Mutational analysis of bacteriophage T4 RNA ligase 1. Different functional groups are required for the nucleotidyl transfer and phosphodiester bond formation steps of the ligation reaction

Programming CircLigase Catalysis for DNA Rings and Topologies

Circular single-stranded (ss) DNA is an essential element in rolling circle amplification and many DNA nanotechnology constructions. It is commonly synthesized from linear ssDNA by a ligase, which nevertheless suffers from low and inconsistent efficiency due to the simultaneous formation of concatemeric byproducts. Here, we design an intramolecular terminal hybridization strategy to program the ring formation catalytic process of CircLigase, a thermostable RNA ligase 1 that can ligate ssDNA in an intramolecular fashion. With the enthalpy gained from the programmed hybridization to override disfavored entropic factors associated with end coupling, we broke the limit of natural CircLigase on circularization of ssDNA, realizing over 75% yields of byproduct-free monomeric rings on a series of hundred-to-half-kilo-based linear DNAs. We found that this hybridization strategy can be twisted from intra- to intermolecular to also program CircLigase to efficiently and predominantly join one ssDNA strand to another. We focused on DNA rings premade by CircLigase and demonstrated their utility in elevating the preparation, quantity, and quality of DNA topologies. We expect that the new insights on engineering CircLigase will further promote the development of nucleic acid biotechnology and nanotechnology.

Caveat mutator: alanine substitutions for conserved amino acids in RNA ligase elicit unexpected rearrangements of the active site for lysine adenylylation

Naegleria gruberi RNA ligase (NgrRnl) exemplifies the Rnl5 family of adenosine triphosphate (ATP)-dependent polynucleotide ligases that seal 3′-OH RNA strands in the context of 3′-OH/5′-PO₄ nicked duplexes. Like all classic ligases, NgrRnl forms a covalent lysyl–AMP intermediate. A two-metal mechanism of lysine adenylylation was established via a crystal structure of the NgrRnl•ATP•(Mn²⁺)₂ Michaelis complex. Here we conducted an alanine scan of active site constituents that engage the ATP phosphates and the metal cofactors. We then determined crystal structures of ligase-defective NgrRnl-Ala mutants in complexes with ATP/Mn²⁺. The unexpected findings were that mutations K170A, E227A, K326A and R149A (none of which impacted overall enzyme structure) triggered adverse secondary changes in the active site entailing dislocations of the ATP phosphates, altered contacts to ATP, and variations in the numbers and positions of the metal ions that perverted the active sites into off-pathway states incompatible with lysine adenylylation. Each alanine mutation elicited a distinctive off-pathway distortion of the ligase active site. Our results illuminate a surprising plasticity of the ligase active site in its interactions with ATP and metals. More broadly, they underscore a valuable caveat when interpreting mutational data in the course of enzyme structure-function studies.

Structure and two-metal mechanism of fungal tRNA ligase

Fungal tRNA ligase (Trl1) is an essential enzyme that repairs RNA breaks with 2′,3′-cyclic-PO₄ and 5′-OH ends inflicted during tRNA splicing and non-canonical mRNA splicing in the fungal unfolded protein response. Trl1 is composed of C-terminal cyclic phosphodiesterase (CPD) and central GTP-dependent polynucleotide kinase (KIN) domains that heal the broken ends to generate the 3′-OH,2′-PO₄ and 5′-PO₄ termini required for sealing by an N-terminal ATP-dependent ligase domain (LIG). Here we report crystal structures of the Trl1-LIG domain from Chaetomium thermophilum at two discrete steps along the reaction pathway: the covalent LIG-(lysyl-Nζ)–AMP•Mn²⁺ intermediate and a LIG•ATP•(Mn²⁺)₂ Michaelis complex. The structures highlight a two-metal mechanism whereby a penta-hydrated metal complex stabilizes the transition state of the ATP α phosphate and a second metal bridges the β and γ phosphates to help orient the pyrophosphate leaving group. A LIG-bound sulfate anion is a plausible mimetic of the essential RNA terminal 2′-PO₄. Trl1-LIG has a distinctive C-terminal domain that instates fungal Trl1 as the founder of an Rnl6 clade of ATP-dependent RNA ligase. We discuss how the Trl1-LIG structure rationalizes the large body of in vivo structure–function data for Saccharomyces cerevisiae Trl1.

Two-step mechanism and step-arrest mutants of Runella slithyformis NAD+-dependent tRNA 2'-phosphotransferase Tpt1

Tpt1 catalyzes the transfer of an internal 2'-monophosphate moiety (2'-PO₄) from a "branched" 2'-PO₄ RNA splice junction to NAD⁺ to form a "clean" 2'-OH, 3'-5' phosphodiester junction, ADP-ribose 1″-2″ cyclic phosphate, and nicotinamide. First discovered as an essential component of the Saccharomyces cerevisiae tRNA splicing machinery, Tpt1 is widely distributed in nature, including in taxa that have no yeast-like RNA splicing system. Here we characterize the RslTpt1 protein from the bacterium Runella slithyformis, in which Tpt1 is encoded within a putative RNA repair gene cluster. We find that (i) expression of RslTpt1 in yeast complements a lethal tpt1Δ knockout, and (ii) purified recombinant RslTpt1 is a bona fide NAD⁺-dependent 2'-phosphotransferase capable of completely removing an internal 2'-phosphate from synthetic RNAs. The in vivo activity of RslTpt1 is abolished by alanine substitutions for conserved amino acids Arg16, His17, Arg64, and Arg119. The R64A, R119A, and H17A mutants accumulate high levels of a 2'-phospho-ADP-ribosylated RNA reaction intermediate (2'-P-ADPR, evanescent in the wild-type RslTpt1 reaction), which is converted slowly to a 2'-OH RNA product. The R16A mutant is 300-fold slower than wild-type RslTpt1 in forming the 2'-P-ADPR intermediate. Whereas wild-type RsTpt1 rapidly converts the isolated 2'-P-ADPR intermediate to 2'-OH product in the absence of NAD⁺, the H17A, R119A, R64A, and R16A mutant are slower by factors of 3, 33, 210, and 710, respectively. Our results identify active site constituents involved in the catalysis of step 1 and step 2 of the Tpt1 reaction pathway.

Structures of DNA-bound human ligase IV catalytic core reveal insights into substrate binding and catalysis

DNA ligase IV (LigIV) performs the final DNA nick-sealing step of classical nonhomologous end-joining, which is critical for immunoglobulin gene maturation and efficient repair of genotoxic DNA double-strand breaks. Hypomorphic LigIV mutations cause extreme radiation sensitivity and immunodeficiency in humans. To better understand the unique features of LigIV function, here we report the crystal structure of the catalytic core of human LigIV in complex with a nicked nucleic acid substrate in two distinct states—an open lysyl-AMP intermediate, and a closed DNA–adenylate form. Results from structural and mutagenesis experiments unveil a dynamic LigIV DNA encirclement mechanism characterized by extensive interdomain interactions and active site phosphoanhydride coordination, all of which are required for efficient DNA nick sealing. These studies provide a scaffold for defining impacts of LigIV catalytic core mutations and deficiencies in human LIG4 syndrome.

