ASCC1 structures and bioinformatics reveal a novel helix-clasp-helix RNA-binding motif linked to a two-histidine phosphodiesterase.

Activating signal co-integrator complex 1 (ASCC1) acts with ASCC-ALKBH3 complex in alkylation damage responses. ASCC1 uniquely combines two evolutionarily ancient domains: nucleotide-binding K-Homology (KH) (associated with regulating splicing, transcriptional, and translation) and two-histidine phosphodiesterase (PDE; associated with hydrolysis of cyclic nucleotide phosphate bonds). Germline mutations link loss of ASCC1 function to spinal muscular atrophy with congenital bone fractures 2 (SMABF2). Herein analysis of The Cancer Genome Atlas (TCGA) suggests ASCC1 RNA overexpression in certain tumors correlates with poor survival, Signatures 29 and 3 mutations, and genetic instability markers. We determined crystal structures of Alvinella pompejana (Ap) ASCC1 and Human (Hs) PDE domain revealing high-resolution details and features conserved over 500 million years of evolution. Extending our understanding of the KH domain Gly-X-X-Gly sequence motif, we define a novel structural Helix-Clasp-Helix (HCH) nucleotide binding motif and show ASCC1 sequence-specific binding to CGCG-containing RNA. The V-shaped PDE nucleotide binding channel has two His-Φ-Ser/Thr-Φ (HXT) motifs (Φ being hydrophobic) positioned to initiate cyclic phosphate bond hydrolysis. A conserved atypical active-site histidine torsion angle implies a novel PDE substrate. Flexible active site loop and arginine-rich domain linker appear regulatory. Small-angle X-ray scattering (SAXS) revealed aligned KH-PDE RNA binding sites with limited flexibility in solution. Quantitative evolutionary bioinformatic analyses of disease and cancer-associated mutations support implied functional roles for RNA binding, phosphodiesterase activity, and regulation. Collective results inform ASCC1's roles in transactivation and alkylation damage responses, its targeting by structure-based inhibitors, and how ASCC1 mutations may impact inherited disease and cancer.

Mechanism of efficient double-strand break repair by a long non-coding RNA.

Mechanistic studies in DNA repair have focused on roles of multi-protein DNA complexes, so how long non-coding RNAs (lncRNAs) regulate DNA repair is less well understood. Yet, lncRNA LINP1 is over-expressed in multiple cancers and confers resistance to ionizing radiation and chemotherapeutic drugs. Here, we unveil structural and mechanistic insights into LINP1's ability to facilitate non-homologous end joining (NHEJ). We characterized LINP1 structure and flexibility and analyzed interactions with the NHEJ factor Ku70/Ku80 (Ku) and Ku complexes that direct NHEJ. LINP1 self-assembles into phase-separated condensates via RNA-RNA interactions that reorganize to form filamentous Ku-containing aggregates. Structured motifs in LINP1 bind Ku, promoting Ku multimerization and stabilization of the initial synaptic event for NHEJ. Significantly, LINP1 acts as an effective proxy for PAXX. Collective results reveal how lncRNA effectively replaces a DNA repair protein for efficient NHEJ with implications for development of resistance to cancer therapy.

RNA Modifications: Reversal Mechanisms and Cancer.

An emerging molecular understanding of RNA alkylation and its removal is transforming our knowledge of RNA biology and its interplay with cancer chemotherapy responses. DNA modifications are known to perform critical functions depending on the genome template, including gene expression, DNA replication timing, and DNA damage protection, yet current results suggest that the chemical diversity of DNA modifications pales in comparison to those on RNA. More than 150 RNA modifications have been identified to date, and their complete functional implications are still being unveiled. These include intrinsic roles such as proper processing and RNA maturation; emerging evidence has furthermore uncovered RNA modification "readers", seemingly analogous to those identified for histone modifications. These modification recognition factors may regulate mRNA stability, localization, and interaction with translation machinery, affecting gene expression. Not surprisingly, tumors differentially modulate factors involved in expressing these marks, contributing to both tumorigenesis and responses to alkylating chemotherapy. Here we describe the current understanding of RNA modifications and their removal, with a focus primarily on methylation and alkylation as functionally relevant changes to the transcriptome. Intriguingly, some of the same RNA modifications elicited by physiological processes are also produced by alkylating agents, thus blurring the lines between what is a physiological mark and a damage-induced modification. Furthermore, we find that a high level of gene expression of enzymes with RNA dealkylation activity is a sensitive readout for poor survival in four different cancer types, underscoring the likely importance of examining RNA dealkylation mechanisms to cancer biology and for cancer treatment and prognosis.

