Deoxyuridylate kinase activity and deoxyuridinetriphosphatase in Escherichia coli

Structure-function analysis of nucleotide housekeeping protein HAM1 from human malaria parasite Plasmodium falciparum

Non-canonical nucleotides, generated as oxidative metabolic by-products, significantly threaten the genome integrity of Plasmodium falciparum and thereby, their survival, owing to their mutagenic effects. PfHAM1, an evolutionarily conserved inosine/xanthosine triphosphate pyrophosphohydrolase, maintains nucleotide homeostasis in the malaria parasite by removing non-canonical nucleotides, although structure-function intricacies are hitherto poorly reported. Here, we report the X-ray crystal structure of PfHAM1, which revealed a homodimeric structure, additionally validated by size-exclusion chromatography-multi-angle light scattering analysis. The two monomeric units in the dimer were aligned in a parallel fashion, and critical residues associated with substrate and metal binding were identified, wherein a notable structural difference was observed in the β-sheet main frame compared to human inosine triphosphate pyrophosphatase. PfHAM1 exhibited Mg++-dependent pyrophosphohydrolase activity and the highest binding affinity to dITP compared to other non-canonical nucleotides as measured by isothermal titration calorimetry. Modifying the pfham1 genomic locus followed by live-cell imaging of expressed mNeonGreen-tagged PfHAM1 demonstrated its ubiquitous presence in the cytoplasm across erythrocytic stages with greater expression in trophozoites and schizonts. Interestingly, CRISPR-Cas9/DiCre recombinase-guided pfham1-null P. falciparum survived in culture under standard growth conditions, indicating its assistive role in non-canonical nucleotide clearance during intra-erythrocytic stages. This is the first comprehensive structural and functional report of PfHAM1, an atypical nucleotide-cleansing enzyme in P. falciparum.

5-Ethynyl-2'-deoxycytidine and 5-ethynyl-2'-deoxyuridine are differentially incorporated in cells infected with HSV-1, HCMV, and KSHV viruses

Nucleoside analogues are a valuable experimental tool. Incorporation of these molecules into newly synthesized DNA (i.e. pulse-labeling) is used to monitor cell proliferation or to isolate nascent DNA. Some of the most common nucleoside analogues used for pulse-labeling of DNA in cells are the deoxypyrimidine analogues 5-ethynyl-2'-deoxyuridine (EdU) and 5-ethynyl-2'-deoxycytidine (EdC). Click chemistry enables conjugation of an azide molecule tagged with a fluorescent dye or biotin to the alkyne of the analog, which can then be used to detect incorporation of EdU and EdC into DNA. The use of EdC is often recommended because of the potential cytotoxicity associated with EdU during longer incubations. Here, by comparing the relative incorporation efficiencies of EdU and EdC during short 30-min pulses, we demonstrate significantly lower incorporation of EdC than of EdU in noninfected human fibroblast cells or in cells infected with either human cytomegalovirus or Kaposi's sarcoma-associated herpesvirus. Interestingly, cells infected with herpes simplex virus type-1 (HSV-1) incorporated EdC and EdU at similar levels during short pulses. Of note, exogenous expression of HSV-1 thymidine kinase increased the incorporation efficiency of EdC. These results highlight the limitations when using substituted pyrimidine analogues in pulse-labeling and suggest that EdU is the preferable nucleoside analogue for short pulse-labeling experiments, resulting in increased recovery and sensitivity for downstream applications. This is an important discovery that may help to better characterize the biochemical properties of different nucleoside analogues with a given kinase, ultimately leading to significant differences in labeling efficiency of nascent DNA.

Structural Insight into African Swine Fever Virus dUTPase Reveals a Novel Folding Pattern in the dUTPase Family

