both hosts. Furthermore, a number of host factors interact with the viral genome to modulate RNA replication. Importantly, the ZIKV genome produces non-coding subgenomic flavivirus RNA (sfRNA) due to stalling of host 5'- 3' ribonucleases on viral RNA structures in the 3' untranslated region (UTR). This sfRNA (sfRNA) exerts important proviral functions such as antagonizing the innate interferon response and RNA interference. Here, we discuss the ZIKV genomic RNA and functional RNAs thereof to assess their significance during ZIKV infection. Understanding the details of the ZIKV infection cycle will aid in the development of effective antiviral strategies and safe vaccines.

Ribosome-based quality control of mRNA and nascent peptides

Quality control processes are widespread and play essential roles in detecting defective molecules and removing them in order to maintain organismal fitness. Aberrant messenger RNA (mRNA) molecules, unless properly managed, pose a significant hurdle to cellular proteostasis. Often mRNAs harbor premature stop codons, possess structures that present a block to the translational machinery, or lack stop codons entirely. In eukaryotes, the three cytoplasmic mRNA-surveillance processes, nonsense-mediated decay (NMD), no-go decay (NGD), and nonstop decay (NSD), evolved to cope with these aberrant mRNAs, respectively. Nonstop mRNAs and mRNAs that inhibit translation elongation are especially problematic as they sequester valuable ribosomes from the translating ribosome pool. As a result, in addition to RNA degradation, NSD and NGD are intimately coupled to ribosome rescue in all domains of life. Furthermore, protein products produced from all three classes of defective mRNAs are more likely to malfunction. It is not surprising then that these truncated nascent protein products are subject to degradation. Over the past few years, many studies have begun to document a central role for the ribosome in initiating the RNA and protein quality control processes. The ribosome appears to be responsible for recognizing the target mRNAs as well as for recruiting the factors required to carry out the processes of ribosome rescue and nascent protein decay. WIREs RNA 2017, 8:e1366. doi: 10.1002/wrna.1366 For further resources related to this article, please visit the WIREs website.

Interaction of host cell microRNAs with the HCV RNA genome during infection of liver cells

It has remained an enigma how hepatitis C viral (HCV) RNA can persist in the liver of infected patients for many decades. With the recent discovery of roles for microRNAs in gene expression, it was reported that the HCV RNA genome subverts liver-specific microRNA miR-122 to protect its 5' end from degradation by host cell exoribonucleases. Sequestration of miR-122 in cultured liver cells and in the liver of chimpanzees by small, modified antisense RNAs resulted in dramatic loss of HCV RNA and viral yield. This finding led to the first successful human trial in which subcutaneous administration of antisense molecules against miR-122 lowered viral yield in HCV patients, without the emergence of resistant virus. In this review, the authors summarize the molecular mechanism by which miR-122 protects the HCV RNA genome from degradation by exoribonucleases Xrn1 and Xrn2 and discuss the application of miR-122 antisense molecules in the clinic.

mRNA quality control at the 5' end

All eukaryotic mRNAs are capped at their 5' end. Capping of mRNAs takes place co-transcriptionally and involves three steps. The intermediates of the capping process, as well as the uncapped 5' tri-phosphate RNA, are resistant to decapping and degradation by known factors, leading to the assumption that the capping process always proceeds to completion. This view was recently drastically changed. A novel family of enzymes, including the yeast proteins Rai1, Dxo1/Ydr370C, and the mammalian protein DXO/Dom3Z, has been identified. These enzymes catalyze the conversion of the improperly capped mRNAs to 5' mono-phosphate RNA, allowing them to be degraded by 5'-3' exoribonucleases. Several of these enzymes also possess 5'-3' exoribonuclease activities themselves, and can single-handedly clear the improperly capped mRNAs. Studying of these enzymes has led to the realization that mRNA capping does not always proceed to completion, and the identification of an mRNA capping quality control mechanism in eukaryotes. In this paper, we briefly review recent advances in this area.

XRN 5'→3' exoribonucleases: structure, mechanisms and functions

The XRN family of 5'→3' exoribonucleases is critical for ensuring the fidelity of cellular RNA turnover in eukaryotes. Highly conserved across species, the family is typically represented by one cytoplasmic enzyme (XRN1/PACMAN or XRN4) and one or more nuclear enzymes (XRN2/RAT1 and XRN3). Cytoplasmic and/or nuclear XRNs have proven to be essential in all organisms tested, and deficiencies can have severe developmental phenotypes, demonstrating that XRNs are indispensable in fungi, plants and animals. XRNs degrade diverse RNA substrates during general RNA decay and function in specialized processes integral to RNA metabolism, such as nonsense-mediated decay (NMD), gene silencing, rRNA maturation, and transcription termination. Here, we review current knowledge of XRNs, highlighting recent work of high impact and future potential. One example is the breakthrough in our understanding of how XRN1 processively degrades 5' monophosphorylated RNA, revealed by its crystal structure and mutational analysis. The expanding knowledge of XRN substrates and interacting partners is outlined and the functions of XRNs are interpreted at the organismal level using available mutant phenotypes. Finally, three case studies are discussed in more detail to underscore a few of the most exciting areas of research on XRN function: XRN4 involvement in small RNA-associated processes in plants, the roles of XRN1/PACMAN in Drosophila development, and the function of human XRN2 in nuclear transcriptional quality control. This article is part of a Special Issue entitled: RNA Decay mechanisms.