Regulation of DNA Double-Strand Break Repair by Non-Coding RNAs.

DNA double-strand breaks (DSBs) are deleterious lesions that are generated in response to ionizing radiation or replication fork collapse that can lead to genomic instability and cancer. Eukaryotes have evolved two major pathways, namely homologous recombination (HR) and non-homologous end joining (NHEJ) to repair DSBs. Whereas the roles of protein-DNA interactions in HR and NHEJ have been fairly well defined, the functions of small and long non-coding RNAs and RNA-DNA hybrids in the DNA damage response is just beginning to be elucidated. This review summarizes recent discoveries on the identification of non-coding RNAs and RNA-mediated regulation of DSB repair.

Structure-specific nucleic acid recognition by L-motifs and their diverse roles in expression and regulation of the genome.

The high-mobility group (HMG) domain containing proteins regulate transcription, DNA replication and recombination. They adopt L-shaped folds and are structure-specific DNA binding motifs. Here, I define the L-motif super-family that consists of DNA-binding HMG-box proteins and the L-motif of the histone mRNA binding domain of stem-loop binding protein (SLBP). The SLBP L-motif and HMG-box domains adopt similar L-shaped folds with three α-helices and two or three small hydrophobic cores that stabilize the overall fold, but have very different and distinct modes of nucleic acid recognition. A comparison of the structure, dynamics, protein-protein and nucleic acid interactions, and regulation by PTMs of the SLBP and the HMG-box L-motifs reveals the versatile and diverse modes by which L-motifs utilize their surfaces for structure-specific recognition of nucleic acids to regulate gene expression.

Assembly of the SLIP1-SLBP complex on histone mRNA requires heterodimerization and sequential binding of SLBP followed by SLIP1.

The SLIP1-SLBP complex activates translation of replication-dependent histone mRNAs. In this report, we describe how the activity of the SLIP1-SLBP complex is modulated by phosphorylation and oligomerization. Biophysical characterization of the free proteins shows that whereas SLIP1 is a homodimer that does not bind RNA, human SLBP is an intrinsically disordered protein that is phosphorylated at 23 Ser/Thr sites when expressed in a eukaryotic expression system such as baculovirus. The bacterially expressed unphosphorylated SLIP1-SLBP complex forms a 2:2 high-affinity (K(D) < 0.9 nM) heterotetramer that is also incapable of binding histone mRNA. In contrast, phosphorylated SLBP from baculovirus has a weak affinity (K(D) ~3 µM) for SLIP1. Sequential binding of phosphorylated SLBP to the histone mRNA stem-loop motif followed by association with SLIP1 is required to form an "active" ternary complex. Phosphorylation of SLBP at Thr171 promotes dissociation of the heterotetramer to the SLIP1-SLBP heterodimer. Using alanine scanning mutagenesis, we demonstrate that the binding site on SLIP1 for SLBP lies close to the dimer interface. A single-point mutant near the SLIP1 homodimer interface abolished interaction with SLBP in vitro and reduced the abundance of histone mRNA in vivo. On the basis of these biophysical studies, we propose that oligomerization and SLBP phosphorylation may regulate the SLBP-SLIP1 complex in vivo. SLIP1 may act to sequester SLBP in vivo, protecting it from proteolytic degradation as an inactive heterotetramer, or alternatively, formation of the SLIP1-SLBP heterotetramer may facilitate removal of SLBP from the histone mRNA prior to histone mRNA degradation.

Interaction of the histone mRNA hairpin with stem-loop binding protein (SLBP) and regulation of the SLBP-RNA complex by phosphorylation and proline isomerization.