The African swine fever virus (ASFV) is the deadly pathogen of African swine fever (ASF) that induces high mortality, approaching 100% in domestic pigs, causes enormous losses to the global pig industry, and threatens food security. Currently, there is no effective treatment or preventive countermeasure. dUTPases (deoxyuridine 5'-triphosphate pyrophosphatases) are ubiquitous enzymes that are essential for the hydrolysis of dUTP and prevent the misincorporation of dUTP into newly synthesized DNA. Here, we present the crystal structures of the ASFV dUTPase in complex with the product dUMP and cofactor Mg²⁺ at a resolution of 2.2 Å. We observed that a unique "turning point" at G125 plays an unexpected critical role in the swapping region of the C-terminal segment, which is further stabilized by the interactions of the last C-terminal β strand with the β1 and β2 strands, thereby positioning the catalytic motif 5 into the active site of its own subunit instead of into a third subunit. Therefore, the ASFV dUTPase employs a novel two-subunit active site that is different than the classic trimeric dUTPase active site, which is composed of all three subunits. Meanwhile, further results confirmed that the configuration of motifs 1 to 5 has high structural homology with and a catalytic mechanism similar to that of the known trimeric dUTPases. In general, our study expands the information not only on the structural diversity of the conserved dUTPase family but also on the details needed to utilize this dUTPase as a novel target in the treatment of ASF.IMPORTANCE African swine fever virus (AFSV), a large enveloped double-stranded DNA virus, causes a deadly infection in domestic pigs. In addition to Africa, Europe, and South America, countries in Asia, such as China, Vietnam, and Mongolia, have suffered the hazards posed by ASFV outbreaks in recent years. Until now, there has been no vaccine for protection from ASFV infection or effective treatments to cure ASF. Here, we solved the crystal structure of the ASFV dUTPase-dUMP-Mg²⁺ complex. The ASFV dUTPase displays a noncanonical folding pattern that differs from that of the classic homotrimeric dUTPase, in which the active site is composed of two subunits. In addition, several nonconserved residues within the 3-fold axis channel play a vital role in ASFV dUTPase homotrimer stability. Our finding on these unique structural features of the ASFV dUTPase could be explored for the design of potential specific inhibitors that target this unique enzyme.

Functional genetic evaluation of DNA house-cleaning enzymes in the malaria parasite: dUTPase and Ap4AH are essential in Plasmodium berghei but ITPase and NDH are dispensable

BACKGROUND

Cellular metabolism generates reactive oxygen species. The oxidation and deamination of the deoxynucleoside triphosphate (dNTP) pool results in the formation of non-canonical, toxic dNTPs that can cause mutations, genome instability, and cell death. House-cleaning or sanitation enzymes that break down and detoxify non-canonical nucleotides play major protective roles in nucleotide metabolism and constitute key drug targets for cancer and various pathogens. We hypothesized that owing to their protective roles in nucleotide metabolism, these house-cleaning enzymes are key drug targets in the malaria parasite.

METHODS

Using the rodent malaria parasite Plasmodium berghei we evaluate here, by gene targeting, a group of conserved proteins with a putative function in the detoxification of non-canonical nucleotides as potential antimalarial drug targets: they are inosine triphosphate pyrophosphatase (ITPase), deoxyuridine triphosphate pyrophosphatase (dUTPase) and two NuDiX hydroxylases, the diadenosine tetraphosphate (Ap4A) hydrolase and the nucleoside triphosphate hydrolase (NDH).

RESULTS

While all four proteins are expressed constitutively across the intraerythrocytic developmental cycle, neither ITPase nor NDH are required for parasite viability. dutpase and ap4ah null mutants, on the other hand, are not viable suggesting an essential function for these proteins for the malaria parasite.

CONCLUSIONS

Plasmodium dUTPase and Ap4A could be drug targets in the malaria parasite.

Structural insights into the mechanism defining substrate affinity in Arabidopsis thaliana dUTPase: the role of tryptophan 93 in ligand orientation

BACKGROUND

Deoxyuridine triphosphate nucleotidohydrolase (dUTPase) hydrolyzes dUTP to dUMP and pyrophosphate to maintain the cellular thymine-uracil ratio. dUTPase is also a target for cancer chemotherapy. However, the mechanism defining its substrate affinity remains unclear. Sequence comparisons of various dUTPases revealed that Arabidopsis thaliana dUTPase has a unique tryptophan at position 93, which potentially contributes to its degree of substrate affinity. To better understand the roles of tryptophan 93, A. thaliana dUTPase was studied.

RESULTS

Enzyme assays showed that A. thaliana dUTPase belongs to a high-affinity group of isozymes, which also includes the enzymes from Escherichia coli and Mycobacterium tuberculosis. Enzymes from Homo sapiens and Saccharomyces cerevisiae are grouped as low-affinity dUTPases. The structure of the homo-trimeric A. thaliana dUTPase showed three active sites, each with a different set of ligand interactions between the amino acids and water molecules. On an α-helix, tryptophan 93 appears to keep serine 89 in place via a water molecule and to specifically direct the ligand. Upon being oriented in the active site, the C-terminal residues close the active site to promote the reaction.

CONCLUSIONS

In the high-affinity group, the prefixed direction of the serine residues was oriented by a positively charged residue located four amino acids away, while low-affinity enzymes possess small hydrophobic residues at the corresponding sites.

