Cloning and verification of the Lactococcus lactis pyrG gene and characterization of the gene product, CTP synthase

The pyrG gene of Lactococcus lactis subsp. cremoris, encoding CTP synthase, has been cloned and sequenced. It is flanked upstream by an open reading frame showing homology to several aminotransferases and downstream by an open reading frame of unknown function. L. lactis strains harboring disrupted pyrG alleles were constructed. These mutants required cytidine for growth, proving that in L. lactis, the pyrG product is the only enzyme responsible for the amination of UTP to CTP. In contrast to the situation in Escherichia coli, an L. lactis pyrG mutant could be constructed in the presence of a functional cdd gene encoding cytidine deaminase. A characterization of the enzyme revealed similar properties as found for CTP synthases from other organisms. However, unlike the majority of CTP synthases the lactococcal enzyme can convert dUTP to dCTP, although a half saturation concentration of 0.6 mm for dUTP makes it unlikely that this reaction plays a significant physiological role. As for other CTP synthases, the oligomeric structure of the lactococcal enzyme was found to be a tetramer, but unlike most of the other previously characterized enzymes, the tetramer was very stable even at dilute enzyme concentrations.     

Changes in Levels of Purine and Pyrimidine Nucleotides During Acute Hypoxia and Recovery in Neonatal Rat Brain

Neonatal rat brains were examined for changes in levels of ATP, ADP, AMP, cyclic AMP, GTP, GDP, UTP, UDP, UMP, and CTP during exposure to 100% nitrogen for 20 min and subsequent recovery in air. During hypoxia, ATP, GTP, UTP, and CTP levels and the GTP/GDP ratio decreased to 38, 50, 26, 21, and 21%, respectively, of control levels. No significant change in cyclic AMP level was observed. The decrease in the total uridine nucleotide pool during hypoxia was markedly greater (to 53% of control levels) than that in the total adenine nucleotide pool (to 92% of control levels). During recovery, ATP and GTP levels were rapidly and almost completely restored. On the other hand, CTP levels returned slowly to control values after a 2-h recovery period. Restoration of the UTP level was slow and incomplete (87% of the control value even after a 3-h recovery period). The GTP/GDP ratio also did not return to normal. These data suggest that hypoxic insult to the neonate may have an effect on the synthesis of nucleotidyl sugars, phospholipids, and proteins in the brain, resulting in significant problems with developmental processes of the brain. The present study also showed that the delayed restorations of the UTP level and the GTP/GDP ratio were not seen in the brains of adult rats subjected to acute severe hypoxic insult.     

Pyrimidine Nucleotide Metabolism and Pathways of Thymidine Triphosphate Biosynthesis in Salmonella typhimurium

The nucleoside triphosphate pools of two cytidine auxotrophic mutants of Salmonella typhimurium LT-2 were studied under different conditions of pyrimidine starvation. Both mutants, DP-45 and DP-55, are defective in cytidine deaminase and cytidine triphosphate (CTP) synthase. In addition, DP-55 has a requirement for uracil (uridine). Cytidine starvation of the mutants results in accumulation of high concentrations of uridine triphosphate (UTP) in the cells, while the pools of CTP and deoxy-CTP drop to undetectable levels within a few minutes. Addition of deoxycytidine to such cells does not restore the dCTP pool, indicating that S. typhimurium has no deoxycytidine kinase. From the kinetics of UTP accumulation during cytidine starvation, it is concluded that only cytidine nucleotides participate in the feedback regulation of de novo synthesis of UTP; both uridine and cytidine nucleotides participate in the regulation of UTP synthesis from exogenously supplied uracil or uridine. Uracil starvation of DP-55 in presence of cytidine results in extensive accumulation of CTP, suggesting that CTP does not regulate its own synthesis from exogenous cytidine. Analysis of the thymidine triphosphate (dTTP) pool of DP-55 labeled for several generations with (32)P-orthophosphate and (3)H-uracil in presence of (12)C-cytidine shows that only 20% of the dTTP pool is derived from uracil (via the methylation of deoxyuridine monophosphate); 80% is apparently synthesized from a cytidine nucleotide.     