Structural and functional control of the eukaryotic mRNA decapping machinery

The regulation of mRNA degradation is critical for proper gene expression. Many major pathways for mRNA decay involve the removal of the 5' 7-methyl guanosine (m(7)G) cap in the cytoplasm to allow for 5'-to-3' exonucleolytic decay. The most well studied and conserved eukaryotic decapping enzyme is Dcp2, and its function is aided by co-factors and decapping enhancers. A subset of these factors can act to enhance the catalytic activity of Dcp2, while others might stimulate the remodeling of proteins bound to the mRNA substrate that may otherwise inhibit decapping. Structural studies have provided major insights into the mechanisms by which Dcp2 and decapping co-factors activate decapping. Additional mRNA decay factors can function by recruiting components of the decapping machinery to target mRNAs. mRNA decay factors, decapping factors, and mRNA substrates can be found in cytoplasmic foci named P bodies that are conserved in eukaryotes, though their function remains unknown. In addition to Dcp2, other decapping enzymes have been identified, which may serve to supplement the function of Dcp2 or act in independent decay or quality control pathways. This article is part of a Special Issue entitled: RNA Decay mechanisms.

Disengaging polymerase: terminating RNA polymerase II transcription in budding yeast

Termination of transcription by RNA polymerase II requires two distinct processes: The formation of a defined 3′ end of the transcribed RNA, as well as the disengagement of RNA polymerase from its DNA template. Both processes are intimately connected and equally pivotal in the process of functional messenger RNA production. However, research in recent years has elaborated how both processes can additionally be employed to control gene expression in qualitative and quantitative ways. This review embraces these new findings and attempts to paint a broader picture of how this final step in the transcription cycle is of critical importance to many aspects of gene regulation. This article is part of a Special Issue entitled: RNA polymerase II Transcript Elongation.

Structures of 5'-3' Exoribonucleases

5'-3' Exoribonucleases (XRNs) have important functions in RNA processing, RNA turnover and decay, RNA interference, RNA polymerase transcription, and other cellular processes. Their sequences share two highly conserved regions, CR1 and CR2. The cytoplasmic Xrn1 and the nuclear Xrn2/Rat1 are found in yeast and animals, and XRNs are found in most other eukaryotes. Crystal structures of Xrn1 and Rat1 have been reported recently, offering the first detailed information on these enzymes. The two conserved regions of XRNs form a single, large domain. CR1 has structural homology with the FEN superfamily of nucleases, while CR2 restricts access to the active site, ensuring that XRNs are exclusive exoribonucleases. The structure of Rai1, the protein partner of Rat1, revealed the presence of an active site, and further studies demonstrated that this activity is a novel mechanism for mRNA 5'-end capping quality surveillance.

XRN1: A Major 5' to 3' Exoribonuclease in Eukaryotic Cells

The degradation of RNA is a critical aspect of gene regulation. Correspondingly, ribonucleases exist within the cell to degrade RNA in specific cellular contexts. An important and conserved ribonuclease is called XRN1. This enzyme, an exoribonuclease, degrades RNA in a processive 5' to 3' direction. Substrates for XRN1 include decapped mRNA, endonucleolytically cleaved mRNA, lncRNA, and some aberrant tRNAs. In addition, XRN1 serves a vital role in the processing and maturation of the 5' ends of rRNA and snoRNAs. In this review, we discuss some of the important roles of XRN1 in the cell.

Dxo1 is a new type of eukaryotic enzyme with both decapping and 5'-3' exoribonuclease activity

Recent studies showed that Rai1 is a crucial component of the mRNA 5′-end capping quality control mechanism in yeast. The yeast genome encodes a weak homolog of Rai1, Ydr370C, but little is known about this protein. Here we report the crystal structures of Kluyveromyces lactis Ydr370C and the first biochemical and functional studies on this protein. The overall structure of Ydr370C is similar to Rai1. Ydr370C has robust decapping activity on RNAs with unmethylated caps but it has no detectable pyrophosphohydrolase activity. Unexpectedly, Ydr370C also possesses distributive, 5′-3′ exoribonuclease activity, and we propose the name Dxo1 for this novel eukaryotic enzyme with both decapping and exonuclease activities. Studies in yeast where both Dxo1 and Rai1 are disrupted reveal that mRNAs with incomplete caps are produced even under normal growth conditions, in sharp contrast to current understanding of the capping process.

