Poly(C) synthesis by class I and class II CCA-adding enzymes

The CCA-adding enzymes [ATP(CTP):tRNA nucleotidyl transferases], which catalyze synthesis of the conserved CCA sequence to the tRNA 3' end, are divided into two classes. Recent studies show that the class II Escherichia coli CCA-adding enzyme synthesizes poly(C) when incubated with CTP alone, but switches to synthesize CCA when incubated with both CTP and ATP. Because the poly(C) activity can shed important light on the mechanism of the untemplated synthesis of CCA, it is important to determine if this activity is also present in the class I CCA enzymes, which differ from the class II enzymes by significant sequence divergence. We show here that two members of the class I family, the archaeal Sulfolobus shibatae and Methanococcus jannaschii CCA-adding enzymes, are also capable of poly(C) synthesis. These two class I enzymes catalyze poly(C) synthesis and display a response of kinetic parameters to the presence of ATP similar to that of the class II E. coli enzyme. Thus, despite extensive sequence diversification, members of both classes employ common strategies of nucleotide addition, suggesting conservation of a mechanism in the development of specificity for CCA. For the E. coli enzyme, discrimination of poly(C) from CCA synthesis in the intact tRNA and in the acceptor-TPsiC domain is achieved by the same kinetic strategy, and a mutation that preferentially affects addition of A76 but not poly(C) has been identified. Additionally, we show that enzymes of both classes exhibit a processing activity that removes nucleotides in the 3' to 5' direction to as far as position 74.     

Divergent evolutions of trinucleotide polymerization revealed by an archaeal CCA-adding enzyme structure

CCA-adding enzyme [ATP(CTP):tRNA nucleotidyltransferase], a template-independent RNA polymerase, adds the defined 'cytidine-cytidine-adenosine' sequence onto the 3' end of tRNA. The archaeal CCA-adding enzyme (class I) and eubacterial/eukaryotic CCA-adding enzyme (class II) show little amino acid sequence homology, but catalyze the same reaction in a defined fashion. Here, we present the crystal structures of the class I archaeal CCA-adding enzyme from Archaeoglobus fulgidus, and its complexes with CTP and ATP at 2.0, 2.0 and 2.7 A resolutions, respectively. The geometry of the catalytic carboxylates and the relative positions of CTP and ATP to a single catalytic site are well conserved in both classes of CCA-adding enzymes, whereas the overall architectures, except for the catalytic core, of the class I and class II CCA-adding enzymes are fundamentally different. Furthermore, the recognition mechanisms of substrate nucleotides and tRNA molecules are distinct between these two classes, suggesting that the catalytic domains of class I and class II enzymes share a common origin, and distinct substrate recognition domains have been appended to form the two presently divergent classes.     

CCA addition by tRNA nucleotidyltransferase: polymerization without translocation?

The CCA-adding enzyme repairs the 3''-terminal CCA sequence of all tRNAs. To determine how the enzyme recognizes tRNA, we probed critical contacts between tRNA substrates and the archaeal Sulfolobus shibatae class I and the eubacterial Escherichia coli class II CCA-adding enzymes. Both CTP addition to tRNA-C and ATP addition to tRNA-CC were dramatically inhibited by alkylation of the same tRNA phosphates in the acceptor stem and TPsiC stem-loop. Both enzymes also protected the same tRNA phosphates in tRNA-C and tRNA-CC. Thus the tRNA substrate must remain fixed on the enzyme surface during CA addition. Indeed, tRNA-C cross-linked to the S. shibatae enzyme remains fully active for addition of CTP and ATP. We propose that the growing 3''-terminus of the tRNA progressively refolds to allow the solitary active site to reuse a single CTP binding site. The ATP binding site would then be created collaboratively by the refolded CC terminus and the enzyme, and nucleotide addition would cease when the nucleotide binding pocket is full. The template for CCA addition would be a dynamic ribonucleoprotein structure.     

