Amplification of the UMP synthase gene and enzyme overproduction in pyrazofurin-resistant rat hepatoma cells. Molecular cloning of a cDNA for UMP synthase

The levels of UMP synthase protein and mRNA are increased in rat hepatoma cells that have acquired resistance to pyrazofurin, a potent inhibitor of pyrimidine biosynthesis. A cDNA plasmid library was prepared from partially purified poly(A)+ mRNA isolated from the resistant cell line. Recombinant plasmids with inserts complementary to UMP synthase mRNA were selected by differential hybridization with cDNA prepared from wild type and resistant cell mRNA and analysis of hybrid-selected mRNA by in vitro translation reactions. One plasmid, pUMPS-2, contains a 850-base pair insert and was used to analyze UMP synthase gene sequences in the wild type and resistant cell lines. Blot hybridization of restricted genomic DNA demonstrated amplification of the UMP synthase gene in the resistant cells. The number of UMP synthase genes is increased 15-fold as determined by a modified dot hybridization procedure. Previous studies have shown that the resistant cells have a 16-fold increase in UMP synthase mRNA but a 40-fold increase in synthase activity (Suttle, D.P. (1983) J. Biol. Chem. 258, 7707-7713). To further investigate this discrepancy between the amount of increase in DNA and mRNA versus the increase in enzyme activity, we have determined the relative rate of synthesis and degradation of UMP synthase. The rate of synthesis was 13-fold faster in the resistant cells. The degradation rate was not significantly different between the two cell lines. These data indicate that gene amplification is the major factor contributing to the enzyme overproduction in the pyrazofurin-resistant cells.     

Coordinate overproduction of orotate phosphoribosyltransferase and orotidine-5'-phosphate decarboxylase in hamster cells resistant to pyrazofurin and 6-azauridine.

Cells resistant to pyrazofurin and 6-azauridine have been selected from a simian virus 40-transformed Syrian hamster line and from a Chinese hamster lung line. By increasing the concentrations of inhibitors in several steps, mutant cells from both lines have been obtained which resist high concentrations (1 to 5 mM) of the two inhibitors separately or together. Orotidine-5'-phosphate decarboxylase (EC 4.1.1.23), the sixth and last enzyme in UMP biosynthesis, is inhibited by the nucleoside monophosphates derived from pyrazofurin or 6-azauridine. The activity of this enzyme is increased in each resistant cell line tested. Furthermore, there is a parallel increase in each case in the activity of the fifth enzyme of the pathway, orotate phosphoribosyltransferase (EC 2.4.2.10), which is not inhibited by pyrazofurin or 6-azauridine monophosphates, and the amount of increase is up to 67 times the level found in wild type cells. In contrast, the activities of the first three enzymes of UMP biosynthesis remain essentially unchanged in the mutants. Resistant Chinese hamster cells remain sensitive to 5-fluorouridine; this indicates that uridine kinase, the enzyme necessary to convert 6-azauridine to the monophosphate, is still functional.     

Resistance to pyrazofurin and 6-azauridine in normal MC3T3-E1 murine osteoblasts

Stable variants resistant to pyrazofurin (PF) and 6-azauridine (AZUrd) were serially selected in increasing drug concentrations from an MC3T3-E1 nontumorigenic murine osteoblastic cell line. Monophosphates of both AZUrd and PF competitively inhibit orotidine-5'-monophosphate decarboxylase (ODCase) activity of the UMP synthase multifunctional enzyme. When compared to the wild type cells, the AZUrdr and PFr lines were 3000- and 10,000-fold more resistant, respectively. Flow cytometry indicated tetraploidy in wild type cells and a reduction of DNA content in both resistant cell lines. DNA dot blot analysis showed no amplification of the gene coding for UMP synthase in either AZUrdr or PFr cells. Measurements of UMP synthase showed a 6-fold higher activity in AZUrdr cells and no significant difference in PFr cells as compared to wild type. Sensitivity to 5-fluorouracil was increased in the AZUrdr line as opposed to PFr and normal cell lines, indicating an increased orotate phosphoribosyltransferase activity in the AZUrdr cells. In comparison to wild type cells, PFr cells were 100-fold resistant to 6-methylmercaptopurine riboside, suggesting a lack of adenosine kinase activity. The control and AZUrdr cells showed equal sensitivity to 5-fluorouridine, thus indicating unchanged uridine kinase levels. While PFr cells were not cross-resistant to AZUrd, the AZUrdr cells were cross-resistant to PF. These results indicate the possibility of an altered ODCase active site. Although amplification of unrelated sequences cannot be excluded, our findings show that bone tetraploid, nontumorigenic cells acquire drug resistance through mechanisms other than the amplification of a target gene and that this resistance is accompanied by the partial loss of a chromosomal complement.     

