RtcB is the RNA ligase component of an Escherichia coli RNA repair operon

RNA 2',3'-cyclic phosphate ends play important roles in RNA metabolism as substrates for RNA ligases during tRNA restriction-repair and tRNA splicing. Diverse bacteria from multiple phyla encode a two-component RNA repair cassette, comprising Pnkp (polynucleotide kinase-phosphatase-ligase) and Hen1 (RNA 3'-terminal ribose 2'-O-methyltransferase), that heals and then seals broken tRNAs with 2',3'-cyclic phosphate and 5'-OH ends. The Pnkp-Hen1 repair operon is absent in the majority of bacterial species, thereby raising the prospect that other RNA repair systems might be extant. A candidate component is RNA 3'-phosphate cyclase, a widely distributed enzyme that transforms RNA 3'-monophosphate termini into 2',3'-cyclic phosphates but cannot seal the ends it produces. Escherichia coli RNA cyclase (RtcA) is encoded in a σ(54)-regulated operon with RtcB, a protein of unknown function. Taking a cue from Pnkp-Hen1, we purified E. coli RtcB and tested it for RNA ligase activity. We report that RtcB per se seals broken tRNA-like stem-loop structures with 2',3'-cyclic phosphate and 5'-OH ends to form a splice junction with a 2'-OH, 3',5'-phosphodiester. We speculate that: (i) RtcB might afford bacteria a means to recover from stress-induced RNA damage; and (ii) RtcB homologs might catalyze tRNA repair or splicing reactions in archaea and eukarya.     

RNA repair: an antidote to cytotoxic eukaryal RNA damage

RNA healing and sealing enzymes drive informational and stress response pathways entailing repair of programmed 2',3' cyclic PO(4)/5'-OH breaks. Fungal, plant, and phage tRNA ligases use different strategies to discriminate the purposefully broken ends of the anticodon loop. Whereas phage ligase recognizes the tRNA fold, yeast and plant ligases do not and are instead hardwired to seal only the tRNA 3'-OH, 2'-PO(4) ends formed by healing of a cyclic phosphate. tRNA anticodon damage inflicted by secreted ribotoxins such as fungal gamma-toxin underlies a rudimentary innate immune system. Yeast cells are susceptible to gamma-toxin because the sealing domain of yeast tRNA ligase is unable to rectify a break at the modified wobble base of tRNA(Glu(UUC)). Plant andphage tRNA repair enzymes protect yeast from gamma-toxin because they are able to reverse the damage. Our studies underscore how a ribotoxin exploits an Achilles' heel in the target cell's tRNA repair system.     

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.     

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.     

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).     

A conditional lethal yeast phosphotransferase (tpt1) mutant accumulates tRNAs with a 2'-phosphate and an undermodified base at the splice junction.

tRNA splicing is essential in yeast and humans and presumably all eukaryotes. The first two steps of yeast tRNA splicing, excision of the intron by endonuclease and joining of the exons by tRNA ligase, leave a splice junction bearing a 2'-phosphate. Biochemical analysis suggests that removal of this phosphate in yeast is catalyzed by a highly specific 2'-phosphotransferase that transfers the phosphate to NAD to form ADP-ribose 1"-2" cyclic phosphate. 2'-Phosphotransferase catalytic activity is encoded by a single essential gene, TPT1, in the yeast Saccharomyces cerevisiae. We show here that Tpt1 protein is responsible for the dephosphorylation step of tRNA splicing in vivo because, during nonpermissive growth, conditional lethal tpt1 mutants accumulate 2'-phosphorylated tRNAs from eight different tRNA species that are known to be spliced. We show also that several of these tRNAs are undermodified at the splice junction residue, which is always located at the hypermodified position one base 3' of the anticodon. This result is consistent with previous results indicating that modification of the hypermodified position occurs after intron excision in the tRNA processing pathway, and implies that modification normally follows the dephosphorylation step of tRNA splicing in vivo.     