Crystal structures of an archaeal class I CCA-adding enzyme and its nucleotide complexes

CCA-adding enzymes catalyze the addition of CCA onto the 3' terminus of immature tRNAs without using a nucleic acid template and have been divided into two classes based on their amino acid sequences. We have determined the crystal structures of a class I CCA-adding enzyme from Archeoglobus fulgidus (AfCCA) and its complexes with ATP, CTP, or UTP. Although it and the class II bacterial Bacillus stearothermophilus CCA enzyme (BstCCA) have similar dimensions and domain architectures (head, neck, body, and tail), only the polymerase domain is structurally homologous. Moreover, the relative orientation of the head domain with respect to the body and tail domains, which appear likely to bind tRNA, differs significantly between the two enzyme classes. Unlike the class II BstCCA, this enzyme binds nucleotides nonspecifically in the absence of bound tRNA. The shape and electrostatic charge distribution of the AfCCA enzyme suggests a model for tRNA binding that accounts for the phosphates that are protected from chemical modification by tRNA binding to AfCCA. The structures of the AfCCA enzyme and the eukaryotic poly(A) polymerase are very similar, implying a close evolutionary relationship between them.     

Isolation and properties of tRNA nucleotidyltransferase from wheat embryos.

From wheat embryos, tRNA nucleotidyltransferase (EC 2.7.7.25) was isolated. By chromatography on Sepharose 6B, DEAE-cellulose and affinity chromatography on tRNA-hydrazyl-Sepharose 4B, 7000-fold purification of the enzyme was achieved. The enzyme required for its activity Mg2+ or Mn2+ ion. ATP inhibited incorporation of CMP from CTP into lupin tRNA, and CTP acted as a competitive inhibitor of AMP incorporation from ATP. The regulatory role of ATP in incorporation of terminal CMP into tRNA is discussed. The incorporation of terminal CMP into tRNA deprived of terminal CCA or CA, was also studied.     

Identification and characterization of mammalian mitochondrial tRNA nucleotidyltransferases

The CCA-adding enzyme (ATP:tRNA adenylyltransferase or CTP:tRNA cytidylyltransferase (EC )) generates the conserved CCA sequence responsible for the attachment of amino acid at the 3' terminus of tRNA molecules. It was shown that enzymes from various organisms strictly recognize the elbow region of tRNA formed by the conserved D- and T-loops. However, most of the mammalian mitochondrial (mt) tRNAs lack consensus sequences in both D- and T-loops. To characterize the mammalian mt CCA-adding enzymes, we have partially purified the enzyme from bovine liver mitochondria and determined cDNA sequences from human and mouse dbESTs by mass spectrometric analysis. The identified sequences contained typical amino-terminal peptides for mitochondrial protein import and had characteristics of the class II nucleotidyltransferase superfamily that includes eukaryotic and eubacterial CCA-adding enzymes. The human recombinant enzyme was overexpressed in Escherichia coli, and its CCA-adding activity was characterized using several mt tRNAs as substrates. The results clearly show that the human mt CCA-adding enzyme can efficiently repair mt tRNAs that are poor substrates for the E. coli enzyme although both enzymes work equally well on cytoplasmic tRNAs. This suggests that the mammalian mt enzymes have evolved so as to recognize mt tRNAs with unusual structures.     

Deoxythymidine kinase induced in the HELA TK- cells by herpes simplex virus type I and type II. Substrate specificity and kinetic behavior.

Deoxythymidine kinases (EC 2.7.1.--) induced in HeLa TK- cells by Herpes simplex Type I and Type II viruses both had a requirement for divalent cations. The enzymes had the highest activities in the presence of Mg2+, followed by Mn2+, Ca2+, Fe2+, and in that order, whereas they were inactive in the presence of Zn2+ and Cu2+. The amount of Mg2+ required for optimal activity was dependent on the amount of ATP present, so that optimal activities were found when the concentration of Mg2+ was equal to that of ATP; an excess of Mg2+ inhibited the reaction. The activities of various nucleoside triphosphates as phosphate donors for Herpes simplex virus Type I deoxythymidine kinase were in the order: ATP = dATP = ara ATP greater than CTP greater than dCTP greater than UTP greater than dUTP greater than GTP greater than dGTP. Those for Herpes simplex virus Type II deoxythymidine kinase were in the order: CTP greater than dCTP = ara CTP greater than dATP greater than ATP greater than UTP greater than GTP greater than dUTP = dGTP. For both deoxythymidine kinases induced by Herpes simplex virus, the nucleoside triphosphates tested exerted cooperative effects. The Km values of ATP and CTP for the Herpes simplex virus Type I enzyme were 30 and 70 muM respectively; whereas those for the Herpes simplex virus Typr II enzyme were 140 and 450 muM. Studies on binding of various thymidine analogs with free 5'-OH to these deoxythymidine kinases indicated that 5-substituted ethyl-, vinyl-, allyl-, propyl-, iodo- and bromo-dUrd as well as iodo5 dCyd and bromo5 dCyd had good affinity to both enzymes. In contrast, vinyl5 Urd, iodo5 Urd and arabinosylthymidine had good affinity only to the Herpes simplex virus Type I enzyme but not to the Herpes simplex virus Type II deoxythymidine kinase. All of these thymidine analogs were competitive inhibitors, with KI values in the range of 0.25 to 1.5 muM. Herpes simplex virus Type I deoxythymidine kinase was less sensitive to either dTTP or iodo dUTP inhibition than Herpes simplex virus Type II. Both dThd and dCyd could serve as substrates and competed with each other for Herpes simplex viruses Type I and Type II induced kinases, but they differed in their Km values for these enzymes. The Km values of dThd and dCyd were 0.59 muM and 25 muM for Herpes simplex virus Type I deoxythymidine kinase; while they were 0.36 muM and 88 muM respectively for the Herpes simplex virus Type II enzyme.     