An RNA pseudoknot is required for production of yellow fever virus subgenomic RNA by the host nuclease XRN1

Cells and mice infected with arthropod-borne flaviviruses produce a small subgenomic RNA that is colinear with the distal part of the viral 3'-untranslated region (UTR). This small subgenomic flavivirus RNA (sfRNA) results from the incomplete degradation of the viral genome by the host 5'-3' exonuclease XRN1. Production of the sfRNA is important for the pathogenicity of the virus. This study not only presents a detailed description of the yellow fever virus (YFV) sfRNA but, more importantly, describes for the first time the molecular characteristics of the stalling site for XRN1 in the flavivirus genome. Similar to the case for West Nile virus, the YFV sfRNA was produced by XRN1. However, in contrast to the case for other arthropod-borne flaviviruses, not one but two sfRNAs were detected in YFV-infected mammalian cells. The smaller of these two sfRNAs was not observed in infected mosquito cells. The larger sfRNA could also be produced in vitro by incubation with purified XRN1. These two YFV sfRNAs formed a 5'-nested set. The 5' ends of the YFV sfRNAs were found to be just upstream of the previously predicted RNA pseudoknot PSK3. RNA structure probing and mutagenesis studies provided strong evidence that this pseudoknot structure was formed and served as the molecular signal to stall XRN1. The sequence involved in PSK3 formation was cloned into the Sinrep5 expression vector and shown to direct the production of an sfRNA-like RNA. These results underscore the importance of the RNA pseudoknot in stalling XRN1 and also demonstrate that it is the sole viral requirement for sfRNA production.

Torpedo nuclease Rat1 is insufficient to terminate RNA polymerase II in vitro

Termination of RNA polymerase (pol) II transcription in vivo requires the 5'-RNA exonuclease Rat1. It was proposed that Rat1 degrades RNA from the 5'-end that is created by transcript cleavage, catches up with elongating pol II, and acts like a Torpedo that removes pol II from DNA. Here we test the Torpedo model in an in vitro system based on bead-coupled pol II elongation complexes (ECs). Recombinant Rat1 complexes with Rai1, and with Rai1 and Rtt103, degrade RNA extending from the EC until they reach the polymerase surface but fail to terminate pol II. Instead, the EC retains an approximately 18-nucleotide RNA that remains with its 3'-end at the active site and can be elongated. Thus, pol II termination apparently requires a factor or several factors in addition to Rat1, Rai1, and Rtt103, post-translational modifications of these factors, or unusual reaction conditions.

Studies of the 5' exonuclease and endonuclease activities of CPSF-73 in histone pre-mRNA processing

Processing of histone pre-mRNA requires a single 3' endonucleolytic cleavage guided by the U7 snRNP that binds downstream of the cleavage site. Following cleavage, the downstream cleavage product (DCP) is rapidly degraded in vitro by a nuclease that also depends on the U7 snRNP. Our previous studies demonstrated that the endonucleolytic cleavage is catalyzed by the cleavage/polyadenylation factor CPSF-73. Here, by using RNA substrates with different nucleotide modifications, we characterize the activity that degrades the DCP. We show that the degradation is blocked by a 2'-O-methyl nucleotide and occurs in the 5'-to-3' direction. The U7-dependent 5' exonuclease activity is processive and continues degrading the DCP substrate even after complete removal of the U7-binding site. Thus, U7 snRNP is required only to initiate the degradation. UV cross-linking studies demonstrate that the DCP and its 5'-truncated version specifically interact with CPSF-73, strongly suggesting that in vitro, the same protein is responsible for the endonucleolytic cleavage of histone pre-mRNA and the subsequent degradation of the DCP. By using various RNA substrates, we define important space requirements upstream and downstream of the cleavage site that dictate whether CPSF-73 functions as an endonuclease or a 5' exonuclease. RNA interference experiments with HeLa cells indicate that degradation of the DCP does not depend on the Xrn2 5' exonuclease, suggesting that CPSF-73 degrades the DCP both in vitro and in vivo.

Recombination and RNA processing: a common strand?

Genetic recombination is a basic cellular process required for altering genome structure. The RecA protein of Escherichia coli has a central role in homologous recombination, and a eukaryotic protein with similar properties has been discovered in the yeast Saccharomyces cerevisiae. Unexpectedly, this RecA-like protein has additional biochemical activities, and its function may not be restricted to recombination.

A nuclear protein in Schizosaccharomyces pombe with homology to the human tumour suppressor Fhit has decapping activity

A number of eukaryotic proteins are already known to orchestrate key steps of mRNA metabolism and translation via interactions with the 5' m7GpppN cap. We have characterized a new type of histidine triad (HIT) motif protein (Nhm1) that co-purifies with the cap-binding complex eIF4F of Schizosaccharomyces pombe. Nhm1 is an RNA-binding protein that binds to m7GTP-Sepharose, albeit with lower specificity and affinity for methylated GTP than is typical for the cap-binding protein known as eukaryotic initiation factor 4E. Sequence searches have revealed that proteins with strong sequence similarity over all regions of the new protein exist in a wide range of eukaryotes, yet none has been characterized up to now. However, other proteins that share specific motifs with Nhm1 include the human Fhit tumour suppressor protein and the diadenosine 5', 5"'-P1, P4-tetraphosphate asymmetrical hydrolase of S. pombe. Our experimental work also reveals that Nhm1 inhibits translation in a cell-free extract prepared from S. pombe, and that it is therefore a putative translational modulator. On the other hand, purified Nhm1 manifests mRNA decapping activity, yet is physically distinct from the Saccharomyces cerevisiae decapping enzyme Dcp1. Moreover, fluorescence and immunofluorescence microscopy show that Nhm1 is predominantly, although not exclusively, nuclear. We conclude that Nhm1 has evolved as a special branch of the HIT motif superfamily that has the potential to influence both the metabolism and the translation of mRNA, and that its presence in S. pombe suggests the utilization of a novel decapping pathway.