In metazoans, the majority of histone proteins are generated from replication-dependent histone mRNAs. These mRNAs are unique in that they are not polyadenylated but have a stem-loop structure in their 3' untranslated region. An early event in 3' end formation of histone mRNAs is the binding of stem-loop binding protein (SLBP) to the stem-loop structure. Here we provide insight into the mechanism by which SLBP contacts the histone mRNA. There are two binding sites in the SLBP RNA binding domain for the histone mRNA hairpin. The first binding site (Glu129-Val158) consists of a helix-turn-helix motif that likely recognizes the unpaired uridines in the loop of the histone hairpin and, upon binding, destabilizes the first G-C base pair at the base of the stem. The second binding site lies between residues Arg180 and Pro200, which appears to recognize the second G-C base pair from the base of the stem and possibly regions flanking the stem-loop structure. We show that the SLBP-histone mRNA complex is regulated by threonine phosphorylation and proline isomerization in a conserved TPNK sequence that lies between the two binding sites. Threonine phosphorylation increases the affinity of SLBP for histone mRNA by slowing the off rate for complex dissociation, whereas the adjacent proline acts as a critical hinge that may orient the second binding site for formation of a stable SLBP-histone mRNA complex. The nuclear magnetic resonance and kinetic studies presented here provide a framework for understanding how SLBP recognizes histone mRNA and highlight possible structural roles of phosphorylation and proline isomerization in RNA binding proteins in remodeling ribonucleoprotein complexes.

Visualizing functional dynamicity in the DNA-dependent protein kinase holoenzyme DNA-PK complex by integrating SAXS with cryo-EM.

Assembly of KU and DNA-dependent protein kinase catalytic subunit (DNA-PKcs) at DNA double strand breaks (DSBs) forms DNA-PK holoenzyme as a critical initiating step for non-homologous end joining (NHEJ) repair of DSBs produced by radiation and chemotherapies. Advanced cryo-electron microscopy (cryo-EM) imaging together with breakthrough macromolecular X-ray crystal (MX) structures of KU and DNA-PKcs recently enabled visualization of the ∼600 kDa DNA-PK assembly at near atomic resolution. These important static structures provide the foundation for definition and interpretation of functional movements crucial to mechanistic understanding that can be tested through solution state structure analysis. We herein therefore leverage Cryo-EM and MX structures for the interpretation of synchrotron small-angle X-ray scattering (SAXS) data on DNA-PK conformations in solution to inform the structural mechanism for NHEJ initiation. SAXS, which measures thermodynamic solution-state conformational states and assemblies outside of cryo- and solid-state conditions, unveils the inherent flexibility of KU, DNA-PKcs and DNA-PK. The combined structural measurements reveal mobility of KU80 C-terminal region (KU80CTR), motion/plasticity of HEAT (DNA-PKcs Huntingtin, Elongation Factor 3, PP2 A, and TOR1) regions, allosteric switching upon DNA-PKcs autophosphorylation, and dimeric arrangements of DNA-PK assembly. Importantly, the results uncover displacement of the N-terminal HEAT domain during autophosphorylation as suitable for a regulated release mechanism of DNA-PKcs from DNA-PK to control unproductive access to toxic and mutagenic DNA repair intermediates. These integrated analyses show that the marriage of SAXS with cryo-EM leverages the strengths of both techniques to enable assessment of functional conformations and flexibility defining atomic-resolution molecular mechanisms for DSB repair.

DSF Guided Refolding As A Novel Method Of Protein Production.

The Anfinsen hypothesis, the demonstration of which led to the Nobel prize in Chemistry, posits that all information required to determine a proteins' three dimensional structure is contained within its amino acid sequence. This suggests that it should be possible, in theory, to fold any protein in vitro. In practice, however, protein production by refolding is challenging because suitable refolding conditions must be empirically determined for each protein and can be painstaking. Here we demonstrate, using a variety of proteins, that differential scanning fluorimetry (DSF) can be used to determine and optimize conditions that favor proper protein folding in a rapid and high-throughput fashion. The resulting method, which we deem DSF guided refolding (DGR), thus enables the production of aggregation-prone and disulfide-containing proteins by refolding from E. coli inclusion bodies, which would not normally be amenable to production in bacteria.

NMR characterization of full-length farnesylated and non-farnesylated H-Ras and its implications for Raf activation.