Possible regulation by nucleosidediphosphate kinase involvement of the synthesis of tRNA 3'-terminal -pCpCpA in mammalian cells

When the cytosol of Ehrlich ascites tumor cells was fractionated by chromatofocusing in the pH range of 9 to 6, two active peaks (I and II) of tRNA nucleotidyltransferase were obtained. Fraction I was a multiple complex with a high molecular weight (M.W. greater than 300K) and fraction II comprised components derived from fraction I. Fraction II was separated into tRNA nucleotidyltransferase (M.W., ca. 46,000) and nucleosidediphosphate kinase (M.W., ca. 74,000) by subsequent Sephacryl S-200 chromatography. The two enzymes appeared to be associated loosely with each other. Using the above fraction II or a mixture of the purified tRNA nucleotidyltransferase and nucleosidediphosphate kinase, it was possible to effectively synthesize the 3'-terminal -pCpCpA of tRNA in a reaction mixture containing [3H]-CDP plus XTP or [3H]ADP plus XTP as substrate. Among the XTPs investigated, dTTP was most effective. In addition, it was found that [3H]AMP + XTP also serves as a substrate. [14C]CMP plus XTP, however, was not utilized. From the antagonism of cold CDP against [3H]CTP, and that of cold ADP and AMP against [3H]ATP with the purified tRNA nucleotidyltransferase, the affinity of CDP to the enzyme was estimated to be 1/100 of that of CTP, while the affinities of ADP and AMP to the enzyme were 3 and 30 times higher, respectively, than that of ATP, suggesting that the subsite which binds ATP also binds ADP or AMP. The tRNA nucleotidyltransferase, which had bound ADP or AMP, could not completely synthesize the 3'-terminus of tRNA.(ABSTRACT TRUNCATED AT 250 WORDS)     

Closely related CC- and A-adding enzymes collaborate to construct and repair the 3'-terminal CCA of tRNA in Synechocystis sp. and Deinococcus radiodurans

The 3'-terminal CCA sequence of tRNA is faithfully constructed and repaired by the CCA-adding enzyme (ATP(CTP):tRNA nucleotidyltransferase) using CTP and ATP as substrates but no nucleic acid template. Until recently, all CCA-adding enzymes from all three kingdoms appeared to be composed of a single kind of polypeptide with dual specificity for adding both CTP and ATP; however, we recently found that in Aquifex aeolicus, which lies near the deepest root of the eubacterial 16 S rRNA-based phylogenetic tree, CCA addition represents a collaboration between closely related CC-adding and A-adding enzymes (Tomita, K. and Weiner, A. M. (2001) Science 294, 1334-1336). Here we show that in Synechocystis sp. and Deinococcus radiodurans, as in A. aeolicus, CCA is added by homologous CC- and A-adding enzymes. We also find that the eubacterial CCA-, CC-, and A-adding enzymes, as well as the related eubacterial poly(A) polymerases, each fall into phylogenetically distinct groups derived from a common ancestor. Intriguingly, the Thermatoga maritima CCA-adding enzyme groups with the A-adding enzymes, suggesting that these distinct tRNA nucleotidyltransferase activities can intraconvert over evolutionary time.     