The sequential 2',3'-cyclic phosphodiesterase and 3'-phosphate/5'-OH ligation steps of the RtcB RNA splicing pathway are GTP-dependent

The RNA ligase RtcB splices broken RNAs with 5'-OH and either 2',3'-cyclic phosphate or 3'-phosphate ends. The 3'-phosphate ligase activity requires GTP and entails the formation of covalent RtcB-(histidinyl)-GMP and polynucleotide-(3')pp(5')G intermediates. There are currently two models for how RtcB executes the strand sealing step. Scheme 1 holds that the RNA 5'-OH end attacks the 3'-phosphorus of the N(3')pp(5')G end to form a 3',5'-phosphodiester and release GMP. Scheme 2 posits that the N(3')pp(5')G end is converted to a 2',3'-cyclic phosphodiester, which is then attacked directly by the 5'-OH RNA end to form a 3',5'-phosphodiester. Here we show that the sealing of a 2',3'-cyclic phosphate end by RtcB requires GTP, is contingent on formation of the RtcB-GMP adduct, and involves a kinetically valid RNA(3')pp(5')G intermediate. Moreover, we find that RtcB catalyzes the hydrolysis of a 2',3'-cyclic phosphate to a 3'-phosphate at a rate that is at least as fast as the rate of ligation. These results weigh in favor of scheme 1. The cyclic phosphodiesterase activity of RtcB depends on GTP and the formation of the RtcB-GMP adduct, signifying that RtcB guanylylation precedes the cyclic phosphodiesterase and 3'-phosphate ligase steps of the RNA splicing pathway.     

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.     

Crystal structure of the 2'-5' RNA ligase from Thermus thermophilus HB8

The 2'-5' RNA ligase family members are bacterial and archaeal RNA ligases that ligate 5' and 3' half-tRNA molecules with 2',3'-cyclic phosphate and 5'-hydroxyl termini, respectively, to the product containing the 2'-5' phosphodiester linkage. Here, the crystal structure of the 2'-5' RNA ligase protein from an extreme thermophile, Thermus thermophilus HB8, was solved at 2.5A resolution. The structure of the 2'-5' RNA ligase superimposes well on that of the Arabidopsis thaliana cyclic phosphodiesterase (CPDase), which hydrolyzes ADP-ribose 1",2"-cyclic phosphate (a product of the tRNA splicing reaction) to the monoester ADP-ribose 1"-phosphate. Although the sequence identity between the two proteins is remarkably low (9.3%), the 2'-5' RNA ligase and CPDase structures have two HX(T/S)X motifs in their corresponding positions. The HX(T/S)X motifs play important roles in the CPDase activity, and are conserved in both the CPDases and 2'-5' RNA ligases. Therefore, the catalytic mechanism of the 2'-5' RNA ligase may be similar to that of the CPDase. On the other hand, the electrostatic potential of the cavity of the 2'-5' RNA ligase is positive, but that of the CPDase is negative. Furthermore, in the CPDase, two loops with low B-factors cover the cavity. In contrast, in the 2'-5' RNA ligase, the corresponding loops form an open conformation and are flexible. These characteristics may be due to the differences in the substrates, tRNA and ADP-ribose 1",2"-cyclic phosphate.     

RtcB, a novel RNA ligase, can catalyze tRNA splicing and HAC1 mRNA splicing in vivo

RtcB enzymes are novel RNA ligases that join 2',3'-cyclic phosphate and 5'-OH ends. The phylogenetic distribution of RtcB points to its candidacy as a tRNA splicing/repair enzyme. Here we show that Escherichia coli RtcB is competent and sufficient for tRNA splicing in vivo by virtue of its ability to complement growth of yeast cells that lack the endogenous "healing/sealing-type" tRNA ligase Trl1. RtcB also protects yeast trl1Δ cells against a fungal ribotoxin that incises the anticodon loop of cellular tRNAs. Moreover, RtcB can replace Trl1 as the catalyst of HAC1 mRNA splicing during the unfolded protein response. Thus, RtcB is a bona fide RNA repair enzyme with broad physiological actions. Biochemical analysis of RtcB highlights the uniqueness of its active site and catalytic mechanism. Our findings draw attention to tRNA ligase as a promising drug target.     