The purification and preliminary characterization of UMP synthase from human placenta

Orotate phosphoribosyltransferase (EC 2.4.2.10) and orotidine 5'-monophosphate decarboxylase (EC 4.1.1.23) are the final two of six enzymatic steps required in the de novo biosynthesis of uridine 5'-monophosphate (UMP). Earlier work of this laboratory showed that, in the mouse Ehrlich ascites carcinoma, both of these enzymatic activities were contained on the single multifunctional polypeptide chain, UMP synthase. We report here that the placenta provided an available human source for UMP synthase with 40-fold higher orotate phosphoribosyltransferase and orotidine 5'-monophosphate decarboxylase specific activities than erythrocytes, a human source previously used by others. By using the placenta as a source of UMP synthase and by developing a novel purification procedure different from that used in the purification of UMP synthase from the Ehrlich ascites carcinoma (the only other homogeneous preparation of a mammalian UMP synthase), we achieved the purification of human UMP synthase to apparent homogeneity. This represents the first publication to homogeneity of UMP synthase from a human source as well as from a source other than malignant cell lines. Contrary to earlier reports human placental UMP synthase was found to be a multifunctional protein containing both enzymatic activities on a single polypeptide of 51,000 molecular weight. Preliminary characterization of the human placental UMP synthase showed it to be similar to UMP synthase from the Ehrlich ascites carcinoma in subunit molecular weight, native molecular weight, isozyme pattern (although not absolute pI values), pH optima of enzymatic activities, and kinetic constants for orotidine 5'-monophosphate (Km) and 6-azauridine 5'-monophosphate (Ki) at the decarboxylase site.     

Regulation of expression of the ADE3 gene for yeast C1-tetrahydrofolate synthase, a trifunctional enzyme involved in one-carbon metabolism

C1-THF (5,6,7,8-tetrahydrofolate) synthase is a trifunctional protein catalyzing the sequential reactions specified by the enzymes 10-formyl-THF synthetase (EC 6.3.4.3), 5,10-methenyl-THF cyclohydrolase (EC 3.5.4.9), and 5,10-methylene-THF dehydrogenase (EC 1.5.1.5). These three activities supply the activated one-carbon units required for the biosynthesis of purines, thymidylate, the amino acids histidine and methionine, the vitamin pantothenic acid, and the formyl group of mitochondrial fMet-tRNAfMet. Extracts of Saccharomyces cerevisiae whose growth is dependent on the three activities of C1-THF synthase contain 2-3 times the level of enzyme activity of extracts from cells grown under conditions where they are independent of this enzyme. Repression of C1-THF synthase activity requires the simultaneous presence of adenine, histidine, methionine, and pantothenic acid. Starvation of the cells for any one of these nutrients leads to derepression of the enzyme. Drug-induced folate starvation also leads to derepression of enzyme activity. The response to changing nutritional conditions occurs within 1 h and is due to changes in the steady-state concentration of C1-THF synthase enzyme, rather than to activation or deactivation of a pre-existing pool of enzyme. Determination of the amount of C1-THF synthase mRNA under the various growth conditions by an in vitro translation/immunoprecipitation assay indicates that regulation of the enzyme occurs predominantly at a pretranslational level since steady-state levels of C1-THF synthase mRNA are 2-3-fold higher in derepressed cells than in repressed cells.     

Control of gene expression during induction of cultured peanut cells: mRNA levels,protein synthesis and enzyme activity of stilbene synthase

Stilbene synthase is an inducible enzyme occurring in a small number of plants. The enzyme is amenable to analysis and biochemical studies only after the cells are subjected to induction. Cell suspension cultures of peanut react very selectively if elicited with biotic inducers. Just as intact peanut plants produce stilbene phytoalexins when attacked by fungi so also do sterile cultured cells when treated with sterilized insoluble fungal cell walls. Both systems react by synthesizing stilbene synthase. The time courses of increase in enzyme activity, protein synthesis and mRNA activity were studied, and their relation to other activities of the cells was elaborated. The results show that, after applying the fungal elicitor, the system responds very quickly and selectively: within 2 hours the synthesis rate of stilbene synthase protein is increased more than 30-fold, the increase being detectable 40 min after induction. The first increase in translatable mRNA for stilbene synthase can be seen 20 min after application of the stimulus. Stilbene synthase synthesized in vivo was compared to stilbene synthase prepared by translation in vitro. There was no difference in size, and limited proteolysis did not indicate significant differences in the peptide structure of the primary translation product and the active enzyme.     