Enzymatic incorporation of ATP and CTP analogues into the 3' end of tRNA

Structural analogues of adenosine 5'-triphosphate and cytidine 5'-triphosphate were investigated as substrates for ATP(CTP):tRNA nucleotidyl transferase. Eight out of 26 ATP analogues and six out of nine CTP analogues were incorporated into the 3' terminus of tRNA. In general, for the recognition of the substrates the modification of the cytidine is less critical than is the modification of adenosine. An isosteric substitution on the ribose residue is possible in both CTP and ATP. The free hydroxyls of these triphosphates can be replaced by an amino group or hydrogen atom without loss of substrate properties. Modifications of positions 1, 2, 6, and 8 on the adenine ring of ATP are not allowed whereas modification on positions 2, 4 and 5 on the cytosine ring of CTP are tolerated by the enzyme. No differences can be observed in the substrate properties of ATP(CTP):tRNA nucleotidyl transferase isolated from different sources. Methods for preparation of tRNA species, which are shortened at their 3' end by one or more nucleotides, and analytical procedures for characterisation of these modified tRNAs are described.     

Mechanism of the activation step of the aminoacylation reaction: a significant difference between class I and class II synthetases

In the present work we report, for the first time, a novel difference in the molecular mechanism of the activation step of aminoacylation reaction between the class I and class II aminoacyl tRNA synthetases (aaRSs). The observed difference is in the mode of nucleophilic attack by the oxygen atom of the carboxylic group of the substrate amino acid (AA) to the αP atom of adenosine triphosphate (ATP). The syn oxygen atom of the carboxylic group attacks the α-phosphorous atom (αP) of ATP in all class I aaRSs (except TrpRS) investigated, while the anti oxygen atom attacks in the case of class II aaRSs. The class I aaRSs investigated are GluRS, GlnRS, TyrRS, TrpRS, LeuRS, ValRS, IleRS, CysRS, and MetRS and class II aaRSs investigated are HisRS, LysRS, ProRS, AspRS, AsnRS, AlaRS, GlyRS, PheRS, and ThrRS. The variation of the electron density at bond critical points as a function of the conformation of the attacking oxygen atom measured by the dihedral angle ψ (C(α)-C') conclusively proves this. The result shows that the strength of the interaction of syn oxygen and αP is stronger than the interaction with the anti oxygen for class I aaRSs. This indicates that the syn oxygen is the most probable candidate for the nucleophilic attack in class I aaRSs. The result is further supported by the computation of the variation of the nonbonded interaction energies between αP atom and anti oxygen as well as syn oxygen in class I and II aaRSs, respectively. The difference in mechanism is explained based on the analysis of the electrostatic potential of the AA and ATP which shows that the relative arrangement of the ATP with respect to the AA is opposite in class I and class II aaRSs, which is correlated with the organization of the active site in respective aaRSs. A comparative study of the reaction mechanisms of the activation step in a class I aaRS (Glutaminyl tRNA synthetase) and in a class II aaRS (Histidyl tRNA synthetase) is carried out by the transition state analysis. The atoms in molecule analysis of the interaction between active site residues or ions and substrates are carried out in the reactant state and the transition state. The result shows that the observed novel difference in the mechanism is correlated with the organizations of the active sites of the respective aaRSs. The result has implication in understanding the experimentally observed different modes of tRNA binding in the two classes of aaRSs.     

A model for C74 addition by CCA-adding enzymes: C74 addition, like C75 and A76 addition, does not involve tRNA translocation

The CCA-adding enzyme adds CCA to the 3'-end of tRNA one nucleotide at a time, using CTP and ATP as substrates. We found previously that tRNA does not rotate or translocate on the enzyme during the addition of C75 and A76. We therefore predicted that the growing 3'-end of tRNA must, upon addition of each nucleotide, refold to reposition the new 3'-hydroxyl equivalently relative to the solitary nucleotidyltransferase motif. Cocrystal structures of the class I archaeal Archaeoglobus fulgidus enzyme, poised for addition of C75 and A76, confirmed this prediction. We have also demonstrated that an evolutionarily flexible beta-turn facilitates progressive refolding of the 3'-terminal C74 and C75 residues during C75 and A76 addition. Although useful cocrystals corresponding to C74 addition have not yet been obtained, we now show experimentally that tRNA does not rotate or translocate during C74 addition. We therefore propose, based on the existing A. fulgidus cocrystal structures, that the same flexible beta-turn functions as a wedge between the discriminator base (N73) and the terminal base pair of the acceptor stem, unstacking and repositioning N73 to attack the incoming CTP. Thus a single flexible beta-turn would orchestrate consecutive addition of all three nucleotides without significant movement of the tRNA on the enzyme surface.     