Modulation of eukaryotic mRNA stability via the cap-binding translation complex eIF4F

Decapping by Dcp1 in Saccharomyces cerevisiae is a key step in mRNA degradation. However, the cap also binds the eukaryotic initiation factor (eIF) complex 4F and its associated proteins. Characterisation of the relationship between decapping and interactions involving eIF4F is an essential step towards understanding polysome disassembly and mRNA decay. Three types of observation suggest how changes in the functional status of eIF4F modulate mRNA stability in vivo. First, partial disruption of the interaction between eIF4E and eIF4G, caused by mutations in eIF4E or the presence of the yeast 4E-binding protein p20, stabilised mRNAs. The interactions of eIF4G and p20 with eIF4E may therefore act to modulate the decapping process. Since we also show that the in vitro decapping rate is not directly affected by the nature of the body of the mRNA, this suggests that changes in eIF4F structure could play a role in triggering decapping during mRNA decay. Second, these effects were seen in the absence of extreme changes in global translation rates in the cell, and are therefore relevant to normal mRNA turnover. Third, a truncated form of eIF4E (Delta196) had a reduced capacity to inhibit Dcp1-mediated decapping in vitro, yet did not change cellular mRNA half-lives. Thus, the accessibility of the cap to Dcp1 in vivo is not simply controlled by competition with eIF4E, but is subject to switching between molecular states with different levels of access.

Exoribonucleases and their multiple roles in RNA metabolism

In recent years there has been a dramatic shift in our thinking about ribonucleases (RNases). Although they were once considered to be nonspecific, degradative enzymes, it is now clear that RNases play a central role in every aspect of cellular RNA metabolism, including decay of mRNA, conversion of RNA precursors to their mature forms, and end-turnover of certain RNAs. Recognition of the importance of this class of enzymes has led to an explosion of work and the establishment of significant new concepts. Thus, we now realize that RNases, both endoribonucleases and exoribonucleases, can be highly specific for particular sequences or structures. It has also become apparent that a single cell can contain a large number of distinct RNases, approaching as many as 20 members, often with overlapping specificities. Some RNases also have been found to be components of supramolecular complexes and to function in concert with other enzymes to carry out their role in RNA metabolism. This review focuses on the exoribonucleases, both prokaryotic and eukaryotic, and details their structure, catalytic properties, and physiological function.

Active-site mutations in the Xrn1p exoribonuclease of Saccharomyces cerevisiae reveal a specific role in meiosis

Xrn1p of Saccharomyces cerevisiae is a major cytoplasmic RNA turnover exonuclease which is evolutionarily conserved from yeasts to mammals. Deletion of the XRN1 gene causes pleiotropic phenotypes, which have been interpreted as indirect consequences of the RNA turnover defect. By sequence comparisons, we have identified three loosely defined, common 5'-3' exonuclease motifs. The significance of motif II has been confirmed by mutant analysis with Xrn1p. The amino acid changes D206A and D208A abolish singly or in combination the exonuclease activity in vivo. These mutations show separation of function. They cause identical phenotypes to that of xrn1Delta in vegetative cells but do not exhibit the severe meiotic arrest and the spore lethality phenotype typical for the deletion. In addition, xrn1-D208A does not cause the severe reduction in meiotic popout recombination in a double mutant with dmc1 as does xrn1Delta. Biochemical analysis of the DNA binding, exonuclease, and homologous pairing activity of purified mutant enzyme demonstrated the specific loss of exonuclease activity. However, the mutant enzyme is competent to promote in vitro assembly of tubulin into microtubules. These results define a separable and specific function of Xrn1p in meiosis which appears unrelated to its RNA turnover function in vegetative cells.

Assays for analyzing exonucleases in vitro

Ribonucleases play essential roles in cell growth, differentiation, and the response to stress. This article deals with exoribonucleases, enzymes that degrade RNAs beginning at either the 5' or 3' end and proceed down the length of the RNA. The preparation of a crude extract of a mammalian 3'-to-5' exonuclease is described. Assay conditions for both 5'-to-3' and 3'-to-5' exonucleases are given. One of these is a yeast enzyme that is known to be involved in mRNA decay. Others are vertebrate exonucleases that are presumed to have a role in mRNA stability but have not yet been proven to do so.

Posttranscriptional control of gene expression in yeast

Studies of the budding yeast Saccharomyces cerevisiae have greatly advanced our understanding of the posttranscriptional steps of eukaryotic gene expression. Given the wide range of experimental tools applicable to S. cerevisiae and the recent determination of its complete genomic sequence, many of the key challenges of the posttranscriptional control field can be tackled particularly effectively by using this organism. This article reviews the current knowledge of the cellular components and mechanisms related to translation and mRNA decay, with the emphasis on the molecular basis for rate control and gene regulation. Recent progress in characterizing translation factors and their protein-protein and RNA-protein interactions has been rapid. Against the background of a growing body of structural information, the review discusses the thermodynamic and kinetic principles that govern the translation process. As in prokaryotic systems, translational initiation is a key point of control. Modulation of the activities of translational initiation factors imposes global regulation in the cell, while structural features of particular 5' untranslated regions, such as upstream open reading frames and effector binding sites, allow for gene-specific regulation. Recent data have revealed many new details of the molecular mechanisms involved while providing insight into the functional overlaps and molecular networking that are apparently a key feature of evolving cellular systems. An overall picture of the mechanisms governing mRNA decay has only very recently begun to develop. The latest work has revealed new information about the mRNA decay pathways, the components of the mRNA degradation machinery, and the way in which these might relate to the translation apparatus. Overall, major challenges still to be addressed include the task of relating principles of posttranscriptional control to cellular compartmentalization and polysome structure and the role of molecular channelling in these highly complex expression systems.