DNA Ligase IV (LigIV) catalyzes nick sealing of DNA double-strand break substrates during non-homologous end-joining. Here the authors present the crystal structures of two human LigIV DNA-bound catalytic states, which provide insights into its catalytic mechanism and the molecular basis of LIG4 syndrome causing disease mutations.

Cleavage of 3'-terminal adenosine by archaeal ATP-dependent RNA ligase

Methanothermobacter thermoautotrophicus RNA ligase (MthRnl) catalyzes formation of phosphodiester bonds between the 5′-phosphate and 3′-hydroxyl termini of single-stranded RNAs. It can also react with RNA with a 3′-phosphate end to generate a 2′,3′-cyclic phosphate. Here, we show that MthRnl can additionally remove adenosine from the 3′-terminus of the RNA to produce 3′-deadenylated RNA, RNA(3′-rA). This 3′-deadenylation activity is metal-dependent and requires a 2′-hydroxyl at both the terminal adenosine and the penultimate nucleoside. Residues that contact the ATP/AMP in the MthRnl crystal structures are essential for the 3′-deadenylation activity, suggesting that 3′-adenosine may occupy the ATP-binding pocket. The 3′-end of cleaved RNA(3′-rA) consists of 2′,3′-cyclic phosphate which protects RNA(3′-rA) from ligation and further deadenylation. These findings suggest that ATP-dependent RNA ligase may act on a specific set of 3′-adenylated RNAs to regulate their processing and downstream biological events.

Two-metal versus one-metal mechanisms of lysine adenylylation by ATP-dependent and NAD+-dependent polynucleotide ligases

Polynucleotide ligases comprise a ubiquitous superfamily of nucleic acid repair enzymes that join 3'-OH and 5'-PO₄ DNA or RNA ends. Ligases react with ATP or NAD⁺ and a divalent cation cofactor to form a covalent enzyme-(lysine-Nζ)-adenylate intermediate. Here, we report crystal structures of the founding members of the ATP-dependent RNA ligase family (T4 RNA ligase 1; Rnl1) and the NAD⁺-dependent DNA ligase family (Escherichia coli LigA), captured as their respective Michaelis complexes, which illuminate distinctive catalytic mechanisms of the lysine adenylylation reaction. The 2.2-Å Rnl1•ATP•(Mg²⁺)₂ structure highlights a two-metal mechanism, whereby: a ligase-bound "catalytic" Mg²⁺(H₂O)₅ coordination complex lowers the pK_a of the lysine nucleophile and stabilizes the transition state of the ATP α phosphate; a second octahedral Mg²⁺ coordination complex bridges the β and γ phosphates; and protein elements unique to Rnl1 engage the γ phosphate and associated metal complex and orient the pyrophosphate leaving group for in-line catalysis. By contrast, the 1.55-Å LigA•NAD⁺•Mg²⁺ structure reveals a one-metal mechanism in which a ligase-bound Mg²⁺(H₂O)₅ complex lowers the lysine pK_a and engages the NAD⁺ α phosphate, but the β phosphate and the nicotinamide nucleoside of the nicotinamide mononucleotide (NMN) leaving group are oriented solely via atomic interactions with protein elements that are unique to the LigA clade. The two-metal versus one-metal dichotomy demarcates a branchpoint in ligase evolution and favors LigA as an antibacterial drug target.

Structural and mutational analysis of archaeal ATP-dependent RNA ligase identifies amino acids required for RNA binding and catalysis

An ATP-dependent RNA ligase from Methanobacterium thermoautotrophicum (MthRnl) catalyzes intramolecular ligation of single-stranded RNA to form a closed circular RNA via covalent ligase-AMP and RNA-adenylylate intermediate. Here, we report the X-ray crystal structures of an MthRnl•ATP complex as well as the covalent MthRnl–AMP intermediate. We also performed structure-guided mutational analysis to survey the functions of 36 residues in three component steps of the ligation pathway including ligase-adenylylation (step 1), RNA adenylylation (step 2) and phosphodiester bond synthesis (step 3). Kinetic analysis underscored the importance of motif 1a loop structure in promoting phosphodiester bond synthesis. Alanine substitutions of Thr117 or Arg118 favor the reverse step 2 reaction to deadenylate the 5′-AMP from the RNA-adenylate, thereby inhibiting step 3 reaction. Tyr159, Phe281 and Glu285, which are conserved among archaeal ATP-dependent RNA ligases and are situated on the surface of the enzyme, are required for RNA binding. We propose an RNA binding interface of the MthRnl based on the mutational studies and two sulfate ions that co-crystallized at the active site cleft in the MthRnl–AMP complex.

A cross-chiral RNA polymerase ribozyme

Thirty years ago it was shown that the non-enzymatic, template-directed polymerization of activated mononucleotides proceeds readily in a homochiral system, but is severely inhibited by the presence of the opposing enantiomer.¹ This finding poses a severe challenge for the spontaneous emergence of RNA-based life, and has led to the suggestion that either RNA was preceded by some other genetic polymer that is not subject to chiral inhibition² or chiral symmetry was broken through chemical processes prior to the origin of RNA-based life.^3,4 Once an RNA enzyme arose that could catalyze the polymerization of RNA, it would have been possible to distinguish among the two enantiomers, enabling RNA replication and RNA-based evolution to occur. It is commonly thought that the earliest RNA polymerase and its substrates would have been of the same handedness, but this is not necessarily the case. Replicating D-and L-RNA molecules may have emerged together, based on the ability of structured RNAs of one handedness to catalyze the templated polymerization of activated mononucleotides of the opposite handedness. Such a cross-chiral RNA polymerase has now been developed using in vitro evolution. The D-RNA enzyme, consisting of 83 nucleotides, catalyzes the joining of L-mono- or oligonucleotide substrates on a complementary L-RNA template, and similarly for the L-enzyme with D-substrates and a D-template. Chiral inhibition is avoided because the 10⁶-fold rate acceleration of the enzyme only pertains to cross-chiral substrates. The enzyme's activity is sufficient to generate full-length copies of its enantiomer through the templated joining of 11 component oligonucleotides.

Structure-function analysis of Methanobacterium thermoautotrophicum RNA ligase - engineering a thermostable ATP independent enzyme

Background

RNA ligases are essential reagents for many methods in molecular biology including NextGen RNA sequencing. To prevent ligation of RNA to itself, ATP independent mutant ligases, defective in self-adenylation, are often used in combination with activated pre-adenylated linkers. It is important that these ligases not have de-adenylation activity, which can result in activation of RNA and formation of background ligation products. An additional useful feature is for the ligase to be active at elevated temperatures. This has the advantage or reducing preferences caused by structures of single-stranded substrates and linkers.