Chromatin 3''-phosphatase/5''-OH kinase cannot transfer phosphate from 3'' to 5'' across a strand nick in DNA.

Rat liver chromatin contains a 3'-phosphatase/5'-OH kinase which may be involved in the repair of DNA strand breaks limited by 3'-phosphate/5'-OH ends. In order to determine whether the phosphate group can be transferred directly from the 3' to the 5' position, a polynucleotide duplex was synthesized between poly (dA) and oligo (dT) segments which had 3'-[32P]phosphate and 5'-OH ends. The oligo (dT) segments were separated by simple nicks as shown by the ability of T4 DNA ligase to seal the nick after the 3'-phosphate was removed by a phosphatase and the 5' end was phosphorylated with a kinase. The chromatin 3'-phosphatase/5'-OH kinase was unable to transfer phosphate directly from the 3' to the 5' end of the oligo (dT) segments in the original duplex; ATP was needed to phosphorylate the 5'-OH end. It is concluded that the chromatin 3'-phosphatase/5'-OH kinase is unable to convert a 3'-phosphate/5'-OH nick which cannot be repaired by DNA ligase directly into a 3'-OH/5'-phosphate nick which can be repaired by DNA ligase; the chromatin enzyme rather acts in two steps: hydrolysis of the 3'-phosphate followed by ATP-mediated phosphorylation of the 5'-OH end.     

Distinctive kinetics and substrate specificities of plant and fungal tRNA ligases

Plant and fungal tRNA ligases are trifunctional enzymes that repair RNA breaks with 2′,3′-cyclic-PO₄ and 5′-OH ends. They are composed of cyclic phosphodiesterase (CPDase) and polynucleotide kinase domains that heal the broken ends to generate the 3′-OH, 2′-PO₄, and 5′-PO₄ required for sealing by a ligase domain. Here, we use short _HORNA>p substrates to determine, in a one-pot assay format under single-turnover conditions, the order and rates of the CPDase, kinase and ligase steps. The observed reaction sequence for the plant tRNA ligase AtRNL, independent of RNA length, is that the CPDase engages first, converting _HORNA>p to _HORNA_2′p, which is then phosphorylated to pRNA_2′p by the kinase. Whereas the rates of the AtRNL CPDase and kinase reactions are insensitive to RNA length, the rate of the ligase reaction is slowed by a factor of 16 in the transition from 10-mer RNA to 8-mer and further by eightfold in the transition from 8-mer RNA to 6-mer. We report that a single ribonucleoside-2′,3′-cyclic-PO₄ moiety enables AtRNL to efficiently splice an otherwise all-DNA strand. Our characterization of a fungal tRNA ligase (KlaTrl1) highlights important functional distinctions vis à vis the plant homolog. We find that (1) the KlaTrl1 kinase is 300-fold faster than the AtRNL kinase; and (2) the KlaTrl1 kinase is highly specific for GTP or dGTP as the phosphate donor. Our findings recommend tRNA ligase as a tool to map ribonucleotides embedded in DNA and as a target for antifungal drug discovery.     

Two reactions of Haloferax volcanii RNA splicing enzymes: joining of exons and circularization of introns

Archaeal RNA splicing involves at least two protein enzymes, a specific endonuclease and a specific ligase. The endonuclease recognizes and cleaves within a characteristic bulge-helix-bulge (BHB) structure formed by pairing of the regions near the two exon-intron junctions, producing 2',3'-cyclic phosphate and 5'-hydroxyl termini. The ligase joins the exons and converts the cyclic phosphate into junction phosphate. The ligated product contains a seven-base hairpin loop, in which the splice junction is in between the two 3' terminal residues of the loop. Archaeal splicing endonucleases are also involved in rRNA processing, cutting within the BHB structures formed by pairing of the 5' and 3' flanking regions of the rRNAs. Large free introns derived from pre-rRNAs have been observed as stable and abundant circular RNAs in certain Crenarchaeota, a kingdom in the domain Archaea. In the present study, we show that the cells of Haloferax volcanii, a Euryarchaeote, contain circular RNAs formed by 3',5'-phosphodiester linkage between the two termini of the introns derived from their pre-tRNAs. H. volcanii ligase, in vitro, can also circularize both endonuclease-cleaved introns, and non-endonuclease-produced substrates. Exon joining and intron circularization are mechanistically similar ligation reactions that can occur independently. The size of the ligated hairpin loop and position of the splice junction within this loop can be changed in in vitro ligation reactions. Overall, archaeal RNA splicing seems to involve two sets of two symmetric transesterification reactions each.     