Homologous recombination is stimulated by a decrease in dUTPase in Arabidopsis

Deoxyuridine triphosphatase (dUTPase) enzyme is an essential enzyme that protects DNA against uracil incorporation. No organism can tolerate the absence of this activity. In this article, we show that dUTPase function is conserved between E. coli (Escherichia coli), yeast (Saccharomyces cerevisiae) and Arabidopsis (Arabidopsis thaliana) and that it is essential in Arabidopsis as in both micro-organisms. Using a RNA interference strategy, plant lines were generated with a diminished dUTPase activity as compared to the wild-type. These plants are sensitive to 5-fluoro-uracil. As an indication of DNA damage, inactivation of dUTPase results in the induction of AtRAD51 and AtPARP2, which are involved in DNA repair. Nevertheless, RNAi/DUT1 constructs are compatible with a rad51 mutation. Using a TUNEL assay, DNA damage was observed in the RNAi/DUT1 plants. Finally, plants carrying a homologous recombination (HR) exclusive substrate transformed with the RNAi/DUT1 construct exhibit a seven times increase in homologous recombination events. Increased HR was only detected in the plants that were the most sensitive to 5-fluoro-uracils, thus establishing a link between uracil incorporation in the genomic DNA and HR. Our results show for the first time that genetic instability provoked by the presence of uracils in the DNA is poorly tolerated and that this base misincorporation globally stimulates HR in plants.

Fermentative production of thymidine by a metabolically engineered Escherichia coli strain

Thymidine is an important precursor in the production of various antiviral drugs, including azidothymidine for the treatment of AIDS. Since thymidine-containing nucleotides are synthesized only by the de novo pathway during DNA synthesis, it is not easy to produce a large amount of thymidine biologically. In order to develop a host strain to produce thymidine, thymidine phosphorylase, thymidine kinase, and uridine phosphorylase genes were deleted from an Escherichia coli BL21 strain to develop BLdtu. Since the genes coding for the enzymes related to the nucleotide salvage pathway were disrupted, BLdtu was unable to utilize thymidine or thymine, and thymidine degradation activity was completely abrogated. We additionally expressed T4 thymidylate synthase, T4 nucleotide diphosphate reductase, bacteriophage PBS2 TMP phosphohydrolase, E. coli dCTP deaminase, and E. coli uridine kinase in the BLdtu strain to develop a thymidine-producing strain (BLdtu24). BLdtu24 produced 649.3 mg liter(-1) of thymidine in a 7-liter batch fermenter for 24 h, and neither thymine nor uridine was detected. However, the dUTP/dTTP ratio was increased in BLdtu24, which could lead to increased double-strand breakages and eventually to cell deaths during fermentation. To enhance thymidine production and to prevent cell deaths during fermentation, we disrupted a gene (encoding uracil-DNA N-glycosylase) involved in DNA excision repair to suppress the consumption of dTTP and developed BLdtug24. Compared with the thymidine production in BLdtu24, the thymidine production in BLdtug24 was increased by approximately 1.2-fold (740.3 mg liter(-1)). Here, we show that a thymidine-producing strain with a relatively high yield can be developed using a metabolic engineering approach.

Synthetic lethality with the dut defect in Escherichia coli reveals layers of DNA damage of increasing complexity due to uracil incorporation

Synthetic lethality is inviability of a double-mutant combination of two fully viable single mutants, commonly interpreted as redundancy at an essential metabolic step. The dut-1 defect in Escherichia coli inactivates dUTPase, causing increased uracil incorporation in DNA and known synthetic lethalities [SL(dut) mutations]. According to the redundancy logic, most of these SL(dut) mutations should affect nucleotide metabolism. After a systematic search for SL(dut) mutants, we did identify a single defect in the DNA precursor metabolism, inactivating thymidine kinase (tdk), that confirmed the redundancy explanation of synthetic lethality. However, we found that the bulk of mutations interacting genetically with dut are in DNA repair, revealing layers of damage of increasing complexity that uracil-DNA incorporation sends through the chromosomal metabolism. Thus, we isolated mutants in functions involved in (i) uracil-DNA excision (ung, polA, and xthA); (ii) double-strand DNA break repair (recA, recBC, and ruvABC); and (iii) chromosomal-dimer resolution (xerC, xerD, and ftsK). These mutants in various DNA repair transactions cannot be redundant with dUTPase and instead reveal "defect-damage-repair" cycles linking unrelated metabolic pathways. In addition, two SL(dut) inserts (phoU and degP) identify functions that could act to support the weakened activity of the Dut-1 mutant enzyme, suggesting the "compensation" explanation for this synthetic lethality. We conclude that genetic interactions with dut can be explained by redundancy, by defect-damage-repair cycles, or as compensation.