Results

To create an RNA ligase with these desirable properties we performed mutational analysis of the archaeal thermophilic RNA ligase from Methanobacterium thermoautotrophicum. We identified amino acids essential for ATP binding and reactivity but dispensable for phosphodiester bond formation with 5’ pre-adenylated donor substrate. The motif V lysine mutant (K246A) showed reduced activity in the first two steps of ligation reaction. The mutant has full ligation activity with pre-adenylated substrates but retained the undesirable activity of deadenylation, which is the reverse of step 2 adenylation. A second mutant, an alanine substitution for the catalytic lysine in motif I (K97A) abolished activity in the first two steps of the ligation reaction, but preserved wild type ligation activity in step 3. The activity of the K97A mutant is similar with either pre-adenylated RNA or single-stranded DNA (ssDNA) as donor substrates but we observed two-fold preference for RNA as an acceptor substrate compared to ssDNA with an identical sequence. In contrast, truncated T4 RNA ligase 2, the commercial enzyme used in these applications, is significantly more active using pre-adenylated RNA as a donor compared to pre-adenylated ssDNA. However, the T4 RNA ligases are ineffective in ligating ssDNA acceptors.

Conclusions

Mutational analysis of the heat stable RNA ligase from Methanobacterium thermoautotrophicum resulted in the creation of an ATP independent ligase. The K97A mutant is defective in the first two steps of ligation but retains full activity in ligation of either RNA or ssDNA to a pre-adenylated linker. The ability of the ligase to function at 65°C should reduce the constraints of RNA secondary structure in RNA ligation experiments.

Reducing ligation bias of small RNAs in libraries for next generation sequencing

BACKGROUND

The use of nucleic acid-modifying enzymes has driven the rapid advancement in molecular biology. Understanding their function is important for modifying or improving their activity. However, functional analysis usually relies upon low-throughput experiments. Here we present a method for functional analysis of nucleic acid-modifying enzymes using next generation sequencing.

FINDINGS

We demonstrate that sequencing data of libraries generated by RNA ligases can reveal novel secondary structure preferences of these enzymes, which are used in small RNA cloning and library preparation for NGS. Using this knowledge we demonstrate that the cloning bias in small RNA libraries is RNA ligase-dependent. We developed a high definition (HD) protocol that reduces the RNA ligase-dependent cloning bias. The HD protocol doubled read coverage, is quantitative and found previously unidentified microRNAs. In addition, we show that microRNAs in miRBase are those preferred by the adapters of the main sequencing platform.

CONCLUSIONS

Sequencing bias of small RNAs partially influenced which microRNAs have been studied in depth; therefore most previous small RNA profiling experiments should be re-evaluated. New microRNAs are likely to be found, which were selected against by existing adapters. Preference of currently used adapters towards known microRNAs suggests that the annotation of all existing small RNAs, including miRNAs, siRNAs and piRNAs, has been biased.

A chloroplastic RNA ligase activity analogous to the bacterial and archaeal 2´-5' RNA ligase

Bacteria and archaea contain a 2'-5' RNA ligase that seals in vitro 2',3'-cyclic phosphodiester and 5'-hydroxyl RNA termini, generating a 2',5'-phosphodiester bond. In our search for an RNA ligase able to circularize the monomeric linear replication intermediates of viroids belonging to the family Avsunviroidae, which replicate in the chloroplast, we have identified in spinach (Spinacea oleracea L.) chloroplasts a new RNA ligase activity whose properties resemble those of the bacterial and archaeal 2'-5' RNA ligase. The spinach chloroplastic RNA ligase recognizes the 5'-hydroxyl and 2',3'-cyclic phosphodiester termini of Avocado sunblotch viroid and Eggplant latent viroid RNAs produced by hammerhead-mediated self-cleavage, yielding circular products linked through an atypical, most likely 2',5'-phosphodiester, bond. The enzyme neither requires divalent cations as cofactors, nor NTPs as substrate. The reaction apparently reaches equilibrium at a low ratio between the final circular product and the linear initial substrate. Even if its involvement in viroid replication seems unlikely, the identification of a 2'-5' RNA ligase activity in higher plant chloroplasts, with properties very similar to an analogous enzyme widely distributed in bacterial and archaeal proteomes, is intriguing and suggests an important biological role so far unknown.

The adenylyltransferase domain of bacterial Pnkp defines a unique RNA ligase family

Pnkp is the end-healing and end-sealing component of an RNA repair system present in diverse bacteria from ten different phyla. To gain insight to the mechanism and evolution of this repair system, we determined the crystal structures of the ligase domain of Clostridium thermocellum Pnkp in three functional states along the reaction pathway: apoenzyme, ligase • ATP substrate complex, and covalent ligase-AMP intermediate. The tertiary structure is composed of a classical ligase nucleotidyltransferase module that is embellished by a unique α-helical insert module and a unique C-terminal α-helical module. Structure-guided mutational analysis identified active site residues essential for ligase adenylylation. Pnkp defines a new RNA ligase family with signature structural and functional properties.

Novel mechanism of RNA repair by RtcB via sequential 2',3'-cyclic phosphodiesterase and 3'-Phosphate/5'-hydroxyl ligation reactions

RtcB enzymes are a newly discovered family of RNA ligases, implicated in tRNA splicing and other RNA repair reactions, that seal broken RNAs with 2',3'-cyclic phosphate and 5'-OH ends. Parsimony and energetics would suggest a one-step mechanism for RtcB sealing via attack by the O5' nucleophile on the cyclic phosphate, with expulsion of the ribose O2' and generation of a 3',5'-phosphodiester at the splice junction. Yet we find that RtcB violates Occam's razor, insofar as (i) it is adept at ligating 3'-monophosphate and 5'-OH ends; (ii) it has an intrinsic 2',3'-cyclic phosphodiesterase activity. The 2',3'-cyclic phosphodiesterase and ligase reactions both require manganese and are abolished by mutation of the RtcB active site. Thus, RtcB executes a unique two-step pathway of strand joining whereby the 2',3'-cyclic phosphodiester end is hydrolyzed to a 3'-monophosphate, which is then linked to the 5'-OH end to form the splice junction. The energy for the 3'-phosphate ligase activity is provided by GTP, which reacts with RtcB in the presence of manganese to form a covalent RtcB-guanylate adduct. This adduct is sensitive to acid and hydroxylamine but resistant to alkali, consistent with a phosphoramidate bond.

Identification and remediation of biases in the activity of RNA ligases in small-RNA deep sequencing

Deep sequencing of small RNAs (sRNA-seq) is now the gold standard for small RNA profiling and discovery. Biases in sRNA-seq have been reported, but their etiology remains unidentified. Through a comprehensive series of sRNA-seq experiments, we establish that the predominant cause of the bias is the RNA ligases. We further demonstrate that RNA ligases have strong sequence-specific biases which distort the small RNA profiles considerably. We have devised a pooled adapter strategy to overcome this bias, and validated the method through data derived from microarray and qPCR. In light of our findings, published small RNA profiles, as well as barcoding strategies using adapter-end modifications, may need to be revisited. Importantly, by providing a wide spectrum of substrate for the ligase, the pooled-adapter strategy developed here provides a means to overcome issues of bias, and generate more accurate small RNA profiles.