The Metabolism of Purine and Pyrimidine Nucleotides in Rat Cortex During Insulin-Induced Hypoglycemia and Recovery

Brains of paralysed rats with insulin-induced hypoglycemia were frozen in situ after spontaneous EEG activity had been absent for 5 or 15 min (“coma”). Recovery (30 min) was achieved in a different group of rats by administering glucose after a 30-min coma period. Purine and pyrimidine nucleotides, nucleosides and free bases were determined in the cortical extracts by high pressure liquid chromatography (HPLC). The ATP values obtained with the HPLC method were in excellent agreement with those obtained using standard enzymatic/fluorometric techniques, while values for ADP and AMP obtained with the HPLC method were significantly lower. Comatose animals showed a severe (40-80%) reduction in the concentrations of all nucleoside triphosphates (ATP. GTP, UTP and CTP) and a simultaneous increase in the concentrations of all nucleoside di- and monophosphates, including that of IMP. The adenine nucleotide pool size decreased to 50% of control level. The concentrations of the nucleosides adenosine, inosine, and uridine increased 50- to 250-fold, while the concentrations of the purine bases, xanthine and hypoxanthine, rose 2- and 30-fold, respectively. There were no increases in the concentrations of adenine, guanine, or xanthosine. Following glucose administration there was a partial (ATP, UTP and CTP) or almost complete (GTP) recovery of the nucleoside triphosphate levels. During recovery, the levels of nucleosidc di- and monophosphates and of adenosine decreased to values close to control; the rise in the inosine level was only partially reversed, and the concentrations of hypoxanthine and xanthine rose further. The adenine nucleotide pool size was only partially restored (to 67% of control value). The adenine nucleotide pool size was not increased by i.p. injection of adenosine or adenine under control condition, or during the posthypoglycemic recovery period.     

Nucleotide sequence of Escherichia coli pyrG encoding CTP synthetase

The amino acid sequence of Escherichia coli CTP synthetase was derived from the nucleotide sequence of pyrG. The derived amino acid sequence, confirmed at the N terminus by protein sequencing, predicts a subunit of 544 amino acids having a calculated Mr of 60,300 after removal of the initiator methionine. A glutamine amide transfer domain was identified which extends from approximately amino acid residue 300 to the C terminus of the molecule. The CTP synthetase glutamine amide transfer domain contains three conserved regions similar to those in GMP synthetase, anthranilate synthase, p-aminobenzoate synthase, and carbamoyl-P synthetase. The CTP synthetase structure supports a model for gene fusion of a trpG-related glutamine amide transfer domain to a primitive NH3-dependent CTP synthetase. The major 5' end of pyrG mRNA was localized to a position approximately 48 base pairs upstream of the translation initiation codon. Translation of the gene eno, encoding enolase, is initiated 89 base pairs downstream of pyrG. The pyrG-eno junction is characterized by multiple mRNA species which are ascribed to monocistronic pyrG and/or eno mRNAs and a pyrG eno polycistronic mRNA.     

Selective high metabolic lability of uridine, guanosine and cytosine triphosphates in response to glucose deprivation and refeeding of untransformed and polyoma virus-transformed hamster fibroblasts.

Sugar deprivation of hamster fibroblasts (NIL) affected the steady state levels (pool sizes) of cellular acid soluble nucleotides in the folloing fashion: the pools of UTP, GTP and CTP decreased to a much greater extent than the cellular ATP pools, with the UTP pools undergoing the most dramatic reduction. Sugar deprivation of polyoma-transformed NIL cells (PyNIL) yielded even sharper decreases in the nucleoside triphosphate pools with relative changes similar to those of the untransformed cells. Inhibition of protein synthesis by cycloheximide, initiated at the onset of (and continued during) sugar deprivation, prevented the reduction in pool sizes and yielded values slightly higher than those observed for pool sizes in cells cultured in sugar-supplemented medium.Refeeding glucose to sugar-depleted hamster fibroblasts led to rapid increases (within 1 hour) in the UTP and CTP pools to levels well above the pool sizes observed in cells which were continuously cultured (16 hours) in sugar supplemented medium. Feeding NIL or PyNIL cells with fructose instead of glucose as the only hexose source did not appreciably affect any of the ribonucleoside triphosphate pool sizes. Measurements of hexose uptake by NIL and PyNIL cells under a variety of conditions suggest that hexose transport is not regulated by the total cellular pools of ATP or any of the other ribonucleoside triphosphates.     