Isolation and characterization of the orotidine 5'-monophosphate decarboxylase domain of the multifunctional protein uridine 5'-monophosphate synthase

The multifunctional protein uridine 5'-monophosphate (UMP) synthase catalyzes the final two reactions of the de novo biosynthesis of UMP in mammalian cells by the sequential action of orotate phosphoribosyltransferase (EC 2.4.2.10) and orotidine 5'-monophosphate (OMP) decarboxylase (EC 4.1.1.23). This protein is composed of one or two identical subunits; the monomer weighs of 51,500 daltons. UMP synthase from mouse Ehrlich ascites cells can exist as three distinct species as determined by sucrose density gradient centrifugation: a 3.6 S monomer, a 5.1 S dimer, and a 5.6 S conformationally altered dimer. Limited digestion of each of these three species with trypsin produced a 28,500-dalton peptide that was relatively resistant to further proteolysis. The peptide appears to be one of the two enzyme domains of UMP synthase for it retained only OMP decarboxylase activity. Similar results were obtained when UMP synthase was digested with elastase. OMP decarboxylase activity was less stable for the domain than for UMP synthase; the domain can rapidly lose activity upon storage or upon dilution. The size of the mammalian OMP decarboxylase domain is similar to that of yeast OMP decarboxylase. If the polypeptides which are cleaved from UMP synthase by trypsin are derived exclusively from either the amino or the carboxyl end of UMP synthase, then the size of a fragment possessing the orotate phosphoribosyltransferase domain could be as large as 23,000 daltons which is similar in size to the orotate phosphoribosyltransferase of yeast and of Escherichia coli.     

Regulation by light and metabolites of ferredoxin-dependent glutamate synthase in maize

The regulation of Fd-glutamate synthase (Fd-GOGAT, EC 1.4.1.7) and NADH-glutamate synthase (NADH-GOGAT, EC 1.4.1.14) was investigated in maize ( Zea mays L. cv. DEA) (1) during development starting from 7- to 11-day-old seedlings, (2) by treatment of 7-day-old etiolated leaves with intermittent light pulses to activate (red) and inactivate (far-red) phytochromes and (3) in 7-day-old green leaves grown under 16-h light/8-h dark cycles. Fd-GOGAT mRNA accumulated 4-fold, and the enzyme polypeptide (3-fold) and activity (3-fold) also increased in leaf cells, while NADH-GOGAT activity remained constantly low. Leaf-specific induction of Fd-GOGAT mRNA (3-fold) occurred in etiolated leaves by low fluence red light, and far-red light reversibly repressed the mRNA accumulation. Red/far-red reversible induction also occurred for Fd-GOGAT polypeptide (2-fold) and activity (2-fold), implicating the phytochrome-dependent induction of Fd-GOGAT. In contrast, NADH-GOGAT activity remained constant, irrespective of red/far-red light treatments. Fd-GOGAT showed diurnal changes under light/dark cycles with the maximum early in the morning and the minimum in the afternoon at the levels of mRNA, enzyme polypeptide and activity. Gln diurnally changed in parallel with Fd-GOGAT mRNA. The induction of Fd-GOGAT provides evidence that light and metabolites are the major signal for the Gln and Glu formation in maize leaf cells.     

Negative control of yeast coproporphyrinogen oxidase synthesis by heme and oxygen

Coproporphyrinogen oxidase (EC 1.3.3.3.) catalyzes the sixth enzymic step of the heme biosynthetic pathway. Coproporphyrinogen oxidase activity is increased in mutant cells of Saccharomyces cerevisiae deficient in heme synthesis and this effect can be partially reversed by the addition of exogenous hemin. A similar increase is found in wild type yeast cells grown anaerobically. The strain-dependent increase varies between 5- and 40-fold. The activity changes are paralleled by similar changes in the steady-state amounts of coproporphyrinogen oxidase protein determined by immunoblotting and the steady-state concentrations of coproporphyrinogen oxidase mRNA estimated by in vitro translation/immunoprecipitation. This demonstrates that coproporphyrinogen oxidase synthesis is regulated by heme and oxygen at a pretranslational level in a negative fashion.     