HSPC117 is the essential subunit of a human tRNA splicing ligase complex

Splicing of mammalian precursor transfer RNA (tRNA) molecules involves two enzymatic steps. First, intron removal by the tRNA splicing endonuclease generates separate 5' and 3' exons. In animals, the second step predominantly entails direct exon ligation by an elusive RNA ligase. Using activity-guided purification of tRNA ligase from HeLa cell extracts, we identified HSPC117, a member of the UPF0027 (RtcB) family, as the essential subunit of a tRNA ligase complex. RNA interference-mediated depletion of HSPC117 inhibited maturation of intron-containing pre-tRNA both in vitro and in living cells. The high sequence conservation of HSPC117/RtcB proteins is suggestive of RNA ligase roles of this protein family in various organisms.

miR-ID: a novel, circularization-based platform for detection of microRNAs

MicroRNAs (miRNAs) are important regulators of gene expression and have great potential as biomarkers, prognostic indicators, and therapeutic targets. Determining the expression patterns of these molecules is essential for elucidating their biogenesis, regulation, relation to disease, and response to therapy. Although PCR-based assays are commonly used for expression profiling of miRNAs, the small size, sequence heterogeneity, and (in some cases) end modifications of miRNAs constrain the performance of existing PCR methods. Here we introduce miR-ID, a novel method that avoids these constraints while providing superior sensitivity and sequence specificity at a lower cost. It also has the unique ability to differentiate unmodified small RNAs from those carrying 2'-OMe groups at their 3'-ends while detecting both forms. miR-ID is comprised of the following steps: (1) circularization of the miRNA by a ligase; (2) reverse transcription of the circularized miRNA (RTC), producing tandem repeats of a DNA sequence complementary to the miRNA; and (3) qPCR amplification of segments of this multimeric cDNA using 5'-overlapping primers and a nonspecific dye such as SYBR Green. No chemically modified probes (e.g., TaqMan) or primers (e.g., LNA) are required. The circular RNA and multimeric cDNA templates provide unmatched flexibility in the positioning of primers, which may include straddling the boundaries between these repetitive miRNA sequences. miR-ID is based on new findings that are themselves of general interest, including reverse transcription of small RNA circles and the use of 5'-overlapping primers for detection of repetitive sequences by qPCR.

Structure-activity relationships in human RNA 3'-phosphate cyclase

RNA 3'-phosphate cyclase (Rtc) enzymes are a widely distributed family that catalyze the synthesis of RNA 2',3' cyclic phosphate ends via an ATP-dependent pathway comprising three nucleotidyl transfer steps: reaction of Rtc with ATP to form a covalent Rtc-(histidinyl-N)-AMP intermediate and release PP(i); transfer of AMP from Rtc1 to an RNA 3'-phosphate to form an RNA(3')pp(5')A intermediate; and attack by the terminal nucleoside O2' on the 3'-phosphate to form an RNA 2',3' cyclic phosphate product and release AMP. Here we used the crystal structure of Escherichia coli RtcA to guide a mutational analysis of the human RNA cyclase Rtc1. An alanine scan defined seven conserved residues as essential for the Rtc1 RNA cyclization and autoadenylylation reactions. Structure-activity relationships were clarified by conservative substitutions. Our results are consistent with a mechanism of adenylate transfer in which attack of the Rtc1 His320 nucleophile on the ATP alpha phosphorus is facilitated by proper orientation of the PP(i) leaving group via contacts to Arg21, Arg40, and Arg43. We invoke roles for Tyr294 in binding the adenine base and Glu14 in binding the divalent cation cofactor. We find that Rtc1 forms a stable binary complex with a 3'-phosphate terminated RNA, but not with an otherwise identical 3'-OH terminated RNA. Mutation of His320 had little impact on RNA 3'-phosphate binding, signifying that covalent adenylylation of Rtc1 is not a prerequisite for end recognition.

Structure-guided Mutational Analysis of the Nucleotidyltransferase Domain of Escherichia coli DNA Ligase (LigA)

NAD(+)-dependent DNA ligases (LigA) are ubiquitous in bacteria, where they are essential for growth and present attractive targets for antimicrobial drug discovery. LigA has a distinctive modular structure in which a nucleotidyltransferase catalytic domain is flanked by an upstream NMN-binding module and by downstream OB-fold, zinc finger, helix-hairpin-helix, and BRCT domains. Here we conducted a structure-function analysis of the nucleotidyltransferase domain of Escherichia coli LigA, guided by the crystal structure of the LigA-DNA-adenylate intermediate. We tested the effects of 29 alanine and conservative mutations at 15 amino acids on ligase activity in vitro and in vivo. We thereby identified essential functional groups that coordinate the reactive phosphates (Arg(136)), contact the AMP adenine (Lys(290)), engage the phosphodiester backbone flanking the nick (Arg(218), Arg(308), Arg(97) plus Arg(101)), or stabilize the active domain fold (Arg(171)). Finer analysis of the mutational effects revealed step-specific functions for Arg(136), which is essential for the reaction of LigA with NAD(+) to form the covalent ligase-AMP intermediate (step 1) and for the transfer of AMP to the nick 5'-PO(4) to form the DNA-adenylate intermediate (step 2) but is dispensable for phosphodiester formation at a preadenylylated nick (step 3).

The structure of an archaeal homodimeric ligase which has RNA circularization activity

The genome of Pyrococcus abyssi contains two open reading frames encoding proteins which had been previously predicted to be DNA ligases, Pab2002 and Pab1020. We show that while the former is indeed a DNA ligase, Pab1020 had no effect on the substrate deoxyoligo-ribonucleotides tested. Instead, Pab1020 catalyzes the nucleotidylation of oligo-ribonucleotides in an ATP-dependent reaction, suggesting that it is an RNA ligase. We have solved the structure of Pab1020 in complex with the ATP analog AMPPNP by single-wavelength anomalous dispersion (SAD), elucidating a structure with high structural similarity to the catalytic domains of two RNA ligases from the bacteriophage T4. Additional carboxy-terminal domains are also present, and one of these mediates contacts with a second protomer, which is related by noncrystallographic symmetry, generating a homodimeric structure. These C-terminal domains are terminated by short domain swaps which themselves end within 5 A of the active sites of the partner molecules. Additionally, we show that the protein is indeed capable of circularizing RNA molecules in an ATP-dependent reaction. These structural and biochemical results provide an insight into the potential physiological roles of Pab1020.

The C-terminal domain of T4 RNA ligase 1 confers specificity for tRNA repair

T4 RNA ligase 1 (Rnl1) is a tRNA repair enzyme that thwarts a tRNA-damaging host response to virus infection. The 374-aa Rnl1 protein consists of an N-terminal nucleotidyltransferase domain fused to a unique C-terminal domain composed of 10 alpha helices. We exploited an in vitro tRNA splicing system to demonstrate that Rnl1 has an inherent specificity for sealing tRNA with a break in the anticodon loop. The tRNA specificity is imparted by the C domain, any deletion of which caused the broken tRNA to be sealed as poorly as the linear intron in vitro and also abolished Rnl1 tRNA splicing activity in vivo. Deletion analysis demarcated Rnl1-(1-254) as a minimal catalytic domain of Rnl1, capable of all chemical steps of the nonspecific RNA ligation reaction. Alanine scanning of the N domain identified Ser103, Leu104, Lys117, and Ser118 as important for pRNA ligation in vitro and tRNA repair in vivo.