The C terminus, also known as the hypervariable region (residues 166-189), of H-, N-, and K-Ras proteins has sequence determinants necessary for full activation of downstream effectors such as Raf kinase and PI-3 kinase as well as for the correct targeting of Ras proteins to lipid rafts and non-raft membranes. There is considerable interest in understanding how residues in the extreme C terminus of the different Ras proteins and farnesylation of the CaaX box cysteine affect Ras membrane localization and allosteric activation of Raf kinase. To provide insights into the structural and dynamic changes that occur in Ras upon farnesylation, we have used NMR spectroscopy to compare the properties of truncated H-Ras (1-166), to non-processed full-length H-Ras (residues 1-185) and full-length (1-189) farnesylated H-Ras. We report that the C-terminal helix alpha-5 extends to residue N172, and the remaining 17 amino acid residues in the C terminus are conformationally averaged in solution. Removal of either 23 or 18 amino acid residues from the C terminus of full length H-Ras generates truncated H-Ras (1-166) and H-Ras (1-171) proteins, respectively, that have been structurally characterized and are biochemical active. Here we report that C-terminal truncation of H-Ras results in minor structural and dynamic perturbations that are propagated throughout the H-Ras protein including increased flexibility of the central beta-sheet and the C-terminal helix alpha-5. Ordering of residues in loop-2, which is involved in Raf CRD binding is also observed. Farnesylation of full-length H-Ras at C186 does not result in detectable conformational changes in H-Ras. Chemical shift mapping studies of farnesylated and non-farnesylated forms of H-Ras with the Raf-CRD show that the farnesyl moiety, the extreme H-Ras C terminus and residues 23-30, contribute to H-Ras:Raf-CRD interactions, thereby increasing the affinity of H-Ras for the Raf-CRD.

Structural basis for regulation of RNA-binding proteins by phosphorylation.

Ribonucleoprotein complexes involved in pre-mRNA splicing and mRNA decay are often regulated by phosphorylation of RNA-binding proteins. Cells use phosphorylation-dependent signaling pathways to turn on and off gene expression. Not much is known about how phosphorylation-dependent signals transmitted by exogenous factors or cell cycle checkpoints regulate RNA-mediated gene expression at the atomic level. Several human diseases are linked to an altered phosphorylation state of an RNA binding protein. Understanding the structural response to the phosphorylation "signal" and its effect on ribonucleoprotein assembly provides mechanistic understanding, as well as new information for the design of novel drugs. In this review, I highlight recent structural studies that reveal the mechanisms by which phosphorylation can regulate protein-protein and protein-RNA interactions in ribonucleoprotein complexes.

Roles of Prolyl Isomerases in RNA-Mediated Gene Expression.

The peptidyl-prolyl cis-trans isomerases (PPIases) that include immunophilins (cyclophilins and FKBPs) and parvulins (Pin1, Par14, Par17) participate in cell signaling, transcription, pre-mRNA processing and mRNA decay. The human genome encodes 19 cyclophilins, 18 FKBPs and three parvulins. Immunophilins are receptors for the immunosuppressive drugs cyclosporin A, FK506, and rapamycin that are used in organ transplantation. Pin1 has also been targeted in the treatment of Alzheimer's disease, asthma, and a number of cancers. While these PPIases are characterized as molecular chaperones, they also act in a nonchaperone manner to promote protein-protein interactions using surfaces outside their active sites. The immunosuppressive drugs act by a gain-of-function mechanism by promoting protein-protein interactions in vivo. Several immunophilins have been identified as components of the spliceosome and are essential for alternative splicing. Pin1 plays roles in transcription and RNA processing by catalyzing conformational changes in the RNA Pol II C-terminal domain. Pin1 also binds several RNA binding proteins such as AUF1, KSRP, HuR, and SLBP that regulate mRNA decay by remodeling mRNP complexes. The functions of ribonucleoprotein associated PPIases are largely unknown. This review highlights PPIases that play roles in RNA-mediated gene expression, providing insight into their structures, functions and mechanisms of action in mRNP remodeling in vivo.

Contribution of protein phosphorylation to binding-induced folding of the SLBP-histone mRNA complex probed by phosphorus-31 NMR.

Phosphorus-31 ((31)P) NMR can be used to characterize the structure and dynamics of phosphorylated proteins. Here, I use (31)P NMR to report on the chemical nature of a phosphothreonine that lies in the RNA binding domain of SLBP (stem-loop binding protein). SLBP is an intrinsically disordered protein and phosphorylation at this threonine promotes the assembly of the SLBP-RNA complex. The data show that the (31)P chemical shift can be a good spectroscopic probe for phosphate-coupled folding and binding processes in intrinsically disordered proteins, particularly where the phosphate exhibits torsional strain and is involved in a network of hydrogen-bonding interactions.