The mutT defect does not elevate chromosomal fragmentation in Escherichia coli because of the surprisingly low levels of MutM/MutY-recognized DNA modifications

Nucleotide pool sanitizing enzymes Dut (dUTPase), RdgB (dITPase), and MutT (8-oxo-dGTPase) of Escherichia coli hydrolyze noncanonical DNA precursors to prevent incorporation of base analogs into DNA. Previous studies reported dramatic AT-->CG mutagenesis in mutT mutants, suggesting a considerable density of 8-oxo-G in DNA that should cause frequent excision and chromosomal fragmentation, irreparable in the absence of RecBCD-catalyzed repair and similar to the lethality of dut recBC and rdgB recBC double mutants. In contrast, we found mutT recBC double mutants viable with no signs of chromosomal fragmentation. Overproduction of the MutM and MutY DNA glycosylases, both acting on DNA containing 8-oxo-G, still yields no lethality in mutT recBC double mutants. Plasmid DNA, extracted from mutT mutM double mutant cells and treated with MutM in vitro, shows no increased relaxation, indicating no additional 8-oxo-G modifications. Our DeltamutT allele elevates the AT-->CG transversion rate 27,000-fold, consistent with published reports. However, the rate of AT-->CG transversions in our mutT(+) progenitor strain is some two orders of magnitude lower than in previous studies, which lowers the absolute rate of mutagenesis in DeltamutT derivatives, translating into less than four 8-oxo-G modifications per genome equivalent, which is too low to cause the expected effects. Introduction of various additional mutations in the DeltamutT strain or treatment with oxidative agents failed to increase the mutagenesis even twofold. We conclude that, in contrast to the previous studies, there is not enough 8-oxo-G in the DNA of mutT mutants to cause elevated excision repair that would trigger chromosomal fragmentation.

Hypoxanthine incorporation is nonmutagenic in Escherichia coli

Endonuclease V, encoded by the nfi gene, initiates removal of the base analogs hypoxanthine and xanthine from DNA, acting to prevent mutagenesis from purine base deamination within the DNA. On the other hand, the RdgB nucleotide hydrolase in Escherichia coli is proposed to prevent hypoxanthine and xanthine incorporation into DNA by intercepting the noncanonical DNA precursors dITP and dXTP. Because many base analogs are mutagenic when incorporated into DNA, it is intuitive to think of RdgB as acting to prevent similar mutagenesis from deaminated purines in the DNA precursor pools. To test this idea, we used a set of Claire Cupples' strains to detect changes in spontaneous mutagenesis spectra, as well as in nitrous acid-induced mutagenesis spectra, in wild-type cells and in rdgB single, nfi single, and rdgB nfi double mutants. We found neither a significant increase in spontaneous mutagenesis in rdgB and nfi single mutants or the double mutant nor any changes in nitrous acid-induced mutagenesis for rdgB mutant strains. We conclude that incorporation of deaminated purines into DNA is nonmutagenic.

dUTPase from Escherichia coli; high-level expression and one-step purification

The dut gene, which encodes Escherichia coli deoxyuridine 5'-triphosphate nucleotidohydrolase (dUTPase), has been recloned to increase overexpression of the enzyme and to enable simplification of the purification protocol into a one-step procedure. The gene was cloned into the vector pET-3a and expressed in E. coli BL21(DE3) pLysS under the control of a bacteriophage T7 promotor. Induction results in production of dUTPase corresponding to 60% of the extracted protein. Phosphocellulose chromatography at low pH was utilised for one-step purification, resulting in a homogenous preparation of the recombinant protein with a highly specific activity. The yield of purified enzyme is 500 mg per litre of bacterial culture, a significant increase compared to previously employed methods.

Crystal structure of dUTPase from equine infectious anaemia virus; active site metal binding in a substrate analogue complex

The X-ray structures of dUTPase from equine infectious anaemia virus (EIAV) in unliganded and complexed forms have been determined to 1.9 and 2.0 A resolution, respectively. The structures were solved by molecular replacement using Escherichia coli dUTPase as search model. The exploitation of a relatively novel refinement approach for the initial model, combining maximum likelihood refinement with stereochemically unrestrained updating of the model, proved to be of crucial importance and should be of general relevance.EIAV dUTPase is a homotrimer where each subunit folds into a twisted antiparallel beta-barrel with the N and C-terminal portions interacting with adjacent subunits. The C-terminal 14 and 17 amino acid residues are disordered in the crystal structure of the unliganded and complexed enzyme, respectively. Interactions along the 3-fold axis include a water-containing volume (size 207 A3) which has no contact with bulk solvent. It has earlier been shown that a divalent metal ion is essential for catalysis. For the first time, a putative binding site for such a metal ion, in this case Sr2+, is established. The positions of the inhibitor (the non-hydrolysable substrate analogue dUDP) and the metal ion in the complex are consistent with the location of the active centre established for trimeric dUTPase structures, in which subunit interfaces form three surface clefts lined with evolutionary conserved residues. However, a detailed comparison of the active sites of the EIAV and E. coli enzymes reveals some structural differences. The viral enzyme undergoes a small conformational change in the uracil-binding beta-hairpin structure upon dUDP binding not observed in the other known dUTPase structures.