Reprogramming the tRNA-splicing activity of a bacterial RNA repair enzyme

Programmed RNA breakage is an emerging theme underlying cellular responses to stress, virus infection and defense against foreign species. In many cases, site-specific cleavage of the target RNA generates 2′,3′ cyclic phosphate and 5′-OH ends. For the damage to be repaired, both broken ends must be healed before they can be sealed by a ligase. Healing entails hydrolysis of the 2′,3′ cyclic phosphate to form a 3′-OH and phosphorylation of the 5′-OH to form a 5′-PO₄. Here, we demonstrate that a polynucleotide kinase-phosphatase enzyme from Clostridium thermocellum (CthPnkp) can catalyze both of the end-healing steps of tRNA splicing in vitro. The route of tRNA repair by CthPnkp can be reprogrammed by a mutation in the 3′ end-healing domain (H189D) that yields a 2′-PO₄ product instead of a 2′-OH. Whereas tRNA ends healed by wild-type CthPnkp are readily sealed by T4 RNA ligase 1, the H189D enzyme generates ends that are spliced by yeast tRNA ligase. Our findings suggest that RNA repair enzymes can evolve their specificities to suit a particular pathway.

Deinococcus radiodurans RNA ligase exemplifies a novel ligase clade with a distinctive N-terminal module that is important for 5'-PO4 nick sealing and ligase adenylylation but dispensable for phosphodiester formation at an adenylylated nick

Deinococcus radiodurans RNA ligase (DraRnl) is a template-directed ligase that seals nicked duplexes in which the 3'-OH strand is RNA. DraRnl is a 342 amino acid polypeptide composed of a C-terminal adenylyltransferase domain fused to a distinctive 126 amino acid N-terminal module (a putative OB-fold). An alanine scan of the C domain identified 9 amino acids essential for nick ligation, which are located within nucleotidyltransferase motifs I, Ia, III, IIIa, IV and V. Seven mutants were dysfunctional by virtue of defects in ligase adenylylation: T163A, H167A, G168A, K186A, E230A, F281A and E305A. Four of these were also defective in phosphodiester formation at a preadenylylated nick: G168A, E230A, F281A and E305A. Two nick sealing-defective mutants were active in ligase adenylylation and sealing a preadenylylated nick, thereby implicating Ser185 and Lys326 in transfer of AMP from the enzyme to the nick 5'-PO(4). Whereas deletion of the N-terminal domain suppressed overall nick ligation and ligase adenylylation, it did not compromise sealing at a preadenylylated nick. Mutational analysis of 15 residues of the N domain identified Lys26, Gln31 and Arg79 as key constituents. Structure-activity relationships at the essential residues were determined via conservative substitutions. We propose that DraRnl typifies a new clade of polynucleotide ligases. DraRnl homologs are detected in several eukaryal proteomes.

RNA ligase structures reveal the basis for RNA specificity and conformational changes that drive ligation forward

T4 RNA ligase 2 (Rnl2) and kinetoplastid RNA editing ligases exemplify a family of RNA repair enzymes that seal 3'OH/5'PO(4) nicks in duplex RNAs via ligase adenylylation (step 1), AMP transfer to the nick 5'PO(4) (step 2), and attack by the nick 3'OH on the 5'-adenylylated strand to form a phosphodiester (step 3). Crystal structures are reported for Rnl2 at discrete steps along this pathway: the covalent Rnl2-AMP intermediate; Rnl2 bound to an adenylylated nicked duplex, captured immediately following step 2; and Rnl2 at an adenylylated nick in a state poised for step 3. These structures illuminate the stereochemistry of nucleotidyl transfer and reveal how remodeling of active-site contacts and conformational changes propel the ligation reaction forward. Mutational analysis and comparison of nick-bound structures of Rnl2 and human DNA ligase I highlight common and divergent themes of substrate recognition that can explain their specialization for RNA versus DNA repair.

Structure-guided mutational analysis of T4 RNA ligase 1

T4 RNA ligase 1 (Rnl1) is a tRNA repair enzyme that circumvents an RNA-damaging host antiviral response. Whereas the three-step reaction scheme of Rnl1 is well established, the structural basis for catalysis has only recently been appreciated as mutational and crystallographic approaches have converged. Here we performed a structure-guided alanine scan of nine conserved residues, including side chains that either contact the ATP substrate via adenine (Leu179, Val230), the 2'-OH (Glu159), or the gamma phosphate (Tyr37) or coordinate divalent metal ions at the ATP alpha phosphate (Glu159, Tyr246) or beta phosphate (Asp272, Asp273). We thereby identified Glu159 and Tyr246 as essential for RNA sealing activity in vitro and for tRNA repair in vivo. Structure-activity relationships at Glu159 and Tyr246 were clarified by conservative substitutions. Eliminating the phosphate-binding Tyr37, and the magnesium-binding Asp272 and Asp273 side chains had little impact on sealing activity in vitro or in vivo, signifying that not all atomic interactions in the active site are critical for function. Analysis of mutational effects on individual steps of the ligation pathway underscored how different functional groups come into play during the ligase-adenylylation reaction versus the subsequent steps of RNA-adenylylation and phosphodiester formation. Moreover, the requirements for sealing exogenous preformed RNA-adenylate are more stringent than are those for sealing the RNA-adenylate intermediate formed in situ during ligation of a 5'-PO4 RNA.

Direct comparison of nick-joining activity of the nucleic acid ligases from bacteriophage T4

The genome of bacteriophage T4 encodes three polynucleotide ligases, which seal the backbone of nucleic acids during infection of host bacteria. The T4Dnl (T4 DNA ligase) and two RNA ligases [T4Rnl1 (T4 RNA ligase 1) and T4Rnl2] join a diverse array of substrates, including nicks that are present in double-stranded nucleic acids, albeit with different efficiencies. To unravel the biochemical and functional relationship between these proteins, a systematic analysis of their substrate specificity was performed using recombinant proteins. The ability of each protein to ligate 20 bp double-stranded oligonucleotides containing a single-strand break was determined. Between 4 and 37 degrees C, all proteins ligated substrates containing various combinations of DNA and RNA. The RNA ligases ligated a more diverse set of substrates than T4Dnl and, generally, T4Rnl1 had 50-1000-fold lower activity than T4Rnl2. In assays using identical conditions, optimal ligation of all substrates was at pH 8 for T4Dnl and T4Rnl1 and pH 7 for T4Rnl2, demonstrating that the protein dictates the pH optimum for ligation. All proteins ligated a substrate containing DNA as the unbroken strand, with the nucleotides at the nick of the broken strand being RNA at the 3'-hydroxy group and DNA at the 5'-phosphate. Since this RNA-DNA hybrid was joined at a similar maximal rate by T4Dnl and T4Rnl2 at 37 degrees C, we consider the possibility that this could be an unexpected physiological substrate used during some pathways of 'DNA repair'.