Translocation of 3-deoxy-<Emphasis Type="SmallCaps">D</Emphasis><Emphasis Type="Italic">-arabino</Emphasis>-heptulosonate 7-phosphate synthase precursor into isolated chloroplasts

A cDNA encoding 3-deoxy-D-arabino-heptulosonate 7-phosphate (DAHP) synthase (EC 4.1.2.15) from potato (Solanum tuberosum L.) presumably specifies a chloroplast transit sequence near its 5'-end. In order to show the function of this transit sequence, we constructed a plasmid that contains the entire coding region of the cDNA downstream from a T7 promoter. Using this plasmid as template, DAHP synthase mRNA was synthesized in vitro with T7 RNA polymerase. The resulting mRNA served as template for the in vitro synthesis of a 59-kDa polypeptide. This translation product was identified as the DAHP synthase precursor by immunoprecipitation with a monospecific polyclonal antibody raised against pure tuber DAHP synthase and by radiosequencing of the [(3)H]leucine-labeled translation product. Incubation of the 59-kDa polypeptide with isolated spinach (Spinacia oleracea L.) chloroplasts resulted in a 53-kDa polypeptide that was resistant to protease treatment. Fractionation of chloroplasts, reisolated after import, showed the mature DAHP synthase in the stroma fraction. Incubation of the 59-kDa polypeptide with a chloroplast precursor-processing enzyme cleaved the precursor between Ser49 and Ala50, generating a mature DAHP synthase of 489 residues. The uptake of the DAHP synthase precursor into isolated chloroplasts was inhibited by anti-DAHP synthase, and the precursor was not processed cotranslationally by canine microsomal membranes. We conclude that the transit sequence is able to direct DAHP synthase into chloroplasts.     

The 1-aminocyclopropane-1-carboxylate synthase of Cucurbita. Purification, properties, expression in Escherichia coli, and primary structure determination by DNA sequence analysis

The key regulatory enzyme in the biosynthetic pathway of the plant hormone ethylene is 1-aminocyclopropane-1-carboxylic acid (ACC) synthase (EC 4.4.1.14). We have partially purified ACC synthase 6,000-fold from Cucurbita fruit tissue treated with indoleacetic acid + benzyladenine + aminooxyacetic acid + LiCl. The enzyme has a specific activity of 35,000 nmol/h/mg protein, a pH optimum of 9.5, an isoelectric point of 5.0, a Km of 17 microM with respect to S-adenosylmethionine, and is a dimer of two identical subunits of approximately 46,000 Da each. The subunit exists in vivo as a 55,000-Da species similar in size to the primary in vitro translation product. DNA sequence analysis of the cDNA clone pACC1 revealed that the coding region of the ACC synthase mRNA spans 493 amino acids corresponding to a 55,779-Da polypeptide; and expression of the coding sequence (pACC1) in Escherichia coli as a COOH terminus hybrid of beta-galactosidase or as a nonhybrid polypeptide catalyzed the conversion of S-adenosylmethionine to ACC (Sato, T., and Theologis, A. (1989) Proc. Natl. Acad. Sci. U.S.A. 86, 6621-6625). Immunoblotting experiments herein show that the molecular mass of the beta-galactosidase hybrid polypeptide is 170,000 Da, and the size of the largest nonhybrid polypeptide is 53,000 Da. The data suggest that the enzyme is post-translationally processed during protein purification.     

Translational and pretranslational control of ornithine aminotransferase synthesis in rat liver

The mitochondrial matrix enzyme, ornithine aminotransferase, is induced in rat liver by the administration of a diet high in protein and by glucagon. The rate of synthesis of the enzyme is increased 100-fold in the livers of rats maintained on a 60% relative to a 0% protein diet, whereas the levels of functional and hybridizable mRNA measured by in vitro translation and through the use of a cloned cDNA probe increased by only 2- to 6-fold and 2- to 3-fold, respectively. Under conditions of glucagon induction that resulted in a 10- to 12-fold increase in the rate of enzyme synthesis, the relative level of functional ornithine aminotransferase mRNA increased by only 2-fold, and the level of hybridizable mRNA actually decreased. The rate of polypeptide chain elongation and the relative number of ornithine aminotransferase nascent chains on polysomes were 2-fold and 23-fold greater, respectively, in hepatocytes derived from 60% relative to 0% protein-fed rats. Using these data, a 23-fold increase in the translational efficiency of the mRNA was calculated. This increase, along with a 2-fold increase in the mRNA level, completely account for the 40-fold increase in the rate of ornithine aminotransferase synthesis observed in hepatocytes derived from 60% protein-fed rats. We conclude that ornithine aminotransferase synthesis is regulated at both a translational and a pretranslational level in rat liver.     