Identifying specific kinase substrates through engineered kinases and ATP analogs

Intracellular signaling by protein kinases controls many aspects of cellular biochemistry and physiology. Determining the direct substrates of protein kinases is important in understanding how these signaling enzymes exert their effect on cellular functions. One of the recent developments in this area takes advantage of the similarity in the ATP binding domains of protein kinases, where a few conserved amino acids containing large side chains come in close contact with the N-6 position of bound ATP. Mutation of one or more of these residues generates a "pocket" in the ATP binding site that allows the mutant kinase, but not other cellular kinases, to utilize analogs of ATP with bulky substituents synthesized onto the N-6 position. The use of such a mutated kinase and radiolabeled ATP analogs allows for the specific labeling of direct substrates of the kinase within a mixture of cellular proteins. We have recently reported the generation of "pocket" mutants of extracellular regulated kinase 2 (ERK2) and their use in the identification of two novel substrates of ERK2. In this report, we discuss the generation and characterization of ERK2 mutants that utilize analogs of ATP and describe the methodology used to identify ERK2-associated substrates. We also describe the direct labeling of ERK2 substrates in cell lysates. These methodologies can be adapted for use with other protein kinases to increase the understanding of intracellular signal transduction.     

Synthesis and properties of fluorescent nucleotide substrates for DNA-dependent RNA polymerases.

A new class of fluorescent nucleotide analogs which contain the fluorophore 1-aminonaphthalene-5-sulfonate attached via a gamma-phosphoamidate bond has been synthesized. Both the purine and pyrimidine analogs have fluorescence emission maxima at 460 nm. Cleavage of the alpha-beta-phosphoryl bond produces change in both the absorption and fluorescence emission spectra. The fluorescence of the pyrimidine analogs is quenched; cleavage of the alpha-beta-phosphoryl bond of the UTP analog produces about a 14-fold increase in fluorescence intensity at 500 nm. Under the same conditions the fluorescence of the CTP analog increases about 8-fold, whereas the fluorescence of the purine analogs shows only a slight change. These derivatives are good substrates for Escherichia coli RNA polymerase with only slightly increased Km values and with Vmax values about 50 to 70% that of the normal nucleotides. They are used less efficiently by wheat germ RNA polymerase II. The ATP analog can be used by E. coli RNA polymerase to initiate RNA chains.     

Efficient aminoacylation of resected RNA helices by class II aspartyl-tRNA synthetase dependent on a single nucleotide.

We show here that small RNA helices which recapitulate part or all of the acceptor stem of yeast aspartate tRNA are efficiently aminoacylated by cognate class II aspartyl-tRNA synthetase. Aminoacylation is strongly dependent on the presence of the single-stranded G73 'discriminator' identity nucleotide and is essentially insensitive to the sequence of the helical region. Substrates which contain as few as 3 bp fused to G73CCAOH are aspartylated. Their charging is insensitive to the sequence of the loop closing the short helical domains. Aminoacylation of the aspartate mini-helix is not stimulated by a hairpin helix mimicking the anticodon domain and containing the three major anticodon identity nucleotides. A thermodynamic analysis demonstrates that enzyme interactions with G73 in the resected RNA substrates and in the whole tRNA are the same. Thus, if the resected RNA molecules resemble in some way the earliest substrates for aminoacylation with aspartate, then the contemporary tRNA(Asp) has quantitatively retained the influence of the major signal for aminoacylation in these substrates.     

An engineered class I transfer RNA with a class II tertiary fold

Structure-based engineering of the tertiary fold of Escherichia coli tRNA(Gln)2 has enabled conversion of this transfer RNA to a class II structure while retaining recognition properties of a class I glutamine tRNA. The new tRNA possesses the 20-nt variable stem-loop of Thermus thermophilus tRNA(Ser). Enlargement of the D-loop appears essential to maintaining a stable tertiary structure in this species, while rearrangement of a base triple in the augmented D-stem is critical for efficient glutaminylation. These data provide new insight into structural determinants distinguishing the class I and class II tRNA folds, and demonstrate a marked sensitivity of glutaminyl-tRNA synthetase to alteration of tRNA tertiary structure.     