Crystal structure and nonhomologous end-joining function of the ligase component of Mycobacterium DNA ligase D

DNA ligase D (LigD) is a large polyfunctional enzyme involved in nonhomologous end-joining (NHEJ) in mycobacteria. LigD consists of a C-terminal ATP-dependent ligase domain fused to upstream polymerase and phosphoesterase modules. Here we report the 2.4 angstroms crystal structure of the ligase domain of Mycobacterium LigD, captured as the covalent ligase-AMP intermediate with a divalent metal in the active site. A chloride anion on the protein surface coordinated by the ribose 3'-OH and caged by arginine and lysine side chains is a putative mimetic of the 5'-phosphate at a DNA nick. Structure-guided mutational analysis revealed distinct requirements for the adenylylation and end-sealing reactions catalyzed by LigD. We found that a mutation of Mycobacterium LigD that ablates only ligase activity results in decreased fidelity of NHEJ in vivo and a strong bias of mutagenic events toward deletions instead of insertions at the sealed DNA ends. This phenotype contrasts with the increased fidelity of double-strand break repair in deltaligD cells or in a strain in which only the polymerase function of LigD is defective. We surmise that the signature error-prone quality of bacterial NHEJ in vivo arises from a dynamic balance between the end-remodeling and end-sealing steps.

Structure-function analysis of the kinase-CPD domain of yeast tRNA ligase (Trl1) and requirements for complementation of tRNA splicing by a plant Trl1 homolog

Trl1 is an essential 827 amino acid enzyme that executes the end-healing and end-sealing steps of tRNA splicing in Saccharomyces cerevisiae. Trl1 consists of two domains—an N-terminal ligase component and a C-terminal 5′-kinase/2′,3′-cyclic phosphodiesterase (CPD) component—that can function in tRNA splicing in vivo when expressed as separate polypeptides. To understand the structural requirements for the kinase-CPD domain, we performed an alanine scan of 30 amino acids that are conserved in Trl1 homologs from other fungi. We thereby identified four residues (Arg463, His515, Thr675 and Glu741) as essential for activity in vivo. Structure–function relationships at these positions, and at four essential or conditionally essential residues defined previously (Asp425, Arg511, His673 and His777), were clarified by introducing conservative substitutions. Biochemical analysis showed that lethal mutations of Asp425, Arg463, Arg511 and His515 in the kinase module abolished polynucleotide kinase activity in vitro. We report that a recently cloned 1104 amino acid Arabidopsis RNA ligase functions in lieu of yeast Trl1 in vivo and identify essential side chains in the ligase, kinase and CPD modules of the plant enzyme. The plant ligase, like yeast Trl1 but unlike T4 RNA ligase 1, requires a 2′-PO₄ end for tRNA splicing in vivo.

Molecular architecture and ligand recognition determinants for T4 RNA ligase

RNA ligase type 1 from bacteriophage T4 (Rnl1) is involved in countering a host defense mechanism by repairing 5'-PO4 and 3'-OH groups in tRNA(Lys). Rnl1 is widely used as a reagent in molecular biology. Although many structures for DNA ligases are available, only fragments of RNA ligases such as Rnl2 are known. We report the first crystal structure of a complete RNA ligase, Rnl1, in complex with adenosine 5'-(alpha,beta-methylenetriphosphate) (AMPcPP). The N-terminal domain is related to the equivalent region of DNA ligases and Rnl2 and binds AMPcPP but with further interactions from the additional N-terminal 70 amino acids in Rnl1 (via Tyr37 and Arg54) and the C-terminal domain (Gly269 and Asp272). The active site contains two metal ions, consistent with the two-magnesium ion catalytic mechanism. The C-terminal domain represents a new all alpha-helical fold and has a charge distribution and architecture for helix-nucleic acid groove interaction compatible with tRNA binding.

An end-healing enzyme from Clostridium thermocellum with 5' kinase, 2',3' phosphatase, and adenylyltransferase activities

We identify and characterize an end-healing enzyme, CthPnkp, from Clostridium thermocellum that catalyzes the phosphorylation of 5'-OH termini of DNA or RNA polynucleotides and the dephosphorylation of 2',3' cyclic phosphate, 2'-phosphate, and 3'-phosphate ribonucleotides. CthPnkp also catalyzes an autoadenylylation reaction via a polynucleotide ligase-type mechanism. These characteristics are consistent with a role in end-healing during RNA or DNA repair. CthPnkp is a homodimer of an 870-amino-acid polypeptide composed of three catalytic domains: an N-terminal module that resembles the polynucleotide kinase domain of bacteriophage T4 Pnkp, a central metal-dependent phosphoesterase module, and a C-terminal module that resembles the nucleotidyl transferase domain of polynucleotide ligases. The distinctive feature of CthPnkp vis-à-vis known RNA repair enzymes is that its 3' end modification component belongs to the calcineurin-type phosphatase superfamily. It contains putative counterparts of the amino acids that form the dinuclear metal-binding site and the phosphate-binding site of bacteriophage lambda phosphatase. As with lambda phosphatase, the 2',3' cAMP phosphatase activity of CthPnkp is specifically dependent on nickel or manganese. We identify homologs of CthPnkp in other bacterial proteomes.

Structure-function analysis of yeast tRNA ligase

Trl 1 is an essential 827-amino-acid enzyme that executes the end-healing and end-sealing steps of tRNA splicing in Saccharomyces cerevisiae. Trl1 consists of two catalytic domains--an N-terminal adenylyltransferase/ligase component (amino acids 1-388) and a C-terminal 5'-kinase/cyclic phosphodiesterase component (amino acids 389-827)--that can function in tRNA splicing in vivo when expressed as separate polypeptides. Sedimentation analysis indicates that the ligase and kinase/CPD domains are monomeric proteins that do not form a stable complex in trans. To understand the structural requirements for the RNA ligase component, we performed a mutational analysis of amino acids that are conserved in Trl1 homologs from other fungi. Alanine scanning identified 23 new residues as essential for Trl1-(1-388) activity in vivo. Structure-activity relationships at these positions, and four essential residues defined previously, were clarified by introducing 50 different conservative substitutions. Lethal mutations of Lys114, Glu184, Glu266, and Lys284 abolished Trl1 adenylyltransferase activity in vitro. The essential elements embrace (1) putative equivalents of nucleotidyltransferase motifs I, Ia, III, IV, and V found in DNA ligases, T4 RNA ligase 2, and mRNA capping enzymes; (2) an N-terminal segment shared with the T4 RNA ligase 1 subfamily only; and (3) a constellation of conserved residues specific to fungal tRNA splicing enzymes. We identify yeastlike tRNA ligases in the proteomes of Leishmania and Trypanosoma. These findings recommend tRNA ligase as a target for antifungal and antiprotozoal drug discovery.

Structure-guided mutational analysis of the nucleotidyltransferase domain of Escherichia coli NAD+-dependent DNA ligase (LigA)

NAD+-dependent DNA ligase (LigA) is essential for bacterial growth and a potential target for antimicrobial drug discovery. Here we queried the role of 14 conserved amino acids of Escherichia coli LigA by alanine scanning and thereby identified five new residues within the nucleotidyltransferase domain as being essential for LigA function in vitro and in vivo. Structure activity relationships were determined by conservative mutagenesis for the Glu-173, Arg-200, Arg-208, and Arg-277 side chains, as well as four other essential side chains that had been identified previously (Lys-115, Asp-117, Asp-285, and Lys-314). In addition, we identified Lys-290 as important for LigA activity. Reference to the structure of Enterococcus faecalis LigA allowed us to discriminate three classes of essential/important side chains that: (i) contact NAD+ directly (Lys-115, Glu-173, Lys-290, and Lys-314); (ii) comprise the interface between the NMN-binding domain (domain Ia) and the nucleotidyltransferase domain or comprise part of a nick-binding site on the surface of the nucleotidyltransferase domain (Arg-200 and Arg-208); or (iii) stabilize the active site fold of the nucleotidyltransferase domain (Arg-277). Analysis of mutational effects on the isolated ligase adenylylation and phosphodiester formation reactions revealed different functions for essential side chains at different steps of the DNA ligase pathway, consistent with the proposal that the active site is serially remodeled as the reaction proceeds.