What determines the intracellular ATP concentration

Analysis is made of the mechanisms that control the intracellular ATP level. The balance between energy production and expenditure determines the energy charge of the cell and the ratio of [ATP] to the adenylate pool. The absolute ATP concentration is determined by the adenylate pool, which, in its turn, depends on the balance between the rates of AMP synthesis and degradation. Experimental data are discussed that demonstrate an increase in the adenylate pool in response to activation of energy-consuming processes. A hypothesis is proposed according to which variation in the adenylate pool and absolute ATP concentration affords a cell the possibility of additional control over processes fulfilling useful work. A mechanism involved in this regulation is described using human erythrocytes as an example. The hypothesis explains why different metabolic pathways (protein and DNA syntheses, polysaccharide synthesis, and lipid synthesis) use different trinucleotides (GTP, UTP, and CTP, respectively) as an energy source. This allows the cell to independently control these metabolic processes by varying the individual nucleotide pools.     

UTP/CTP ratio,an important regulatory parameter for ATCase expression

Intracellular nucleotides of Salmonella typhimurium were separated and quantified by high performance liquid chromatography (HPLC). Wild type and specially constructed strains of S. typhimurium, in which uridine and cytidine nucleotides could be manipulated independently, were used in this study. By varying growth conditions it was possible to create different concentrations of uridine and cytidine nucleotides in the cell. The specific activity of ATCase was determined for each condition. Generally, a direct correlation was found: at high nucleotide (UTP) concentrations, maximal repression of ATCase was usually seen; at low nucleotide (UTP) concentrations ATCase was derepressed. However, it was the ratio of the concentrations of UTP-to-CTP rather than either the concentration of UTP or CTP alone that best determined the extent of ATCase expression. This applied to all conditions in the present work as well as to all conditions in work hitherto reported by others. The ratio of UTP/CTP is proposed as a key regulatory parameter for pyr enzyme expression.Dedicated to Professor John Ingraham in appreciation for his guidance during my graduate work and in admiration for his confidence and eternal optimism     

Effect of D-galactosamine on nucleotides in the rat heart. Effects of cytidine and uridine administration

The treatment of rats by galactosamine (2 mmol/kg i.p.), which dramatically alters the metabolism of pyrimidine nucleotides in the liver, has been used to investigate the dynamics of pyrimidine nucleotides in the rat heart. Six hours after administration of the drug, the UTP and UDPG myocardial contents were decreased by respectively 40 and 52% while the sum of uracil nucleotides was increased by 66% and that of cytosine nucleotides by 15%. When administered 5 h after galactosamine treatment, cytidine (750 nmol/rat i.v.) induced a further increase in cytosine nucleotides (46% above control value 1 h later) without however effect on uracil nucleotides. On the other hand, the administration of uridine (250 nmol/rat, i.v. 5 h after galactosamine), while restoring UTP, UDPG and the pool of uracil nucleotides, provoked a decrease in cytosine nucleotide level (-17%). In the absence of galactosamine treatment, the administration of uridine and cytidine did not induce changes in nucleotide levels despite a rise in blood cytidine concentration. All these observations support the hypothesis that: 1. the pathway for cytosine nucleotide synthesis predominant in the heart is that utilizing preformed exogenous cytidine and 2. this pathway is mainly controlled by the intracellular concentration of UTP rather than that of CTP.     

Mononucleotide Metabolism in the Rat Brain After Transient Ischemia

Nucleotide metabolism was studied in rats during and following the induction of 10 min of forebrain ischemia (four-vessel occlusion model). Purine and pyrimidine nucleotides, nucleotides, and bases in forebrain extracts were quantitated by HPLC with an ultraviolet detector. Ischemia resulted in a severe reduction in the concentration of nucleoside triphosphates (ATP, GTP, UTP, and CTP) and an increase in the concentration of AMP, IMP, adenosine, inosine, hypoxanthine, and guanosine. During the recovery period, both the phosphocreatine level and adenylate energy charge were rapidly and completely restored to the normal range. ATP was only 78% of the control value at 180 min after ischemic reperfusion. Levels of nucleosides and bases were elevated during ischemia but decreased to values close to those of control animals following recirculation. Both the decrease in the adenine nucleotide pool and the incomplete ATP recovery were caused by insufficient reutilization of hypoxanthine via the purine salvage system. The content of cyclic AMP, which transiently accumulated during the early recirculation period, returned to the control level, paralleling the decrease of adenosine concentration, which suggested that adenylate cyclase activity during reperfusion is modulated by adenosine A2 receptors. The recovery of CTP was slow but greater than that of ATP, GTP, and UTP. The GTP/GDP ratio was higher than that of the control animals following recirculation.     