Ribonuclease E is a 5'-end-dependent endonuclease

The selective degradation of messenger RNAs enables cells to regulate the levels of particular mRNAs in response to changes in the environment. Ribonuclease (RNase) E, a single-strand-specific endonuclease that is found in a multi-enzyme complex known as the 'degradosome', initiates the degradation of many mRNAs in Escherichia coli. Its relative lack of sequence specificity and the presence of many potential cleavage sites in mRNA substrates cannot explain why mRNA decay frequently proceeds in a net 5'-to-3' direction. I have prepared covalently closed circular derivatives of natural substrates, the rpsT mRNA encoding ribosomal protein S20 and the 9S precursor to 5S ribosomal RNA, and find that these derivatives are considerably more resistant to cleavage in vitro by RNase E than are linear molecules. Moreover, antisense oligo-deoxynucleotides complementary to the 5' end of linear substrates significantly reduce the latter's susceptibility to attack by RNase E. Finally, natural substrates with terminal 5'-triphosphate groups are poorly cleaved by RNase E in vitro, whereas 5' monophosphorylated substrates are strongly preferred. These results show that RNase E has inherent vectorial properties, with its activity depending on the 5' end of its substrates; this can account for the direction of mRNA decay in E. coli, the phenomenon of 'all or none' mRNA decay, and the stabilization provided by 5' stem-loop structures.

A mouse cytoplasmic exoribonuclease (mXRN1p) with preference for G4 tetraplex substrates

Exoribonucleases are important enzymes for the turnover of cellular RNA species. We have isolated the first mammalian cDNA from mouse demonstrated to encode a 5'-3' exoribonuclease. The structural conservation of the predicted protein and complementation data in Saccharomyces cerevisiae suggest a role in cytoplasmic mRNA turnover and pre-rRNA processing similar to that of the major cytoplasmic exoribonuclease Xrn1p in yeast. Therefore, a key component of the mRNA decay system in S. cerevisiae has been conserved in evolution from yeasts to mammals. The purified mouse protein (mXRN1p) exhibited a novel substrate preference for G4 RNA tetraplex-containing substrates demonstrated in binding and hydrolysis experiments. mXRN1p is the first RNA turnover function that has been localized in the cytoplasm of mammalian cells. mXRN1p was distributed in small granules and was highly enriched in discrete, prominent foci. The specificity of mXRN1p suggests that RNAs containing G4 tetraplex structures may occur in vivo and may have a role in RNA turnover.

mRNA turnover in yeast promoted by the MATalpha1 instability element

The decay rates of eukaryotic transcripts can be determined by sequence elements within an mRNA. One example of this phenomenon is the rapid degradation of the yeast MATalpha1 mRNA, which is promoted by a 65 nt segment of its coding region termed the MATalpha1 instability element (MIE). The MIE is also capable of destabilizing the stable PGK1 transcript. To determine how the MIE accelerates mRNA turnover we examined the mechanism of degradation of the MATalpha1 transcript. These experiments indicated that the MATalpha1 mRNA was degraded by a deadenylation-dependent decapping reaction which exposed the transcript to 5'-->3' exonucleolytic digestion. Deletion of the MIE from the MATalpha1 mRNA decreased the rate at which this mRNA was decapped. In contrast, insertion of the MIE into the PGK1 transcript caused an increase in the rate of deadenylation of the resulting chimeric mRNA. These observations suggest that the MIE promotes rapid mRNA decay by increasing the rates of deadenylation and decapping, with its primary effect on mRNA turnover depending on additional features of a given transcript. These results also strengthen the hypothesis that deadenylation-dependent decapping is a common pathway of mRNA decay in yeast and indicate that an instability element within the coding region of an mRNA can effect nucleolytic events that occur at both the 5'- and 3'-ends of an mRNA.

Purification and characterization of a 5' to 3' exoribonuclease from rabbit reticulocytes that degrades capped and uncapped RNAs

The cytoplasm of mammalian cells of undoubtedly contain a number of different ribonuclease activities, few if any of which have been well characterized. We describe the purification of an exoribonuclease from rabbit reticulocytes which is able to degrade capped RNAs in a 5' to 3' manner. The purified enzyme contains polypeptides of 62 and 58 kDa and may contain an additional polypeptide of 54 kDa. It behaves as a complex of 150 kDa when analyzed by HPLC gel retardation on Superdex 200HR. It is heat-labile, dependent upon divalent cations (Mg2+) for activity, resistant to placental ribonuclease inhibitor, and active over a broad range (10-200 mM) of monovalent cation (K+) concentrations. The enzyme requires a polynucleotide chain of at least 10 bases for activity and cleaves oligonucleotides, up to an octamer long, from the 5' end of an appropriate substrate. In the case of a capped RNA substrate, product analysis by TLC and PAGE indicates that a capped trinucleotide or tetranucleotide or both is produced. Examination of the kinetics of the enzyme with capped and triphosphate-terminated substrates shows that that the cap structure inhibits the action of the enzyme. Furthermore, the data suggest that the rate-limiting step involves the positioning of the enzyme at the 5' end of the substrate and/or cleavage of the first internucleotide bond.