Plant tRNA ligases are multifunctional enzymes that have diverged in sequence and substrate specificity from RNA ligases of other phylogenetic origins

Pre-tRNA splicing is an essential process in all eukaryotes. It requires the concerted action of an endonuclease to remove the intron and a ligase for joining the resulting tRNA halves as studied best in the yeast Saccharomyces cerevisiae. Here, we report the first characterization of an RNA ligase protein and its gene from a higher eukaryotic organism that is an essential component of the pre-tRNA splicing process. Purification of tRNA ligase from wheat germ by successive column chromatographic steps has identified a protein of 125 kDa by its potentiality to covalently bind AMP, and by its ability to catalyse the ligation of tRNA halves and the circularization of linear introns. Peptide sequences obtained from the purified protein led to the elucidation of the corresponding proteins and their genes in Arabidopsis and Oryza databases. The plant tRNA ligases exhibit no overall sequence homologies to any known RNA ligases, however, they harbour a number of conserved motifs that indicate the presence of three intrinsic enzyme activities: an adenylyltransferase/ligase domain in the N-terminal region, a polynucleotide kinase in the centre and a cyclic phosphodiesterase domain at the C-terminal end. In vitro expression of the recombinant Arabidopsis tRNA ligase and functional analyses revealed all expected individual activities. Plant RNA ligases are active on a variety of substrates in vitro and are capable of inter- and intramolecular RNA joining. Hence, we conclude that their role in vivo might comprise yet unknown essential functions besides their involvement in pre-tRNA splicing.

Isolation and characterization of a thermostable RNA ligase 1 from a Thermus scotoductus bacteriophage TS2126 with good single-stranded DNA ligation properties

We have recently sequenced the genome of a novel thermophilic bacteriophage designated as TS2126 that infects the thermophilic eubacterium Thermus scotoductus. One of the annotated open reading frames (ORFs) shows homology to T4 RNA ligase 1, an enzyme of great importance in molecular biology, owing to its ability to ligate single-stranded nucleic acids. The ORF was cloned, and recombinant protein was expressed, purified and characterized. The recombinant enzyme ligates single-stranded nucleic acids in an ATP-dependent manner and is moderately thermostable. The recombinant enzyme exhibits extremely high activity and high ligation efficiency. It can be used for various molecular biology applications including RNA ligase-mediated rapid amplification of cDNA ends (RLM-RACE). The TS2126 RNA ligase catalyzed both inter- and intra-molecular single-stranded DNA ligation to >50% completion in a matter of hours at an elevated temperature, although favoring intra-molecular ligation on RNA and single-stranded DNA substrates. The properties of TS2126 RNA ligase 1 makes it very attractive for processes like adaptor ligation, and single-stranded solid phase gene synthesis.

Structure and mechanism of RNA ligase

T4 RNA ligase 2 (Rnl2) exemplifies an RNA ligase family that includes the RNA editing ligases (RELs) of Trypanosoma and Leishmania. The Rnl2/REL enzymes are defined by essential signature residues and a unique C-terminal domain, which we show is essential for sealing of 3'-OH and 5'-PO4 RNA ends by Rnl2, but not for ligase adenylation or phosphodiester bond formation at a preadenylated AppRNA end. The N-terminal segment Rnl2(1-249) of the 334 aa Rnl2 protein comprises an autonomous adenylyltransferase/AppRNA ligase domain. We report the 1.9 A crystal structure of the ligase domain with AMP bound at the active site, which reveals a shared fold, catalytic mechanism, and evolutionary history for RNA ligases, DNA ligases, and mRNA capping enzymes.

An RNA ligase from Deinococcus radiodurans

Although DNA repair pathways have been the focus of much attention, there is an emerging appreciation that distinct pathways exist to maintain or manipulate RNA structure in response to breakage events. Here we identify an RNA ligase (DraRnl) from the radiation-resistant bacterium Deinococcus radiodurans. DraRnl seals 3'-OH/5'-PO4 RNA nicks in either a duplex RNA or an RNA: DNA hybrid, but it cannot seal 3'-OH/5'-PO4 DNA nicks. The specificity of DraRnl arises from a requirement for RNA on the 3'-OH side of the nick. DraRnl is a 342-amino acid monomeric protein with a distinctive structure composed of a C-terminal adenylyltransferase domain linked to an N-terminal module that resembles the OB-fold of phenylalanyl-tRNA synthetases. RNA sealing activity was abolished by mutation of the predicted lysine adenylylation site (Lys-165) in the C-terminal domain and was reduced by an order of magnitude by deletion of the N-terminal OB module. Our findings highlight the existence of an RNA repair capacity in bacteria and support the hypothesis that contemporary DNA ligases, RNA ligases, and RNA capping enzymes evolved by the fusion of ancillary effector domains to an ancestral catalytic module involved in RNA repair.

RNA ligase; picking up the pieces

In the current issue of Structure, Ho and coworkers report the crystal structure and mechanism of a T4 RNA ligase. These studies provide valuable insights on the mechanism and origin of RNA and DNA ligases, and RNA capping enzymes.

Characterization of bacteriophage KVP40 and T4 RNA ligase 2

Bacteriophage T4 RNA ligase 2 (Rnl2) exemplifies a subfamily of RNA strand-joining enzymes that includes the trypanosome RNA editing ligases. A homolog of T4 Rnl2 is encoded in the 244-kbp DNA genome of vibriophage KVP40. We show that the 335-amino acid KVP40 Rnl2 is a monomeric protein that catalyzes RNA end-joining through ligase-adenylate and RNA-adenylate (AppRNA) intermediates. In the absence of ATP, pre-adenylated KVP40 Rnl2 reacts with an 18-mer 5'-PO(4) single-strand RNA (pRNA) to form an 18-mer RNA circle. In the presence of ATP, Rnl2 generates predominantly AppRNA. Isolated AppRNA can be circularized by KVP40 Rnl2 in the absence of ATP. The reactivity of phage Rnl2 and the distribution of the products are affected by the length of the pRNA substrate. Whereas 18-mer and 15-mer pRNAs undergo intramolecular sealing by T4 Rnl2 to form monomer circles, a 12-mer pRNA is ligated intermolecularly to form dimers, and a 9-mer pRNA is unreactive. In the presence of ATP, the 15-mer and 12-mer pRNAs are converted to AppRNAs, but the 9-mer pRNA is not. A single 5' deoxynucleotide substitution of an 18-mer pRNA substrate has no apparent effect on the 5' adenylation or circularization reactions of T4 Rnl2. In contrast, a single deoxyribonucleoside at the 3' terminus strongly and selectively suppresses the sealing step, thereby resulting in accumulation of high levels of AppRNA in the absence of ATP. The ATP-dependent "capping" of RNA with AMP by Rnl2 is reminiscent of the capping of eukaryotic mRNA with GMP by GTP:RNA guanylyltransferase and suggests an evolutionary connection between bacteriophage Rnl2 and eukaryotic RNA capping enzymes.