Essential constituents of the 3'-phosphoesterase domain of bacterial DNA ligase D, a nonhomologous end-joining enzyme

DNA ligase D (LigD) catalyzes end-healing and end-sealing steps during nonhomologous end joining in bacteria. Pseudomonas aeruginosa LigD consists of a central ATP-dependent ligase domain fused to a C-terminal polymerase domain and an N-terminal 3'-phosphoesterase (PE) module. The PE domain catalyzes manganese-dependent phosphodiesterase and phosphomonoesterase reactions at a duplex primer-template with a short 3'-ribonucleotide tract. The phosphodiesterase, which cleaves a 3'-terminal diribonucleotide to yield a primer strand with a ribonucleoside 3'-PO4 terminus, requires the vicinal 2'-OH of the penultimate ribose. The phosphomonoesterase converts the terminal ribonucleoside 3'-PO4 to a 3'-OH. Here we show that the PE domain has a 3'-phosphatase activity on an all-DNA primer-template, signifying that the phosphomonoesterase reaction does not depend on a 2'-OH. The distinctions between the phosphodiesterase and phosphomonoesterase activities are underscored by the results of alanine-scanning, limited proteolysis, and deletion analysis, which show that the two reactions depend on overlapping but nonidentical ensembles of protein functional groups, including: (i) side chains essential for both ribonuclease and phosphatase activity (His-42, His-48, Asp-50, Arg-52, His-84, and Tyr-88); (ii) side chains important for 3'-phosphatase activity but not for 3' ribonucleoside removal (Arg-14, Asp-15, Glu-21, Gln-40, and Glu-82); and (iii) side chains required selectively for the 3'-ribonuclease (Lys-66 and Arg-76). These constellations of critical residues are unique to LigD-like proteins, which we propose comprise a new bifunctional phosphoesterase family.     

Human RNA 5'-kinase (hClp1) can function as a tRNA splicing enzyme in vivo

Yeast and human Clp1 proteins are homologous components of the mRNA 3′-cleavage-polyadenylation machinery. Recent studies highlighting an association of human Clp1 (hClp1) with tRNA splicing endonuclease and an intrinsic RNA-specific 5′-OH polynucleotide kinase activity of hClp1 have prompted speculation that Clp1 might play a catalytic role in tRNA splicing in animal cells. Here, we show that expression of hClp1 in budding yeast can complement conditional and lethal mutations in the essential 5′-OH RNA kinase module of yeast or plant tRNA ligases. The tRNA splicing activity of hClp1 in yeast is abolished by mutations in the kinase active site. In contrast, overexpression of yeast Clp1 (yClp1) cannot rescue kinase-defective tRNA ligase mutants, and, unlike hClp1, the purified recombinant yClp1 protein has no detectable RNA kinase activity in vitro. Mutations of the yClp1 ATP-binding site do not affect yeast viability. These findings, and the fact that hClp1 cannot complement growth of a yeast clp1Δ strain, indicate that yeast and human Clp1 proteins are not functional orthologs, despite their structural similarity. Although hClp1 can perform the 5′-end-healing step of a yeast-type tRNA splicing pathway in vivo, it is uncertain whether its kinase activity is necessary for tRNA splicing in human cells, given that other mammalian counterparts of yeast-type tRNA repair enzymes are nonessential in vivo.     

