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

Properties of Leishmania major dUTP nucleotidohydrolase, a distinct nucleotide-hydrolysing enzyme in kinetoplastids

We have previously reported the presence, in the parasitic protozoan Leishmania major, of an enzyme involved in controlling intracellular dUTP levels. The gene encoding this enzyme has now been overexpressed in Escherichia coli, and the recombinant enzyme was purified to homogeneity. Biochemical and enzymic analyses of the Leishmania enzyme show that it is a novel nucleotidohydrolase highly specific for deoxyuridine 5'-triphosphate. The enzyme has proved to be a dimer by gel filtration and is able to hydrolyse both dUTP and dUDP quite efficiently, acting as a dUTP nucleotidohydrolase (dUTPase)-dUDP nucleotidohydrolase but has a limited capacity to act upon other nucleoside di- or triphosphates. The reaction products are dUMP and PP(i) when dUTP is the substrate and dUMP and P(i) in the case of dUDP. The enzyme is sensitive to inhibition by the reaction product dUMP but not by PP(i). dUTPase activity is highly dependent on Mg(2+) concentrations and markedly sensitive to the phosphatase inhibitor, NaF. In summary, Leishmania dUTPase appears to be markedly different to other proteins characterized previously that accomplish the same function.

A mollicute (mycoplasma) DNA repair enzyme: purification and characterization of uracil-DNA glycosylase

The DNA repair enzyme uracil-DNA glycosylase from Mycoplasma lactucae (831-C4) was purified 1,657-fold by using affinity chromatography and chromatofocusing techniques. The only substrate for the enzyme was DNA that contained uracil residues, and the Km of the enzyme was 1.05 +/- 0.12 microM for dUMP containing DNA. The product of the reaction was uracil, and it acted as a noncompetitive inhibitor of the uracil-DNA glycosylase with a Ki of 5.2 mM. The activity of the enzyme was insensitive to Mg2+, Mn2+, Zn2+, Ca2+, and Co2+ over the concentration range tested, and the activity was not inhibited by EDTA. The enzyme activity exhibited a biphasic response to monovalent cations and to polyamines. The enzyme had a pI of 6.4 and existed as a nonspherical monomeric protein with a molecular weight of 28,500 +/- 1,200. The uracil-DNA glycosylase from M. lactucae was inhibited by the uracil-DNA glycosylase inhibitor from bacteriophage PBS-2, but the amount of inhibitor required for 50% inhibition of the mycoplasmal enzyme was 2.2 and 8 times greater than that required to cause 50% inhibition of the uracil-DNA glycosylases from Escherichia coli and Bacillus subtilis, respectively. Previous studies have reported that some mollicutes lack uracil-DNA glycosylase activity, and the results of this study demonstrate that the uracil-DNA glycosylase from M. lactucae has a higher Km for uracil-containing DNA than those of the glycosylases of other procaryotic organisms. Thus, the low G + C content of the DNA from some mollicutes and the A.T-biased mutation pressure observed in these organisms may be related to their decreased capacity to remove uracil residues from DNA.

Acholeplasma laidlawii B-PG9 adenine-specific purine nucleoside phosphorylase that accepts ribose-1-phosphate, deoxyribose-1-phosphate, and xylose-1-phosphate

An adenylate-specific purine nucleoside phosphorylase (purine nucleoside:orthophosphate ribosyltransferase, EC12.4.2.1) (PNP) was isolated from a cytoplasmic fraction of Acholeplasma laidlawii B-PG9 and partially purified (820-fold). This partially purified PNP could only ribosylate adenine and deribosylate adenosine and deoxyadenosine. The A. laidlawii partially purified PNP could not use hypoxanthine, guanine, uracil, guanosine, deoxyguanosine, or inosine as substrates, but could use ribose-1-phosphate, deoxyribose-1-phosphate, or xylose-1-phosphate as the pentose donor. Mg2+ and a pH of 7.6 were required for maximum activity for each of the pentoses. The partially purified enzyme in sucrose density gradient experiments had an approximate molecular weight of 108,000 and a sedimentation coefficient of 6.9, and in gel filtration experiments it had an approximate molecular weight of 102,000 and a Stoke's radius of 4.1 nm. Nondenaturing polyacrylamide tube gels of the enzyme preparation produced one major and one minor band. The major band (Rf, 0.57) corresponded to all enzyme activity. The Kms for the partially purified PNP with ribose-1-phosphate, deoxyribose-1-phosphate, and xylose-1-phosphate were 0.80, 0.82, and 0.81 mM, respectively. The corresponding Vmaxs were 12.5, 14.3, and 12.0 microM min-1, respectively. The Hill or interaction coefficients (n) for all three pentose phosphates were close to unity. The characterization data suggest the possibility of one active site on the enzyme which is equally reactive toward each of the three pentoses. This is the first report of an apparently adenine-specific PNP activity.