Portability and fidelity of RNA-repair systems

Yeast tRNA ligase (Trl1) is an essential enzyme that converts cleaved tRNA half-molecules into spliced tRNAs containing a 2'-PO(4), 3'-5' phosphodiester at the splice junction. Trl1 also catalyzes splicing of HAC1 mRNA during the unfolded protein response. Trl1 performs three reactions: the 2',3'-cyclic phosphate of the proximal RNA fragment is hydrolyzed to a 3'-OH, 2'-PO(4) by a cyclic phosphodiesterase; the 5'-OH of the distal RNA fragment is phosphorylated by a GTP-dependent polynucleotide kinase; and the 3'-OH, 2'-PO(4), and 5'-PO(4) ends are then sealed by an ATP-dependent RNA ligase. The removal of the 2'-PO(4) at the splice junction is catalyzed by the essential enzyme Tpt1, which transfers the RNA 2'-PO(4) to NAD(+) to form ADP-ribose 1"-2"-cyclic phosphate. Here, we show that the bacteriophage T4 enzymes RNA ligase 1 and polynucleotide kinase/phosphatase can fulfill the tRNA and HAC1 mRNA splicing functions of yeast Trl1 in vivo and bypass the requirement for Tpt1. These results attest to the portability of RNA-repair systems, notwithstanding the significant differences in the specificities, mechanisms, and reaction intermediates of the individual yeast and T4 enzymes responsible for the RNA healing and sealing steps. We surmise that Tpt1 and its unique metabolite ADP-ribose 1"-2"-cyclic phosphate do not play essential roles in yeast independent of the tRNA-splicing reaction. Our finding that one-sixth of spliced HAC1 mRNAs in yeast cells containing the T4 RNA-repair system suffered deletion of a single nucleotide at the 3' end of the splice-donor site suggests a model whereby the yeast RNA-repair system evolved a requirement for the 2'-PO(4) for RNA ligation to suppress inappropriate RNA recombination.

Characterization of a baculovirus enzyme with RNA ligase, polynucleotide 5'-kinase, and polynucleotide 3'-phosphatase activities

The end-healing and end-sealing steps of the phage T4-induced RNA restriction-repair pathway are performed by two separate enzymes, a bifunctional polynucleotide 5'-kinase/3'-phosphatase and an ATP-dependent RNA ligase. Here we show that a single trifunctional baculovirus enzyme, RNA ligase 1 (Rnl1), catalyzes the identical set of RNA repair reactions. Three enzymatic activities of baculovirus Rnl1 are organized in a modular fashion within a 694-amino acid polypeptide consisting of an autonomous N-terminal RNA-specific ligase domain, Rnl1-(1-385), and a C-terminal kinase-phosphatase domain, Rnl1-(394-694). The ligase domain is itself composed of two functional units. The N-terminal module Rnl1-(1-270) contains essential nucleotidyltransferase motifs I, IV, and V and suffices for both enzyme adenylylation (step 1 of the ligation pathway) and phosphodiester bond formation at a preactivated RNA-adenylate end (step 3). The downstream module extending to residue 385 is required for ligation of a phosphorylated RNA substrate, suggesting that it is involved specifically in the second step of the end-joining pathway, the transfer of AMP from the ligase to the 5'-PO(4) end to form RNA-adenylate. The end-healing domain Rnl1-(394-694) consists of a proximal 5'-kinase module with an essential P-loop motif ((404)GSGKS(408)) and a distal 3'-phosphatase module with an essential acylphosphatase motif ((560)DLDGT(564)). Our findings have implications for the evolution of RNA repair systems and their potential roles in virus-host dynamics.

Discovery and characterization of a thermostable bacteriophage RNA ligase homologous to T4 RNA ligase 1

Thermophilic viruses represent a novel source of genetic material and enzymes with great potential for use in biotechnology. We have isolated a number of thermophilic viruses from geothermal areas in Iceland, and by combining high throughput genome sequencing and state of the art bioinformatics we have identified a number of genes with potential use in biotechnology. We have also demonstrated the existence of thermostable counterparts of previously known bacteriophage enzymes. Here we describe a thermostable RNA ligase 1 from the thermophilic bacteriophage RM378 that infects the thermophilic eubacterium Rhodothermus marinus. The RM378 RNA ligase 1 has a temperature optimum of 60-64 degrees C and it ligates both RNA and single-stranded DNA. Its thermostability and ability to work under conditions of high temperature where nucleic acid secondary structures are removed makes it an ideal enzyme for RNA ligase-mediated rapid amplification of cDNA ends (RLM-RACE), and other RNA and DNA ligation applications.

Genetic and biochemical analysis of the functional domains of yeast tRNA ligase

Yeast tRNA ligase (Trl1) converts cleaved tRNA half-molecules into spliced tRNAs containing a 2'-PO4, 3'-5' phosphodiester at the splice junction. Trl1 performs three reactions: (i) the 2',3'-cyclic phosphate of the proximal fragment is hydrolyzed to a 3'-OH, 2'-PO4 by a cyclic phosphodiesterase (CPD); (ii) the 5'-OH of the distal fragment is phosphorylated by an NTP-dependent polynucleotide kinase; and (iii) the 3'-OH, 2'-PO4, and 5'-PO4 ends are sealed by an ATP-dependent RNA ligase. Trl1 consists of an N-terminal adenylyltransferase domain that resembles T4 RNA ligase 1, a central domain that resembles T4 polynucleotide kinase, and a C-terminal CPD domain that resembles the 2H phosphotransferase enzyme superfamily. Here we show that all three domains are essential in vivo, although they need not be linked in the same polypeptide. We identify five amino acids in the adenylyltransferase domain (Lys114, Glu266, Gly267, Lys284, and Lys286) that are essential for Trl1 activity and are located within motifs I (114KANG117), IV (266EGFVI270), and V (282FFKIK286) that comprise the active sites of DNA ligases, RNA capping enzymes, and T4 RNA ligases 1 and 2. Mutations K404A and T405A in the P-loop (401GXGKT405) of the central kinase-like domain had no effect on Trl1 function in vivo. The K404A and T405A mutations eliminated ATP-dependent kinase activity but preserved GTP-dependent kinase activity. A double alanine mutant in the P-loop was lethal in vivo and abolished GTP-dependent kinase activity. These results suggest that GTP is the physiological substrate and that the Trl1 kinase has a single NTP binding site of which the P-loop is a component. Two other mutations in the central domain were lethal in vivo and either abolished (D425A) or severely reduced (R511A) GTP-dependent RNA kinase activity in vitro. Mutations of the signature histidines of the CPD domain were either lethal (H777A) or conferred a ts growth phenotype (H673A).