In vivo functions of the auxiliary genes and regulatory elements of feline immunodeficiency virus

Feline immunodeficiency virus (FIV) is a widespread lentivirus of domestic cats that causes an acquired immunodeficiency syndrome (AIDS)-like disease similar to human AIDS caused by human immunodeficiency virus. FIV has a complex genome structure including structural, enzymatic and auxiliary genes and regulatory elements. In this article, we review the in vivo roles of some of these FIV auxiliary genes and regulatory elements, especially focusing on the dUTPase, vif, and ORF-A genes and AP-1 binding site in the enhancer region of the long terminal repeat, by comparison with those of other non-primate lentiviruses. These genes and elements are considered to be important for viral replication, immunological response and pathogenesis in cats.

Description of a novel eukaryotic deoxyuridine 5'-triphosphate nucleotidohydrolase in Leishmania major

A Leishmania major full-length cDNA encoding a functional dUTP nucleotidohydrolase (dUTPase; EC 3.6.1.23) was isolated from a cDNA expression library by genetic complementation of dUTPase deficiency in Escherichia coli. The cDNA contained an open reading frame that encoded a protein of 269 amino acid residues with a calculated molecular mass of 30.3 kDa. Although eukaryotic dUTPases exhibit extensive similarity and there are five amino acid motifs that are common to all currently known dUTPase enzymes, the sequence of the protozoan gene differs significantly from its eukaryotic counterparts. None of the characteristic motifs were readily identifiable and the sequence encoded a larger polypeptide. However, the products of the reaction were dUMP and PPi, competition experiments with other deoxyribonucleoside triphosphates showed that the reaction is specific for dUTP, and the protozoan gene complemented dUTPase deficiency in Escherichia coli. The gene is of single copy; Northern blots indicated a transcript of the same size as the cDNA isolated in the screening procedure. The enzyme can be efficiently overexpressed in a highly active form by using the expression vector pET-11c. The availability of recombinant enzyme in large quantities will now permit detailed mechanistic and structural studies, which might contribute to a rational design of specifically targeted inhibitors against dUTPase from L. major.

Replication in vitro and in vivo of an equine infectious anemia virus mutant deficient in dUTPase activity

As an important enzyme in DNA synthesis, dUTPase is present in a wide variety of organisms and viruses and has been identified as a component of the equine infectious anemia virus (EIAV) pol gene. The role of EIAV dUTPase, designated DU, in virus replication in vitro and in vivo was investigated with a recently described infectious molecular clone of EIAV. A deletion mutant that was deficient in dUTPase activity was constructed, and its replication kinetics was examined in fetal equine kidney (FEK) cells and primary equine bone marrow macrophage (EBMM) cells. In FEK cells, which are permissive for EIAV replication, the mutant virus replicated as well as the parental virus. In primary cultures of EBMM cells, which are primary targets of EIAV infection in vivo, the DU mutant showed delayed replication kinetics and replicated to a lower extent than did the parental virus. As the multiplicity of infection decreased, the difference between the parental and mutant viruses increased, such that at the lowest multiplicity of infection tested, there was over a 100-fold difference in virus production. The mutant virus was also much less cytopathic. The role of DU in replication in vivo was examined using a Shetland pony model of EIAV infection. Shetland ponies that were infected with the parental and mutant viruses showed transient virus RNA levels in plasma approximately 5 to 10 days postinfection. The peak virus levels in plasma (as measured by a quantitative reverse transcriptase PCR assay) were 10- to 100-fold lower in the mutant virus-infected animals than in the animals infected with the parental virus. However, ponies infected with the mutant virus mounted similar antibody responses despite the marked differences in virus replication. These studies demonstrate that EIAV DU is important for the efficient replication of the virus in macrophages in vitro and in vivo and suggests that variations in the DU sequence could markedly affect the biological and pathogenic properties of EIAV.