Identification of functional domains in the Sep1 protein (= Kem1, Xrn1), which is required for transition through meiotic prophase in Saccharomyces cerevisiae

The Sep1 (also known as Kem1, Xrn1, Rar5, DST2/Stpbeta) protein of Saccharomyces cerevisiae is an Mr 175,000 multifunctional exonuclease with suspected roles in RNA turnover and in the microtubular cytoskeleton as well as in DNA recombination and DNA replication. The most striking phenotype of SEP1 null mutations is quantitative arrest during meiotic prophase at the pachytene stage. We have constructed a set of N- and C-terminal as well as internal deletions of the large SEP1 gene. Analysis of these deletion mutations on plasmids in a host carrying a null allele (sep1 ) revealed that at least 270 amino acids from the C-terminus of the wild-type protein were dispensable for complementing the slow growth and benomyl hypersensitivity of a null mutant. In contrast, any deletion at the N-terminus abrogated complementing activity for these phenotypes. The sequences essential for function correspond remarkably well with the regions of Sep1 that are homologous to its Schizosaccharomyces pombe counterpart Exo2. In addition, these experiments showed that, despite the high intracellular levels of Sep1, over-expression of this protein above these levels is detrimental to the cell. We discuss the potential cellular roles of the Sep1 protein as a microtubule-nucleic acid interface protein linking its suspected function in the microtubular cytoskeleton with its role as a nucleic acid binding protein.

mRNA stability in mammalian cells

This review concerns how cytoplasmic mRNA half-lives are regulated and how mRNA decay rates influence gene expression. mRNA stability influences gene expression in virtually all organisms, from bacteria to mammals, and the abundance of a particular mRNA can fluctuate manyfold following a change in the mRNA half-life, without any change in transcription. The processes that regulate mRNA half-lives can, in turn, affect how cells grow, differentiate, and respond to their environment. Three major questions are addressed. Which sequences in mRNAs determine their half-lives? Which enzymes degrade mRNAs? Which (trans-acting) factors regulate mRNA stability, and how do they function? The following specific topics are discussed: techniques for measuring eukaryotic mRNA stability and for calculating decay constants, mRNA decay pathways, mRNases, proteins that bind to sequences shared among many mRNAs [like poly(A)- and AU-rich-binding proteins] and proteins that bind to specific mRNAs (like the c-myc coding-region determinant-binding protein), how environmental factors like hormones and growth factors affect mRNA stability, and how translation and mRNA stability are linked. Some perspectives and predictions for future research directions are summarized at the end.

Regulation and intracellular localization of Saccharomyces cerevisiae strand exchange protein 1 (Sep1/Xrn1/Kem1), a multifunctional exonuclease

The Saccharomyces cerevisiae strand exchange protein 1 (Sep1; also referred to as Xrn1, Kem1, Rar5, or Stp beta) catalyzes the formation of hybrid DNA from model substrates in vitro. The protein is also a 5'-to-3' exonuclease active on DNA and RNA. Multiple roles for the in vivo function of Sep1, ranging from DNA recombination and cytoskeleton to RNA turnover, have been proposed. We show that Sep1 is an abundant protein in vegetative S. cerevisiae cells, present at about 80,000 molecules per diploid cell. Protein levels were not changed during the cell cycle or in response to DNA-damaging agents but increased twofold during meiosis. Cell fractionation and indirect immunofluorescence studies indicated that > 90% of Sep1 was cytoplasmic in vegetative cells, and indirect immunofluorescence indicated a cytoplasmic localization in meiotic cells as well. The localization supports the proposal that Sep1 has a role in cytoplasmic RNA metabolism. Anti-Sep1 monoclonal antibodies detected cross-reacting antigens in the fission yeast Schizosccharomyces pombe, in Drosophila melanogaster embryos, in Xenopus laevis, and in a mouse pre-B-cell line.

Synthetic lethality of sep1 (xrn1) ski2 and sep1 (xrn1) ski3 mutants of Saccharomyces cerevisiae is independent of killer virus and suggests a general role for these genes in translation control

Strand exchange protein 1 (Sep1) (also referred to as exoribonuclease I [Xrn1]) from Saccharomyces cerevisiae has been implicated in DNA recombination, RNA turnover, karyogamy, and G4 DNA pairing among other disparate cellular processes. Using a genetic approach to study the role of SEP1/XRN1 in mitotic yeast cells, we identified mutations in the genes superkiller 2 (SKI2) and superkiller 3 (SKI3) as synthetically lethal with an sep1 null mutation. The SKI genes are thought to comprise an intracellular antiviral system controlling the expression of killer toxin from double-stranded RNA virus found in many yeast strains. However, the lethality of sep1 ski2 and sep1 ski3 mutants was independent of the L-A and M viruses, suggesting that the SKI genes act in a general cellular process in addition to virus control. We propose that Sep1/Xrn1 and Ski2 both act to block translation on transcripts targeted for degradation. Using a temperature-sensitive allele of SEP1/XRN1, we show that double mutants display a synthetic cell cycle arrest in late G1 at Start.