Recent Advances in Metabolic Profiling And Imaging of Prostate Cancer.

Cancer is a metabolic disease. Cancer cells, being highly proliferative, show significant alterations in metabolic pathways such as glycolysis, respiration, the tricarboxylic acid (TCA) cycle, oxidative phosphorylation, lipid metabolism, and amino acid metabolism. Metabolites like peptides, nucleotides, products of glycolysis, the TCA cycle, fatty acids, and steroids can be an important read out of disease when characterized in biological samples such as tissues and body fluids like urine, serum, etc. The cancer metabolome has been studied since the 1960s by analytical techniques such as mass spectrometry (MS) and nuclear magnetic resonance (NMR) spectroscopy. Current research is focused on the identification and validation of biomarkers in the cancer metabolome that can stratify high-risk patients and distinguish between benign and advanced metastatic forms of the disease. In this review, we discuss the current state of prostate cancer metabolomics, the biomarkers that show promise in distinguishing indolent from aggressive forms of the disease, the strengths and limitations of the analytical techniques being employed, and future applications of metabolomics in diagnostic imaging and personalized medicine of prostate cancer.

The prolyl isomerase pin1 regulates mRNA levels of genes with short half-lives by targeting specific RNA binding proteins.

The peptidyl-prolyl isomerase Pin1 is over-expressed in several cancer tissues is a potential prognostic marker in prostate cancer, and Pin1 ablation can suppress tumorigenesis in breast and prostate cancers. Pin1 can co-operate with activated ErbB2 or Ras to enhance tumorigenesis. It does so by regulating the activity of proteins that are essential for gene expression and cell proliferation. Several targets of Pin1 such as c-Myc, the Androgen Receptor, Estrogen Receptor-alpha, Cyclin D1, Cyclin E, p53, RAF kinase and NCOA3 are deregulated in cancer. At the posttranscriptional level, emerging evidence indicates that Pin1 also regulates mRNA decay of histone mRNAs, GM-CSF, Pth, and TGFß mRNAs by interacting with the histone mRNA specific protein SLBP, and the ARE-binding proteins AUF1 and KSRP, respectively. To understand how Pin1 may affect mRNA abundance on a genome-wide scale in mammalian cells, we used RNAi along with DNA microarrays to identify genes whose abundance is significantly altered in response to a Pin1 knockdown. Functional scoring of differentially expressed genes showed that Pin1 gene targets control cell adhesion, leukocyte migration, the phosphatidylinositol signaling system and DNA replication. Several mRNAs whose abundance was significantly altered by Pin1 knockdown contained AU-rich element (ARE) sequences in their 3' untranslated regions. We identified HuR and AUF1 as Pin1 interacting ARE-binding proteins in vivo. Pin1 was also found to stabilize all core histone mRNAs in this study, thereby validating our results from a previously published study. Statistical analysis suggests that Pin1 may target the decay of essential mRNAs that are inherently unstable and have short to medium half-lives. Thus, this study shows that an important biological role of Pin1 is to regulate mRNA abundance and stability by interacting with specific RNA-binding proteins that may play a role in cancer progression.

Recognition modes of RNA tetraloops and tetraloop-like motifs by RNA-binding proteins.

RNA hairpins are the most commonly occurring secondary structural elements in RNAs and serve as nucleation sites for RNA folding, RNA-RNA, and RNA-protein interactions. RNA hairpins are frequently capped by tetraloops, and based on sequence similarity, three broad classes of RNA tetraloops have been defined: GNRA, UNCG, and CUYG. Other classes such as the UYUN tetraloop in histone mRNAs, the UGAA in 16S rRNA, the AUUA tetraloop from the MS2 bacteriophage, and the AGNN tetraloop that binds RNase III have also been characterized. The tetraloop structure is compact and is usually characterized by a paired interaction between the first and fourth nucleotides. The two unpaired nucleotides in the loop are usually involved in base-stacking or base-phosphate hydrogen bonding interactions. Several structures of RNA tetraloops, free and complexed to other RNAs or proteins, are now available and these studies have increased our understanding of the diverse mechanisms by which this motif is recognized. RNA tetraloops can mediate RNA-RNA contacts via the tetraloop-receptor motif, kissing hairpin loops, A-minor interactions, and pseudoknots. While these RNA-RNA interactions are fairly well understood, how RNA-binding proteins recognize RNA tetraloops and tetraloop-like motifs remains unclear. In this review, we summarize the structures of RNA tetraloop-protein complexes and the general themes that have emerged on sequence- and structure-specific recognition of RNA tetraloops. We highlight how proteins achieve molecular recognition of this nucleic acid motif, the structural adaptations observed in the tetraloop to accommodate the protein-binding partner, and the role of dynamics in recognition.