Characterization of equine infectious anemia virus dUTPase: growth properties of a dUTPase-deficient mutant

The putative dUTPase domain was deleted from the polymerase (pol) gene of equine infectious anemia virus (EIAV) to produce a recombinant delta DUpol Escherichia coli expression cassette and a delta DU proviral clone. Expression of the recombinant delta DUpol polyprotein yielded a properly processed and enzymatically active reverse transcriptase, as determined by immunoblot analysis and DNA polymerase activity gels. Transfection of delta DU provirus into feline (FEA) cells resulted in production of virus that replicated to wild-type levels in both FEA cells and fetal equine kidney cells. In contrast, the delta DU virus replicated poorly (less than 1% of wild-type levels) in primary equine macrophage cultures, as measured by reverse transcriptase assays. Preparations of delta DU virus contained negligible dUTPase activity, which confirms that virion-associated dUTPase is encoded in the pol gene region between the RNase H domain and integrase, as has been demonstrated previously for feline immunodeficiency virus (J. H. Elder, D. L. Lerner, C. S. Hasselkus-Light, D. J. Fontenot, E. Hunter, P. A. Luciw, R. C. Montelaro, and T. R. Phillips, J. Virol. 66:1791-1794, 1992). Our results suggest that virus-encoded dUTPase is dispensable for virus replication in dividing cells in vitro but may be required for efficient replication of EIAV in nondividing equine macrophages, the natural host cells for this virus.

Purification and characterization of a dUTPase from Acholeplasma laidlawii B-PG9

dUTP was purified 120-fold from extracts of Acholeplasma laidlawii B-PG9 by Blue-Sepharose, Phenyl-Sepharose, hydroxyapatite, and DEAE-Sephacel chromatography techniques. The only substrate for the enzyme was dUTP with an apparent Km of 4.5 microM. The only reaction products were dUMP and PPi. The dUTPase did not exhibit any specific divalent cation requirement, but it was inhibited by EDTA. The enzyme was not inhibited by Pi or p-hydroxymercuribenzoate. The molecular weight of the enzyme was estimated by gel filtration chromatography to be 48,000, and its isoelectric point was 5.3. The enzyme was thermostable at 55 degrees C for 1 h. A. laidlawii dUTPase was distinguishable from KB (human epidermoid carcinoma) dUTPase by differences in electrophoretic migration, isoelectric point, and thermostability. The enzyme is important in preventing dUTP from being incorporated into DNA and may have a significant role in both the synthesis of thymidine- and PPi-dependent phosphorylations.

Methotrexate-induced misincorporation of uracil into DNA

A line of human lymphoid cells was tested for the presence of dUMP in DNA with or without treatment with the dihydrofolate reductase inhibitor, methotrexate. Cells treated with methotrexate and labeled with [(3)H]dUrd contained dUMP in DNA in readily detectable amounts ( approximately 0.8 pmol of dUMP per mumol of total DNA nucleotide), and this was increased approximately 3-fold if the cells were also treated with Ura at the same time. No dUMP (<1 fmol/mumol of DNA) could be detected by these methods in DNA from cells not treated with methotrexate, regardless of whether Ura was present or absent. The presence of dUMP in DNA from cells treated with methotrexate is a result of the great increase in intracellular concentration of dUTP and the fall in dTTP that accompany inhibition of thymidylate synthetase (5,10-methylenetetrahydrofolate:dUMP C-methyltransferase; EC 2.1.1.45) by the drug. These changes are apparently sufficient to overcome the normal mechanisms that exclude dUMP from DNA, and the enhancement by Ura reflects suppression of one of the mechanisms, Ura removal from DNA by the enzyme Ura-DNA glycosylase. The results suggest an active lesion of DNA in cells in which thymidylate synthetase is inhibited. Under these conditions there appears to be a cyclic incorporation and removal of dUMP resulting from reinsertion of dUMP during gap repair at sites of Ura removal. This consequence of the normal excision-repair process, which occurs when intracellular levels of dUTP approach those of dTTP, may have effects related to the cytotoxicity of drug inhibitors of thymidylate synthetase, clinical deficiencies of folate and vitamin B-12, and thymineless death, in general.