Responses of adenine nucleotides in germinating soybean embryonic axes to exogenously applied adenine and adenosine

The ATP content of soybean (Glycine max [L.] Merr. cv. Kent) axes incubated for 3 hours in 1 mm solutions of adenine and adenosine increased over 100% and 75%, respectively, over axes incubated in water. The increase in ATP was primarily due to the conversion of these purines to nucleotides via the nucleotide salvage pathway. The ATP formed was in a metabolically active pool because label from adenine was incorporated into acid-insoluble material. Adenine also increased the levels of GTP, UTP, and CTP, but not to the extent of the ATP level.     

In situ regulation of mammalian CTP synthetase by allosteric inhibition

The regulatory role of the allosteric site of CTP synthetase on flux through the enzyme in situ and on pyrimidine nucleotide triphosphate (NTP) pool balance was investigated using a mutant mouse T lymphoblast (S49) cell line which contains a CTP synthetase refractory to complete inhibition by CTP. Measurements of [3H]uridine incorporation into cellular pyrimidine NTP pools as a function of time indicated that CTP synthesis in intact wild type cells was markedly inhibited in a cooperative fashion by small increases in CTP pools, whereas flux across the enzyme in mutant cells was much less affected by changes in CTP levels. The cooperativity of the allosteric inhibition of the enzyme was greater in situ than in vitro. Exogenous manipulation of levels of GTP, an activator of the enzyme, indicated that GTP had a moderate effect on enzyme activity in situ, and changes in pools of ATP, a substrate of the enzyme, had small effects on CTP synthetase activity. The consequences of incubation with actinomycin D, cycloheximide, dibutyryl cyclic AMP, and 6-azauridine on the flux across CTP synthetase and on NTP pools differed considerably between wild type and mutant cells. Under conditions of growth arrest, an intact binding site for CTP on CTP synthetase was required to maintain a balance between the CTP and UTP pools in wild type cells. Moreover, wild type cells failed to incorporate H14CO3- into pyrimidine pools following growth arrest. In contrast, mutant cells incorporated the radiolabel at a high rate indicating loss of a regulatory function. These results indicated that uridine nucleotides are important regulators of pyrimidine nucleotide synthesis in mouse S49 cells, and CTP regulates the balance between UTP and CTP pools.     

Nutritional effects on precursor uptake and compartmentalization of intracellular pools in relation to RNA synthesis.

The effects of nutritional variables on the processing of exogenous precursors into RNA was examined. General nutritional deprivation, or asparagine depletion, led to significant changes in the absolute pool sizes, especially of ATP, UTP and CTP. Fluctuations were found depending on the elapsed time after the nutritional perturbations occurred, and the cell density of the cultures. Depletion of the medium by 28 h of growth, or 1 h of guinea pig asparaginase action, led to considerable inhibition of the conversion of exogenous uridine to CTP by the cells. A series of experiments indicated that in 6C3HED lymphoma cells the uridine nucleotide pool which provided the immediate precursors to RNA (denoted UTP-NA) behaves as a small compartment in rapid equilibrium with exogenously supplied nucleosides. The resemblance to the compartmentation model described by Plagemann (Plagemann, P.G.W. (1972) J. Cell Biol. 52, 131-146 and (1971) J. Cell. Physiol. 77, 241-258) for rat hepatoma cells was close. The UTP-NA pool of the 6C3HED cells constitutes no more than 5% of the cellular UTP pool and is relatively slow in equilibrating with the general cell pool. Correction of the rates of incorporation of isotope into RNA by using some function of the whole cell UTP specific activity to normalize the pool effects, was shown to be invalid.     

D-glucosamine-induced changes in nucleotide metabolism and growth of colon-carcinoma cells in culture.