Deadenylation of the unstable mRNA encoded by the yeast MFA2 gene leads to decapping followed by 5'-->3' digestion of the transcript

The first step in the decay of some eukaryotic mRNAs is the shortening of the poly(A) tail. To examine how the transcript body was degraded after deadenylation, we followed the decay of a pulse of newly synthesized MFA2 transcripts while utilizing two strategies to trap intermediates in the degradation pathway. First, we inserted strong RNA secondary structures, which can slow exonucleolytic digestion and thereby trap decay intermediates, into the MFA2 5' UTR. Following deadenylation, fragments of the MFA2 mRNA trimmed from the 5' end to the site of secondary structure accumulated as full-length mRNA levels decreased. In addition, in cells deleted for the XRN1 gene, which encodes a major 5' to 3' exonuclease in yeast, the MFA2 transcript is deadenylated normally but persists as a full-length mRNA lacking the 5' cap structure. These results define a mRNA decay pathway in which deadenylation leads to decapping of the mRNA followed by 5'-->3' exonucleolytic degradation of the transcript body. Because the poly(A) tail and the cap structure are found on essentially all mRNAs, this pathway could be a general mechanism for the decay of many eukaryotic transcripts.

Yeast cells lacking 5'-->3' exoribonuclease 1 contain mRNA species that are poly(A) deficient and partially lack the 5' cap structure

Analysis of the slowed turnover rates of several specific mRNA species and the higher cellular levels of some of these mRNAs in Saccharomyces cerevisiae lacking 5'-->3' exoribonuclease 1 (xrn1 cells) has led to the finding that these yeast contain higher amounts of essentially full-length mRNAs that do not bind to oligo(dT)-cellulose. On the other hand, the length of mRNA poly(A) chains found after pulse-labeling of cells lacking the exoribonuclease, the cellular rate of synthesis of oligo(dT)-bound mRNA, and the initial rate of its deadenylation appeared quite similar to the same measurements in wild-type yeast cells. Examination of the 5' cap structure status of the poly(A)-deficient mRNAs by comparative analysis of the m7G content of poly(A)- and poly(A)+ RNA fractions of wild-type and xrn1 cells suggested that the xrn1 poly(A)- mRNA fraction is low in cap structure content. Further analysis of the 5' termini by measurements of the rate of 5'-->3' exoribonuclease 1 hydrolysis of specific full-length mRNA species showed that approximately 50% of the xrn1 poly(A)-deficient mRNA species lack the cap structure. Primer extension analysis of the 5' terminus of ribosomal protein 51A (RP51A) mRNA showed that about 30% of the poly(A)-deficient molecules of the xrn1 cells are slightly shorter at the 5' end. The finding of some accumulation of poly(A)-deficient mRNA species partially lacking the cap structure together with the reduction of the rate of mRNA turnover in cells lacking the enzyme suggest a possible role for 5'-->3' exoribonuclease 1 in the mRNA turnover process.

Yeast cells lacking 5'-->3' exoribonuclease 1 contain mRNA species that are poly(A) deficient and partially lack the 5' cap structure

Analysis of the slowed turnover rates of several specific mRNA species and the higher cellular levels of some of these mRNAs in Saccharomyces cerevisiae lacking 5'-->3' exoribonuclease 1 (xrn1 cells) has led to the finding that these yeast contain higher amounts of essentially full-length mRNAs that do not bind to oligo(dT)-cellulose. On the other hand, the length of mRNA poly(A) chains found after pulse-labeling of cells lacking the exoribonuclease, the cellular rate of synthesis of oligo(dT)-bound mRNA, and the initial rate of its deadenylation appeared quite similar to the same measurements in wild-type yeast cells. Examination of the 5' cap structure status of the poly(A)-deficient mRNAs by comparative analysis of the m7G content of poly(A)- and poly(A)+ RNA fractions of wild-type and xrn1 cells suggested that the xrn1 poly(A)- mRNA fraction is low in cap structure content. Further analysis of the 5' termini by measurements of the rate of 5'-->3' exoribonuclease 1 hydrolysis of specific full-length mRNA species showed that approximately 50% of the xrn1 poly(A)-deficient mRNA species lack the cap structure. Primer extension analysis of the 5' terminus of ribosomal protein 51A (RP51A) mRNA showed that about 30% of the poly(A)-deficient molecules of the xrn1 cells are slightly shorter at the 5' end. The finding of some accumulation of poly(A)-deficient mRNA species partially lacking the cap structure together with the reduction of the rate of mRNA turnover in cells lacking the enzyme suggest a possible role for 5'-->3' exoribonuclease 1 in the mRNA turnover process.