Escherichia coli mutants deficient in deoxyuridine triphosphatase

Mutants deficient in deoxyuridine triphosphatase (dUTPase) were identified by enzyme assays of randomly chosen heavily mutagenized clones. Five mutants of independent origin were obtained. One mutant produced a thermolabile enzyme, and it was presumed to have a mutation in the structural gene for dUTPase, designated dut. The most deficient mutant had the following associated phenotypes: less than 1% of parental dUTPase activity, prolonged generation time, increased sensitivity to 5'-fluorodeoxyuridine, increased rate of spontaneous mutation, increased rate of recombination (hyper-Rec), an inhibition of growth in the presence of 2 mM uracil, and a decreased ability to support the growth of phage P1 (but not T4 or lambda). This mutation also appeared to be incompatible with pyrE mutations. A revertant selected by its faster growth had regained dUTPase activity and lost its hyper-Rec phenotype. Many of the properties of the dut mutants are compatible with their presumed increased incorporation of uracil into DNA and the subsequent transient breakage of the DNA by excision repair.

In vivo synthesis and properties of uracil-containing DNA

T4 bacteriophage DNA containing as much as 30% of its thymine replaced by uracil can be synthesised in Escherichia coli deficient in both dUTPase and uracil--DNA glycosidase. This uracil-containing DNA is competent for RNA transcription, and can be packaged into phage which are viable, if the host cells are deficient in uracil--DNA glycosidase activity. If the host cells are not deficient in this glycosidase activity the infecting phage DNA is rapidly attacked, resulting in more than 50% acid-solubilisation of the DNA. The infected cells are inefficiently killed, presumably because of very limited, if any, expression of the phage DNA. These results indicate that this replacement of thymine by uracil in DNA does not seriously impair the biological functionality of T4 DNA, provided the DNA is not subjected to the breakdown (repair) pathway initiated by uracil--DNA glycosidase.

Transient accumulation of Okazaki fragments as a result of uracil incorporation into nascent DNA

Strains of Escherichia coli with a mutation in the sof (dnaS) locus show a higher than normal frequency of recombination (are hyper rec) and incorporate label into short (4-5S) DNA fragments following brief [3H]thymidine pulses [Konrad and Lehman, Proc. Natl. Acad. Sci. USA 72, 2150 (1975)]. These mutant strains have now been found to be defective in deoxyuridinetriphosphate diphosphohydrolase (dUTPase; deoxyuridinetriphosphatase, EC 3.6.1.23), the enzyme that catalyzes the hydrolysis of dUTP to dUMP and PPi. Reversion of one sof- mutation to sof+ restores dUTPase activity and abolishes the accumulation of labeled 4-5S DNA fragments. Mutants initially isolated as defective in dUTPase (dut-) are also hyper rec and show transient accumulation of short DNA fragments. Both the sof and dut mutations are located at 81 min on the E. coli map, closely linked to the pyrE locus. The sof and dut loci thus appear to be identical. A decrease in dUTPase as a consequence of a sof or dut mutation may result in the increased incorporation of uracil into DNA. Rapid removal of the uracil by an excision-repair process could then lead to the transient accumulation of short DNA fragments. It is possible that at least a portion of the Okazaki fragments seen in wild-type cells may originate in this way.

ITP pyrophosphohydrolase and IDP phosphohydrolase in rat tissue

The existence of a nucleoside triphosphate pyrophosphohydrolase specific for ITP has been demonstrated in the cytosol fraction of a variety of rat tissues. The enzyme, stable to moderate heat treatment, was present in erythrocytes as well as brain, heart, kidney, liver, lung, muscle, ovaries, spleen, testes and thymus. The specific activity of the enzyme ranges from 26 to 150 mumoles/min/g protein. In addition, evidence is given for a heat labile nucleoside diphosphate (IDP) phosphohydrolase present in most rat tissues, and particularly high in the adrenal (137 mumoles/min/g protein). An "ITP-IMP cycle" is proposed as a rgulating mechanism for intracellular levels of ATP.

Direct participation of dCMP hydroxymethylase in synthesis of bacteriophage T4 DNA

In order to retain in an in situ system the control mechanisms involved in synthesis of bacteriophage T4 DNA, infected cells were made permeable to nucleotides by plasmolysis with concentrated sucrose. Such preparations use exogenous deoxyribonucleotides to synthesize T4 phage DNA. As has been observed with in vivo studies, DNA synthesis was drastically reduced in plasmolyzed preparations from cells infected by amber mutants of genes 1, 32, 41, 42, 43, 44, or 45. Added 5-hydroxymethyl dCTP did not bypass either a mutant of gene 42 (dCMP hydroxymethylase) or of gene 1 (phage-induced deoxyribonucleotide kinase). In a phage system lacking deoxycytidine triphosphatase (gene 56) and the gene-46 product, and therefore incorporating dCTP into DNA, dCTP incorporation did not require dCMP hydroxymethylase, in keeping with in vivo results. With a triple amber mutant of genes 1, 46, and 56 only slight incorporation of dCTP occurred. By contrast, in experiments performed in vivo the synthesis of cytosine-containing DNA was unaffected by an amber mutation in gene 1. These studies provide evidence that dCMP hydroxymethylase, in addition to its known catalytic function, has a second, more direct, role in phage T4 DNA synthesis, apparently in recognition of hydroxymethyl dCTP. The role of the phage-induced deoxyribonucleotide kinase in T4 DNA synthesis in the plasmolyzed system remains unresolved.