Human colon-carcinoma cells were exposed to D-glucosamine at 2.5, 5 and 10 mM, concentrations that were growth-inhibitory but not cytocidal in the presence of a physiological glucose concentration. Labelling of these HT-29 cells with D-[14C]-glucosamine, followed by nucleotide analyses, demonstrated that UDP-N-acetyl-hexosamines represented the major intracellular nucleotide pool and the predominant metabolite of the amino sugar. D-[14C]Glucosamine was not a precursor of UDP-glucosamine. After 4h exposure to D-glucosamine (2.5 mM), the pool of UDP-N-acetylhexosamines was increased more than 6-fold, whereas UTP and CTP were markedly decreased. UDP-glucuronate content increased by more than 2-fold, whereas purine nucleotide content was little altered. Uridine (0.1 mM) largely reversed the decrease in UTP, CTP, UDP-glucose and UDP-galactose, while intensifying the expansion of the UDP-N-acetylhexosamine pool. Uridine did not reverse the D-glucosamine-induced retardation of growth in culture. A 50% decrease in growth also persisted when uridine and cytidine, cytidine alone, or UDP, were added together with D-glucosamine. The growth-inhibitory effect of the amino sugar could therefore be best correlated with the quantitative change in the pattern of sugar nucleotides, and, in particular, with the many-fold increase in UDP-N-acetylglucosamine and UDP-N-acetylgalactosamine.     

Mutational analysis of conserved glycine residues 142, 143 and 146 reveals Gly(142) is critical for tetramerization of CTP synthase from Escherichia coli

CTPS (cytidine 5'-triphosphate synthase) catalyses the ATP-dependent formation of CTP from UTP using either ammonia or L-glutamine as the nitrogen source. Binding of the substrates ATP and UTP, or the product CTP, promotes oligomerization of CTPS from inactive dimers to active tetramers. In the present study, site-directed mutagenesis was used to replace the fully conserved glycine residues 142 and 143 within the UTP-binding site and 146 within the CTP-binding site of Escherchia coli CTPS. CD spectral analyses of wild-type CTPS and the glycine mutants showed a slight reduction of approximately 15% in alpha-helical content for G142A and G143A relative to G146A and wild-type CTPS, suggesting some local alterations in structure. Relative to wild-type CTPS, the values of k(cat)/K(m) for ammonia-dependent and glutamine-dependent CTP formation catalysed by G143A were reduced 22- and 16-fold respectively, whereas the corresponding values for G146A were reduced only 1.4- and 1.8-fold respectively. The glutaminase activity (k(cat)) of G146A was similar to that exhibited by the wild-type enzyme, whereas that of G143A was reduced 7.5-fold. G146A exhibited substrate inhibition at high concentrations of ammonia and a partial uncoupling of glutamine hydrolysis from CTP production. Although the apparent affinity (1/[S](0.5)) of G143A and G146A for UTP was reduced approximately 4-fold, G146A exhibited increased co-operativity with respect to UTP. Thus mutations in the CTP-binding site can affect UTP-dependent activity. Surprisingly, G142A was inactive with both ammonia and glutamine as substrates. Gel-filtration HPLC experiments revealed that both G143A and G146A were able to form active tetramers in the presence of ATP and UTP; however, nucleotide-dependent tetramerization of G142A was significantly impaired. Our observations highlight the sensitivity of the structure of CTPS to mutations in the UTP- and CTP-binding sites, with Gly(142) being critical for nucleotide-dependent oligomerization of CTPS to active tetramers. This 'structural sensitivity' may limit the number and/or types of mutations that could be selected for during the development of resistance to cytotoxic pyrimidine nucleotide analogues.     

Effect of various precursors on the synthesis of adenine and uracil nucleotides in the rat heart (author's transl)

The dynamics of cardiac adenine and uracil nucleotides, following a subcutaneous injection of isoproterenol, was studied on the rat in vivo. The effect of continuous supply of adenosine, uridine, or ribose on the level of ATP and UTP was investigated on control rats and on isoproterenol-treated animals. The precursors were administered by continuous infusion (1 ml.h-1) into the superior caval vein. 1. ATP and UTP levels were decreased within one hour after a single dose of isoproterenol (5 mg.kg-1) (Fig. 1). 2. Then, the level of ATP rose slowly toward the control value. The normal level was not reached within 48 h (Fig. 1). 3. On the contrary, the initial drop in UTP concentration was followed by a rapid restoration. The control value was reached in 3 h, and then the UTP pool was increased to 180% of the normal level, 12 h after isoproterenol application. 4. As previously shown by other authors, the restoration of ATP was accelerated by a continuous supply of adenosine (37 micromoles per hour) or ribose (170 micromoles per hour) (Fig. 2). 5. The infusion of ribose (170 micromoles per hour) or uridine (41 micromoles per hour) completely suppressed the initial decrease in UTP level caused by beta-receptor stimulation. The further enlargement of the UTP pool was greatly enhanced by ribose or uridine (Fig. 3). 6. The infusion of adenosine was also positive on UTP regeneration. On the contrary, uridine had no effect on the ATP pool (Fig. 3). 7. When supplied to non-treated animals, all precursors caused an enhancement of the UTP level. Adenosine and ribose increased the ATP pool (Fig. 2 and 3). These results contribute to the comparison of the efficiency of the various pathways of cardiac nucleotide synthesis. They show that both de novo synthesis and salvage pathways are limited by the amount of precursors. The increase in UTP synthesis caused by ribose is consistent with the theory put forward for purines (ZIMMER et GERLACH, 1974) that phosphoribosyl-pyrophosphate availability limits the efficiency of de novo synthesis of nucleotides; it demonstrates that this concept is also true for de novo synthesis of pyrimidine nucleotides.     