Signaling pathways that control mRNA turnover.

Cells regulate their genomes mainly at the level of transcription and at the level of mRNA decay. While regulation at the level of transcription is clearly important, the regulation of mRNA turnover by signaling networks is essential for a rapid response to external stimuli. Signaling pathways result in posttranslational modification of RNA binding proteins by phosphorylation, ubiquitination, methylation, acetylation etc. These modifications are important for rapid remodeling of dynamic ribonucleoprotein complexes and triggering mRNA decay. Understanding how these posttranslational modifications alter gene expression is therefore a fundamental question in biology. In this review we highlight recent findings on how signaling pathways and cell cycle checkpoints involving phosphorylation, ubiquitination, and arginine methylation affect mRNA turnover.

The prolyl isomerase Pin1 targets stem-loop binding protein (SLBP) to dissociate the SLBP-histone mRNA complex linking histone mRNA decay with SLBP ubiquitination.

Histone mRNAs are rapidly degraded at the end of S phase, and a 26-nucleotide stem-loop in the 3' untranslated region is a key determinant of histone mRNA stability. This sequence is the binding site for stem-loop binding protein (SLBP), which helps to recruit components of the RNA degradation machinery to the histone mRNA 3' end. SLBP is the only protein whose expression is cell cycle regulated during S phase and whose degradation is temporally correlated with histone mRNA degradation. Here we report that chemical inhibition of the prolyl isomerase Pin1 or downregulation of Pin1 by small interfering RNA (siRNA) increases the mRNA stability of all five core histone mRNAs and the stability of SLBP. Pin1 regulates SLBP polyubiquitination via the Ser20/Ser23 phosphodegron in the N terminus. siRNA knockdown of Pin1 results in accumulation of SLBP in the nucleus. We show that Pin1 can act along with protein phosphatase 2A (PP2A) in vitro to dephosphorylate a phosphothreonine in a conserved TPNK sequence in the SLBP RNA binding domain, thereby dissociating SLBP from the histone mRNA hairpin. Our data suggest that Pin1 and PP2A act to coordinate the degradation of SLBP by the ubiquitin proteasome system and the exosome-mediated degradation of the histone mRNA by regulating complex dissociation.

Genetic and biochemical characterization of Drosophila Snipper: A promiscuous member of the metazoan 3'hExo/ERI-1 family of 3' to 5' exonucleases.

The DnaQ-H family exonuclease Snipper (Snp) is a 33-kDa Drosophila melanogaster homolog of 3'hExo and ERI-1, exoribonucleases implicated in the degradation of histone mRNA in mammals and in the negative regulation of RNA interference (RNAi) in Caenorhabditis elegans, respectively. In metazoans, Snp, Exod1, 3'hExo, ERI-1, and the prpip nucleases define a new subclass of structure-specific 3'-5' exonucleases that bind and degrade double-stranded RNA and/or DNA substrates with 3' overhangs of 2-5 nucleotides (nt) in the presence of Mg2+ with no apparent sequence specificity. These nucleases are also capable of degrading linear substrates. Snp efficiently degrades structured RNA and DNA substrates as long as there exists a minimum 3' overhang of 2 nt to initiate degradation. We identified a Snp mutant and used it to test whether Snp plays a role in regulating histone mRNA degradation or RNAi in vivo. Snp mutant flies are viable, and display no obvious developmental abnormalities. The expression pattern and level of histone H3 mRNA in Snp mutant embryos and third instar imaginal eye discs was indistinguishable from wild type, suggesting that Snp does not play a significant role in the turnover of histone mRNA at the end of the S phase. The loss of Snp was also unable to enhance the silencing capability of two different RNAi transgenes targeting the white and yellow genes, suggesting that Snp does not negatively modulate RNAi. Therefore, Snp is a nonessential exonuclease that is not a functional ortholog of either 3'hExo or ERI-1.