Deoxycytidine triphosphate deaminase: characterization of an Escherichia coli mutant deficient in the enzyme

A mutant of Escherichia coli, previously shown to contain abnormal nucleoside triphosphate pools, was found to be defective in its ability to synthesize thymidine nucleotides. The defect is not in the enzyme thymidylate synthetase but in deoxycytidine triphosphate deaminase, an enzyme that supplies deoxyuridine monophosphate, the substrate for thymidylate synthetase.

Deoxycytidine triphosphate deaminase: identification and function in Salmonella typhimurium

The biosynthesis of 2'-deoxyuridine monophosphate (dUMP) has been studied in a cytidine- and uracil-requiring mutant of Salmonella typhimurium (DP-55). The dUMP pool and the thymidine monophosphate (dTMP) pool of DP-55, grown in the presence of (3)H-uracil and unlabeled cytidine, are found to have the same specific activities. However, only 30% of the dUMP and the dTMP is synthesized from a uridine nucleotide. Seventy per cent is derived directly from a cytosine compound. The identification and partial purification of a Mg(2+)-dependent 2'-deoxycytidine triphosphate (dCTP) deaminase from S. typhimurium suggests that the combined action of dCTP deaminase and 2'-deoxyuridine triphosphate pyrophosphatase accounts for 70% of the dUMP, and therefore the dTMP, synthesized in vivo. The introduction of a thymine requirement (i.e., a block in thymidylate synthetase) into DP-55 results in a 100-fold increase in the size of the dUMP pool. However, the relative contribution of the uridine and cytidine pathways to dUMP synthesis is unaltered. The high dUMP pool is accompanied by extensive catabolism of dUMP to uracil. Partial thymine starvation of the cells results in a significant increase in the dUMP and dCTP pools. Moreover, an increase in the contribution of the dCTP pathway to dUMP synthesis is observed. As a result of these changes the catabolism of dUMP to uracil is augmented.

Biological effects of substituting cytosine for 5-hydroxymethylcytosine in the deoxyribonucleic acid of bacteriophage T4

Previous work from this laboratory has shown that the cytosine-containing T4 deoxyribonucleic acid (DNA) made by deoxycytidine triphosphatase (dCTPase) amber mutants is extensively degraded, and that nucleases controlled by genes 46 and 47 participate in this process. In this paper, we examine other consequences of a defective dCTPase. Included are studies of DNA synthesis and phage production, and of the control of both early and late protein synthesis after infection of Escherichia coli B with various T4 mutants defective in genes 56 (dCTPase), 42 (dCMP hydroxymethylase), 1 (deoxynucleotide kinase), 43 (DNA polymerase), 30 (polynucleotide ligase), 46 and 47 (DNA breakdown) or e(lysozyme). By varying the temperature of infection with a temperature-sensitive dCTPase mutant, we have been able to control intracellular dCTPase activity, and thus vary the cytosine content of the phage DNA. We have produced and characterized viable T4 phage in which cytosine replaces 20% of the 5-hydroxymethylcytosine (HMC) in the DNA. We present evidence which suggests that intact, cytosine-containing T4 DNA is much less efficient than is normal T4 DNA in directing the synthesis of tail-fiber antigen. Lysozyme production is much less affected by progressively decreasing dCTPase activity; however, complete substitution of cytosine is correlated with a depression of lysozyme synthesis greater than expected from the defective synthesis of DNA. Low but significant lysozyme synthesis is observed late after infection of E. coli B with T4 amber mutants defective in a number of genes controlling DNA synthesis. The "20% cytosine" T4 phage, once produced, can initiate an apparently normal infection at permissive temperatures; the synthesis of early enzymes, DNA, and phage does not appear to be impaired. Two roles for HMC in T4 DNA have been indicated previously: (i) involvement in host-controlled restriction of the phage, in which glucosylation of the hydroxymethyl group plays a crucial role (16, 29, 53, 58), and (ii) protection of vegetative DNA against phage-controlled nucleases, a protection not dependent on glucosylation (41, 66, 67). A third role is suggested by our present results: transcription of at least some late genes can occur only from HMC-containing DNA and not from cytosine-containing DNA.