Characterization of the XRN1 gene encoding a 5'-->3' exoribonuclease: sequence data and analysis of disparate protein and mRNA levels of gene-disrupted yeast cells

Sequencing of the XRN1 gene of Saccharomyces cerevisiae, cloned in this laboratory as a gene encoding a 160-kDa 5'-->3' exoribonuclease (XRN1), shows that it is identical to a gene (DST2 or SEP1) encoding a DNA strand transferase and to genes involved in nuclear fusion, KEM1, and plasmid stability, RAR5. To better understand the various phenotypes associated with loss of XRN1 and the enzymatic activities associated with the protein, certain characteristics of our yeast cells lacking an active gene (xrn1) have been examined. Cells are larger (average volume is x 1.5-1.8) and have an increased doubling time (x1.9-2.1). The protein synthesis rate per cell is 80-90% that of wild-type (wt) cells, and the resultant cellular protein levels are higher. The rate of the 25S and 18S rRNA synthesis is approximately 45% that of wt cells and its cellular level is about 90% that of wt cells. Levels of protein bands resolved by one-dimensional PAGE show substantial differences. Synthesis rates observed for the same protein bands, as well as measurements of several specific mRNA levels by Northern analysis, suggest disparities in mRNA levels. Results show two to four times longer half-lives of specific short-lived mRNAs. The variations in levels of protein and RNA species found in the xrn1 cells may be the cause of some of the phenotypes found associated with gene loss.

Turnover of mRNA in prokaryotes and lower eukaryotes

The turnover of mRNA plays an important role in the regulation of gene expression. The two best understood model systems are those of the prokaryote Escherichia coli and the lower eukaryote Saccharomyces cerevisiae. Considerable progress in recent years has helped define the general pathways by which mRNA is degraded in E coli. Much less is known about the pathways of decay, or the enzymes involved, in eukaryotic cells. However, both cis-acting sequences and trans-acting factors have recently been characterized in S. cerevisiae and an indispensable role for translation has been identified. A comparison of these model species highlights both similarities and differences in mRNA turnover between prokaryotic and eukaryotic systems.

Disruption of the gene XRN1, coding for a 5'----3' exoribonuclease, restricts yeast cell growth

As a step toward determining the metabolic role(s) of a 5'----3' exoribonuclease (XRN1), a yeast gene, XRN1, encoding XRN1, was first cloned, then disrupted to test its essentially or effect on yeast cell growth. Clones in the high-copy-number plasmid YEp24 cause overproduction (fivefold) of XRN1 in yeast cells, as measured by either poly(A) hydrolytic activity or immunoreactivity. Restriction mapping and deletion analysis showed that the XRN1 gene is located on a 6.7-kb XbaI-XhoI fragment of chromosome VII. The normal gene was disrupted in two haploid yeast strains by integrating a fragment with a BglII-deleted segment replaced with the yeast URA3 gene, and the disrupted strains lack XRN1. Successful transformation of haploid cells showed that the gene is not essential, but its absence markedly affected the cell growth rate. The growth defect is corrected by introduction of the XRN1 gene on a plasmid back into the disrupted yeast.

A 5'----3' exoribonuclease of Saccharomyces cerevisiae: size and novel substrate specificity

The purification scheme for a 5'----3' exoribonuclease of Saccharomyces cerevisiae has been modified to facilitate purification of larger amounts of enzyme and further extended to yield highly purified enzyme by use of poly(A)-agarose chromatography. As determined by either sodium dodecyl sulfate-polyacrylamide gel electrophoresis or physical characterization, the enzyme has a molecular weight of about 160,000. Further studies of its substrate specificity show that poly(C) and poly(U) preparations require 5' phosphorylation for activity and that poly(A) with a 5'-triphosphate end group is hydrolyzed at only 12% of the rate of poly(A) with a 5'-monophosphate end group. DNA is not hydrolyzed, but synthetic polydeoxyribonucleotides are strong competitive inhibitors of the hydrolysis of noncomplementary ribopolymers. Poly(A).poly(U) and poly(A).poly(dT) are hydrolyzed at 60 and 50%, respectively, of the rate of poly(A) at 37 degrees C. The RNase H activity of the enzyme can also be demonstrated using an RNA X M13 DNA hybrid as a substrate. When poly(dT).poly(dA) with a 5'-terminal poly(A) segment on the poly(dA) is used as a substrate, the enzyme hydrolyzes the poly(A) "tail," removing the last ribonucleotide, but does not hydrolyze the poly(dA).

A 5'----3' exoribonuclease of human placental nuclei: purification and substrate specificity

An exoribonuclease that hydrolyzes single-stranded RNA by a 5'----3' mode yielding 5'-mononucleotides has been purified from human placental nuclei. Chromatographic studies of crude placental nuclear extracts suggest that the enzyme is a relatively abundant nuclear RNase. Poly(A) is degraded by a processive mechanism while rRNA is degraded in a partially non-processive manner, possibly because of its secondary structure. The enzyme has an apparent molecular weight of 113,000, derived from determinations of the Stokes radius (43 A) and sedimentation coefficient (6.3 S). Substrates with 5'-phosphomonoester end groups are 10-20 times better than 5'-dephosphorylated substrates. The locale of the enzyme in nuclei of normal human cells as well as its mode of action suggest a role in nuclear RNA processing or turnover.