Breaking the stereo barrier of amino acid attachment to tRNA by a single nucleotide

Aminoacyl-tRNA synthetases are responsible for attaching amino acid residues to the tRNA 3'-end. The two classes of synthetases approach tRNA as mirror images, with opposite but symmetrical stereochemistries that allow the class I enzymes to attach amino acid residues to the 2'-hydroxyl group of the terminal ribose, whereas, the class II enzymes attach amino acid residues to the 3'-hydroxyl group. However, we show here that the attachment of cysteine to tRNA(Cys) by the class I cysteinyl-tRNA synthetase (CysRS) is flexible; the enzyme is capable of using either the 2' or 3'-hydroxyl group as the attachment site. The molecular basis for this flexibility was investigated. Introduction of the nucleotide U73 of tRNA(Cys) into tRNA(Val) was found to confer the flexibility. While valylation of the wild-type tRNA(Val) by the class I ValRS was strictly dependent on the terminal 2'-hydroxyl group, that of the U73 mutant of tRNA(Val) occurred at either the 2' or 3'-hydroxyl group. Thus, the single nucleotide U73 of tRNA has the ability to break the stereo barrier of amino acid attachment to tRNA, by mobilizing the 2' and 3'-hydroxyl groups of A76 in flexible geometry with respect to the tRNA acceptor stem.     

Complementary substrate specificities of class I and class II collagenases from Clostridium histolyticum

The substrate specificities of three class I (beta, gamma, and eta) and three class II (sigma, epsilon, and zeta) collagenases from Clostridium histolyticum have been investigated by quantitating the kcat/KM values for the hydrolysis of 53 synthetic peptides with collagen-like sequences covering the P3 through P3 subsites of the substrate. For both classes of collagenases, there is a strong preference for Gly in subsites P1' and P3. All six enzymes also prefer substrates that contain Pro and Ala in subsites P2 and P2' and Hyp, Ala, or Arg in subsite P3'. This agrees well with the occupancies of these sites by these residues in type I collagen. However, peptides with Glu in subsites P2 or P2' are not good substrates, even though Glu occurs frequently in these positions in collagen. Conversely, all six enzymes prefer aromatic amino acids in subsite P1, even though such residues do not occur in this position in type I collagen. In general, the class II enzymes have a broader specificity than the class I enzymes. However, they are much less active toward sequences containing Hyp in subsites P1 and P3'. Thus, the two classes of collagenases have similar but complementary sequence specificities. This accounts for the ability of the two classes of enzymes to synergistically digest collagen.     

Evidence for the absence of the terminal adenine nucleotide at the amino acid-acceptor end of transfer ribonucleic acid in non-lactating bovine mammary gland and its inhibitory effect on the aminoacylation of rat liver transfer ribonucleic acid

1. tRNA isolated from non-lactating bovine mammary gland competitively inhibits the formation of aminoacyl-tRNA in the rat liver system. 2. Non-lactating bovine mammary gland tRNA and twice-pyrophosphorolysed rat liver tRNA are unable to accept amino acids in a reaction catalysed by aminoacyl-tRNA synthetases from either rat liver or bovine mammary gland. Deacylated rat liver tRNA can however be aminoacylated in the presence of either enzyme. 3. Bovine mammary gland tRNA lacks the terminal adenine nucleotide at the 3′-terminus amino acid acceptor end, which can be replaced by incubation in the presence of rat liver nucleotide-incorporating enzyme, ATP and CTP. 4. The enzymically modified bovine tRNA (tRNApCpCpA) can bind labelled amino acids to form aminoacyl-tRNA, which can then transfer its labelled amino acids to growing polypeptide chains on ribosomes. 5. Molecules of rat liver tRNA or bovine mammary gland tRNA that lack the terminal adenine nucleotide or the terminal cytosine and adenine nucleotides inhibit the aminoacylation of normal rat liver tRNA to varying degrees. tRNA molecules lacking the terminal −pCpCpA nucleotide sequence exhibit the major inhibitory effect. 6. The enzyme fraction from bovine mammary gland corresponding to that containing the nucleotide-incorporating enzyme in rat liver is unable to catalyse the incorporation of cytosine and adenine nucleotides in pyrophosphorolysed rat liver tRNA and deacylated bovine tRNA. This fraction also markedly inhibits the action of the rat liver nucleotide-incorporating enzyme.