Characterization of a herpes simplex virus type 2 deoxyuridine triphosphate nucleotidohydrolase and mapping of a gene conferring type specificity for the enzyme

The herpes simplex virus type 2 (HSV-2)-induced deoxyuridine triphosphate nucleotidohydrolase (dUTPase) was purified approximately 600 +/- 43-fold using a combination of affinity, hydrophobic, absorption, and ion-exchange chromatography techniques. The only substrate for the dUTPase was dUTP with a Km of 3.6 +/- 1.1 microM. There was no apparent divalent cation requirement, but the HSV-2-induced dUTPase was inhibited by EDTA (0.1 mM) and this inhibition was reversed by either Co2+ (0.5 mM) or Mg2+ (0.5 mM). The HSV-2-induced dUTPase was distinguished from the HSV-1-induced and cellular dUTPases based upon differences in sensitivity to substrate inhibition, thermostability, and electrophoretic migration in nondenaturing polyacrylamide gels. Analysis of HSV-1 temperature-sensitive (ts) mutants demonstrated that ts A15 and ts K13 did not induce significant amounts of dUTPase activity at the permissive or nonpermissive temperatures. Mutants with defects in HSV-induced DNA polymerase or in the major DNA binding protein induced dUTPase at both temperatures. In contrast ts mutants defective in the alpha polypeptide VP175 (ICP4) did not induce normal levels of dUTPase at the nonpermissive temperature. The location of a gene encoding for the type specificity of the HSV induced dUTPase was mapped to the left 20% of the genome in Us in the region 0.060 to 0.100 or from 0.148 to 0.204.

PPi-dependent phosphofructotransferase (phosphofructokinase) activity in the mollicutes (mycoplasma) Acholeplasma laidlawii

A PPi-dependent phosphofructotransferase (PPi-fructose 6-phosphate 1-phosphotransferase, EC 2.7.1.90) which catalyzes the conversion of fructose 6 phosphate (F-6-P) to fructose 1,6-bisphosphate (F-1, 6-P2) was isolated from a cytoplasmic fraction of Acholeplasma laidlawii B-PG9 and partially purified (430-fold). PPi was required as the phosphate donor. ATP, dATP, CTP, dCTP, GTP, dGTP, UTP, dUTP, ITP, TTP, ADP, or Pi could not substitute for PPi. The PPi-dependent reaction (2.0 mM PPi) was not altered in the presence of any of these nucleotides (2.0 mM) or in the presence of smaller (less than or equal to 300 microM) amounts of fructose 2,6-bisphosphate, (NH4)2SO4, AMP, citrate, GDP, or phosphoenolpyruvate. Mg2+ and a pH of 7.4 were required for maximum activity. The partially purified enzyme in sucrose density gradient experiments had an approximate molecular weight of 74,000 and a sedimentation coefficient of 6.7. A second form of the enzyme (molecular weight, 37,000) was detected, although in relatively smaller amounts, by using Blue Sepharose matrix when performing electrophoresis experiments. The back reaction, F-1, 6-P2 to F-6-P, required Pi; arsenate could substitute for Pi, but not PPi or any other nucleotide tested. The computer-derived kinetic constants (+/- standard deviation) for the reaction in the PPi-driven direction of F-1, 6-P2 were as follows: v, 38.9 +/- 0.48 mM min-1; Ka(PPi), 0.11 +/- 0.04 mM; Kb(F-6-P), 0.65 +/- 0.15 mM; and Kia(PPi), 0.39 +/- 0.11 mM. A. laidlawii B-PG9 required PPi not only for the PPi-phosphofructotransferase reaction which we describe but also for purine nucleoside kinase activity. a dependency unknown in any other organism. In A. laidlawii B-PG9, the PPi requirement may be met by reactions in this organism already known to synthesize PPi (e.g., dUTPase and purine nucleobase phosphoribosyltransferases). In almost all other cells, the conversion of F-6-P to F-1,6-P2 is ATP dependent, and the reaction is generally considered to be the rate-limiting step of glycolysis. The ability of A. laidlawii B-PG9 and one other acholeplasma to use PPi instead of ATP as an energy source may offer these cytochrome-deficient organisms some metabolic advantage and may represent a conserved metabolic remnant of an earlier evolutionary process.

Induction of a deoxyuridine triphosphate nucleotidohydrolase activity in Epstein-Barr virus-infected cells

Superinfection of Raji cells with Epstein-Barr virus (EBV) or chemical induction of HR-1 cells with 12-O-tetradecanoyl-phorbol-13-acetate (TPA) results in the induction of a deoxyuridine triphosphate nucleotidohydrolase (dUTPase) which is not observed in mock-treated cells or TPA-treated EBV genome-negative BJAB cells. The EBV-induced dUTPase could be distinguished from the host dUTPase based upon differences in their migration in polyacrylamide gels and sensitivity to the 5-mercurithioguanosine derivitive of dUTP. The expression of the EBV-specified dUTPase is prevented by phosphonoacetic acid indicating that its expression is dependent upon EBV-DNA replication.

Pyrimidine deoxyribonucleotide metabolism in Acholeplasma laidlawii B-PG9

Extracts of Acholeplasma laidlawii B-PG9 were examined for the enzymes associated with the interconversion of the pyrimidine deoxyribonucleotides and the biosynthesis of thymidine nucleotides. A. laidlawii B-PG9 possessed deaminases for deoxycytidine and dCMP, pyrophosphatases for dUTP, phosphorylases for thymidine and uridine, and a membrane-associated pyrimidine deoxyribonucleoside monophosphate phosphatase activity. The role these enzyme activities have in the generation of deoxyribose-1-phosphate during growth may explain the ability of A. laidlawii B-PG9 to utilize either thymine or thymidine for biosynthesis.