Alteration in de novo pyrimidine biosynthesis during uridine reversal of pyrazofurin-inhibited DNA synthesis

Pyrazofurin, a pyrimidine nucleoside analogue with antineoplastic activity, inhibits cell proliferation and DNA synthesis in cells by inhibiting uridine 5'-phosphate (UMP) synthase. It has been previously shown in concanavalin A (con A)-stimulated guinea pig lymphocytes (23) that pyrazofurin-inhibited DNA synthesis could be selectively reversed by exogenous uridine (Urd). In this report, we have examined possible mechanisms for the Urd reversal with experiments that determine the ability of exogenous Urd to (a) interfere with either the intracellular transport of pyrazofurin, or the conversion of pyrazofurin to its intracellularly active form, pyrazofurin-5'-phosphate; (b) reverse the pyrazofurin block of [14C]orotic acid incorporation into DNA; and (c) alter the pattern of exogenous [3H]Urd incorporation into DNA-thymine (DNA-Thy) and DNA-cytosine (DNA-Cyt) during pyrazofurin inhibition of pyrimidine de novo biosynthesis. The results of these experiments showed that Urd reversal does not occur through altered pyrazofurin transport or intracellular conversion to pyrazofurin-5'-phosphate, nor does it alter the distribution of [3H]Urd in DNA-Thy and DNA-Cyt. Instead, these findings indicate that the primary mechanism for exogenous Urd reversal of pyrazofurin inhibition of DNA synthesis involves the reversal of pyrazofurin inhibition of UMP synthase, thus restoring orotic acid incorporation into lymphocyte DNA through the pyrimidine de novo pathway.     

Toxoplasma gondii: characterization of a mutant resistant to sulfonamides.

Sulfadiazine was a potent inhibitor of the in vitro growth of Toxoplasma gondii, although it had little effect during the first 24 hr of treatment. A mutant parasite (R-SulR-5) with a 300-fold increase in sulfadiazine resistance was selected by a combination of chemical mutagenesis and growth in gradually increased sulfadiazine concentrations. This mutant was completely cross-resistant to several other sulfonamides and to dapsone. The same concentration of p-aminobenzoic acid reversed the sulfadiazine inhibition of both mutant and wild-type parasites even though much higher concentrations of sulfadiazine were used to inhibit the mutant. Dihydropteroate synthase, a sulfonamide-sensitive enzyme in the pathway leading to dihydrofolic acid, had similar activities in wild-type and R-SulR-5 parasites. However, the mutant enzyme was 40-fold more resistant to sulfadiazine and had higher apparent Kms for both substrates, p-aminobenzoic acid and dihydropteridine pyrophosphate. The mutant was slightly less active than the wild type in the uptake of sulfadiazine.     

Resistance to copper toxicity of cultured hepatoma cells. Characterization of resistant cell lines

A series of four cell lines resistant to the toxic effect of copper were developed from Morris rat hepatoma cells by gradually increasing the concentration of copper in the growth medium. The EC50, that concentration of copper that kills and/or inhibits the growth of 50% of the cells after 72 h, increased 4-fold over that for wild type cells in the most resistant cell line. These cells were also resistant to zinc, cadmium, and mercury toxicity, but not to nickel or cobalt. The amount of copper in the soluble protein pool of the resistant cells increased proportionally with the concentration of copper in the medium in which they were maintained. Associated with copper accumulation was the production of an 18-kDa cysteine-rich protein which complexes a significant amount of the metal. It is suggested that resistance to copper toxicity is due to sequestration of the metal by this protein. When resistant cells were removed from the copper-enriched environment, cellular copper levels rapidly fell to that observed for wild type cells, but no reduction in either the EC50 or the level of the cysteine-rich protein was noted. This suggests that a permanent change responsible for copper resistance had occurred which is maintained in the absence of the metal.     

An investigation of the mechanism of activation of tryptophan by tryptophanyl-tRNA synthetase from beef pancreas

Exposure of dark-grown Euglena to white or red light, but not blue light, produced a twofold increase in the specific activity of citrate synthase. A 400-fold purification of mitochondrial citrate synthase (subunit Mr = 44000) was achieved from cells of Euglena gracilis by affinity chromatography on ATP-activated agarose. Antisera, raised against the homogeneously pure enzyme, were used to demonstrate that the increase in citrate synthase activity on exposure of dark-grown cells to light resulted from an increase in citrate synthase protein. Anti-(citrate synthase) was used to detect precursor citrate synthase resulting from the translation of total polyadenylated RNA from Euglena in a cell-free rabbit reticulocyte lysate system. Citrate synthase mRNA was found to be present in cells at all stages of regreening. However, extraction and translation of polyadenylated RNA from free polysomes isolated from darkgrown and regreening cells demonstrated that appreciable translation of citrate synthase mRNA was only occurring in regreening cells.