Bacteriophage T4 anticodon nuclease, polynucleotide kinase and RNA ligase reprocess the host lysine tRNA.

Host tRNAs cleaved near the anticodon occur specifically in T4-infected Escherichia coli prr strains which restrict polynucleotide kinase (pnk) or RNA ligase (rli) phage mutants. The cleavage products are transient with wt but accumulate in pnk- or rli- infections, implicating the affected enzymes in repair of the damaged tRNAs. Their roles in the pathway were elucidated by comparing the mutant infection intermediates with intact tRNA counterparts before or late in wt infection. Thus, the T4-induced anticodon nuclease cleaves lysine tRNA 5' to the wobble position, yielding 2':3'-P greater than and 5'-OH termini. Polynucleotide kinase converts them into a 3'-OH and 5' P pair joined in turn by RNA ligase. Presumably, lysine tRNA depletion, in the absence of polynucleotide kinase and RNA ligase mediated repair, underlies prr restriction. However, the nuclease, kinase and ligase may benefit T4 directly, by adapting levels or decoding specificities of host tRNAs to T4 codon usage.     

Structure-function analysis of the yeast NAD+-dependent tRNA 2'-phosphotransferase Tpt1

Tpt1 is an essential 230-amino-acid enzyme that catalyzes the final step in yeast tRNA splicing: the transfer of the 2'-PO4 from the splice junction to NAD+ to form ADP-ribose 1'-2'cyclic phosphate and nicotinamide. To understand the structural requirements for Saccharomyces cerevisiae Tpt1 activity, we performed an alanine-scanning mutational analysis of 14 amino acids that are conserved in homologous proteins from fungi, metazoa, protozoa, bacteria, and archaea. We thereby identified four residues-Arg23, His24, Arg71, and Arg138-as essential for Tpt1 function in vivo. Structure-activity relationships at these positions were clarified by introducing conservative substitutions. The activity of the Escherichia coli ortholog KptA in complementing tpt1Delta was abolished by alanine substitutions at the equivalent side chains, Arg21, His22, Arg69, and Arg125. Deletion analysis of Tpt1 shows that the C-terminal 20 amino acids, which are not conserved, are not essential for activity in vivo at 30 degrees C. These findings attest to the structural and functional conservation of Tpt1-like 2'-phosphotransferases and identify likely constituents of the active site.     

The enzymatic conversion of 3'-phosphate terminated RNA chains to 2',3'-cyclic phosphate derivatives

The enzyme, RNA cyclase, has been purified from cell-free extracts of HeLa cells approximately 6000-fold. The enzyme catalyzes the conversion of 3'-phosphate ends of RNA chains to the 2',3'-cyclic phosphate derivative in the presence of ATP or adenosine 5'-(gamma-thio)triphosphate (ATP gamma S) and Mg2+. The formation of 1 mol of 2',3'-cyclic phosphate ends is associated with the disappearance of 1 mol of 3'-phosphate termini and the hydrolysis of 1 mol of ATP gamma S to AMP and thiopyrophosphate. No other nucleotides could substitute for ATP or ATP gamma S in the reaction. The reaction catalyzed by RNA cyclase was not reversible and exchange reactions between [32P]pyrophosphate and ATP were not detected. However, an enzyme-AMP intermediate could be identified that was hydrolyzed by the addition of inorganic pyrophosphate or 3'-phosphate terminated RNA chains but not by 3'-OH terminated chains or inorganic phosphate. 3'-[32P](Up)10Gp* could be converted to a form that yielded, (Formula: see text) after degradation with nuclease P1, by the addition of wheat germ RNA ligase, 5'-hydroxylpolynucleotide kinase, RNA cyclase, and ATP. This indicates that the RNA cyclase had catalyzed the formation of the 2',3'-cyclic phosphate derivative, the kinase had phosphorylated the 5'-hydroxyl end of the RNA, and the wheat germ RNA ligase had catalyzed the formation of a 3',5'-phosphodiester linkage concomitant with the conversion of the 2',3'-cyclic end to a 2'-phosphate terminated residue.