The synergistic inhibition of Escherichia coli aspartate carbamoyltransferase by UTP in the presence of CTP is due to the binding of UTP to the low affinity CTP sites

Escherichia coli aspartate carbamoyltransferase controls pyrimidine biosynthesis by feedback inhibition involving both CTP and UTP, although UTP only inhibits the enzyme in the presence of CTP (Wild, J. R., Loughrey-Chen, S. J., and Corder, T. S. (1989) Proc. Natl. Acad. Sci. U. S. A. 86, 46-50). The mechanism by which the enzyme can discriminate between these two pyrimidines is unknown, as well as where UTP binds and its mode of action. A mutant version of the enzyme with a single amino acid substitution in the regulatory site (Asp-19----Ala) causes loss of the synergistic inhibition of UTP in the presence of CTP, and furthermore, this enzyme is inhibited by UTP alone. Analysis of CTP binding to the mutant enzyme reveals that UTP can bind to the mutant enzyme in the absence of CTP but not in its presence. This is completely opposite to the wild-type enzyme in which case UTP only exhibits significant binding in the presence of CTP. Further analysis of the binding data for the wild-type enzyme reveals that, in the presence of UTP, CTP only binds to three sites, although CTP binds to six sites, three with high affinity and three with low affinity in the absence of UTP. Parallel UTP binding experiments in the presence of CTP suggest that UTP binds to the three weak CTP sites. The Asp-19----Ala substitution prevents UTP binding in the presence of CTP and allows UTP to bind and inhibit the enzyme in the absence of CTP. Since the x-ray data indicate no specific interactions between the amino group of cytosine and amino acid side chains in the regulatory binding site, the discrimination between UTP and CTP by the wild-type enzyme must be due to subtle differences in the binding sites rather than direct side chain contacts.     

Inactivation and covalent modification of CTP synthetase by thiourea dioxide.

Thiourea dioxide was used in chemical modification studies to identify functionally important amino acids in Escherichia coli CTP synthetase. Incubation at pH 8.0 in the absence of substrates led to rapid, time dependent, and irreversible inactivation of the enzyme. The second-order rate constant for inactivation was 0.18 M-1 s-1. Inactivation also occurred in the absence of oxygen and in the presence of catalase, thereby ruling out mixed-function oxidation/reduction as the mode of amino acid modification. Saturating concentrations of the substrates ATP and UTP, and the allosteric activator GTP prevented inactivation by thiourea dioxide, whereas saturating concentrations of glutamine (a substrate) did not. The concentration dependence of nucleotide protection revealed cooperative behavior with respect to individual nucleotides and with respect to various combinations of nucleotides. Mixtures of nucleotides afforded greater protection against inactivation than single nucleotides alone, and a combination of the substrates ATP and UTP provided the most protection. The Hill coefficient for nucleotide protection was approximately 2 for ATP, UTP, and GTP. In the presence of 1:1 ratios of ATP:UTP, ATP:GTP, and UTP:GTP, the Hill coefficient was approximately 4 in each case. Fluorescence and circular dichroism measurements indicated that modification by thiourea dioxide causes detectable changes in the structure of the protein. Modification with [14C]thiourea dioxide demonstrated that complete inactivation correlates with incorporation of 3 mol of [14C]thiourea dioxide per mole of CTP synthetase monomer. The specificity of thiourea dioxide for lysine residues indicates that one or more lysines are most likely involved in CTP synthetase activity. The data further indicate that nucleotide binding prevents access to these functionally important residues.