AppppA and related adenylylated nucleotides are synthesized as a consequence of oxidation stress

Stringency in the absence of ppGpp accumulation in Rhodobacter sphaeroides

Leucine deprivation of either phototrophically or chemotrophically growing cells of Rhodobacter sphaeroides resulted in a restriction in the continued accumulations of cellular RNA, phospholipids, and protein. Phototrophically growing cells also displayed restrictions in the accumulations of cellular carotenoids and bacteriochlorophyll. Leucine deprivation, however, did not provoke the accumulation of cellular ppGpp or alter the steady-state levels of ppGpp, ATP, or GTP in cells of R. sphaeroides.

In vivo levels of diadenosine tetraphosphate and adenosine tetraphospho-guanosine in Physarum polycephalum during the cell cycle and oxidative stress

Cellular levels of diadenosine tetraphosphate (Ap4A) and adenosine tetraphospho-guanosine (Ap4G) were specifically measured during the cell cycle of Physarum polycephalum by a high-pressure liquid chromatographic method. Ap4A was also measured indirectly by a coupled phosphodiesterase-luciferase assay. No cell cycle-specific changes in either Ap4A or Ap4G were detected in experiments involving different methods of assay, different strains of P. polycephalum, or different methods of fixation of macroplasmodia. Our results on Ap4A are in contrast with those reported previously (C. Weinmann-Dorsch, G. Pierron, R. Wick, H. Sauer, and F. Grummt, Exp. Cell Res. 155:171-177, 1984). Weinmann-Dorsch et al. reported an 8- to 30-fold increase in Ap4A in early S phase in P. polycephalum, as measured by the phosphodiesterase-luciferase assay. We also measured levels of Ap4A, Ap4G, and ATP in macroplasmodia treated with 0.1 mM dinitrophenol. Ap4A and Ap4G transiently increased three- to sevenfold after 1 h and then decreased concomitantly with an 80% decrease in the level of ATP after 2 h in the presence of dinitrophenol. These results do not support the hypothesis that Ap4A is a positive pleiotypic activator that modulates DNA replication, but they are consistent with the hypothesis proposed for procaryotes that Ap4A and Ap4G are signal nucleotides or alarmones of oxidative stress (B.R. Bochner, P.C. Lee, S.W. Wilson, C.W. Cutler, and B.N. Ames, Cell 37:225-232, 1984).

Differential induction of heat shock, SOS, and oxidation stress regulons and accumulation of nucleotides in Escherichia coli

Heat and various inhibitory chemicals were tested in Escherichia coli for the ability to cause accumulation of adenylylated nucleotides and to induce proteins of the heat shock (htpR-controlled), the oxidation stress (oxyR-controlled), and the SOS (lexA-controlled) regulons. Under the conditions used, heat and ethanol initiated solely a heat shock response, hydrogen peroxide and 6-amino-7-chloro-5,8-dioxoquinoline (ACDQ) induced primarily an oxidation stress response and secondarily an SOS response, nalidixic acid and puromycin induced primarily an SOS and secondarily a heat shock response, isoleucine restriction induced a poor heat shock response, and CdCl2 strongly induced all three stress responses. ACDQ, CdCl2, and H2O2 each stimulated the synthesis of approximately 35 proteins by factors of 5- to 50-fold, and the heat shock, oxidation stress, and SOS regulons constituted a minor fraction of the overall cellular response. The pattern of accumulation of adenylylated nucleotides during these treatments was inconsistent with a simple role for these nucleotides as alarmones sufficient for triggering the heat shock response, but was consistent with a role in the oxyR-mediated response.

Intracellular 5',5'-dinucleoside polyphosphate levels remain constant during the Escherichia coli cell cycle

All AppppN and ApppN nucleotides (N = A, C, G, or U) occur in Escherichia coli. Measured cellular concentrations were 2.42 microM AppppA, 0.61 microM AppppC, 0.95 microM AppppG, 1.17 microM AppppU, 0.47 microM ApppA, 0.14 microM ApppC, 0.20 microM ApppG, and 0.12 microM ApppU. These concentrations remained constant during the cell cycle in synchronized exponentially growing cells.

HSP 26 and 27 are phosphorylated in response to heat shock and ecdysterone in Drosophila melanogaster cells

Protein phosphorylation has been studied in Drosophila melanogaster 8.9 K cells following heat shock. By in vivo double labelling with [35S]-methionine and [32P]-orthophosphate, we observed that two proteins are newly phosphorylated among the 26,000-27,000 dalton heat-shock proteins group. These two proteins are also phosphorylated after ecdysterone treatment, albeit at a lower level. That this phosphorylation event is induced by two different treatments, i.e. ecdysterone, a key steroid hormone of development, and heat-shock, a cellular stress suggests a possible common pathway for those two events and an important function for the phosphorylated heat-shock proteins.

The gene for Escherichia coli diadenosine tetraphosphatase is located immediately clockwise to folA and forms an operon with ksgA

The DNA sequences of the diadenosine tetraphosphatase gene (apaH) and of the flanking regions were determined. Three other genes were identified in the flanking regions: ksgA, apaG and folA encoding, respectively, a 16 S rRNA methyltransferase, an unidentified protein of Mr 13,826 and dihydrofolate reductase, with the order folA-apaH-apaG-ksgA. The apaH gene is thus located between folA and ksgA at 1 min on the Escherichia coli chromosome linkage map and folA is transcribed clockwise, whereas ksgA, apaG and apaH are transcribed in the opposite direction. It was shown that ksgA, apaG and apaH can be expressed from a polycistronic mRNA originating from a promoter (p1) located upstream of ksgA. However, another promoter (p2) was found within the ksgA structural gene. This promoter, active in vivo, can account for p1-independent expression of the two distal cistrons, apaG and apaH. Finally, the effect on diadenosine tetraphosphatase over-production of a frameshift mutation causing premature translational termination of apaG suggests that expression of apaG and apaH is coupled at the translational level.

Oxygen-dependent mutagenesis in Escherichia coli lacking superoxide dismutase

Escherichia coli double mutants (sodA sodB) completely lacking superoxide dismutase (SOD) have greatly enhanced mutation rates during aerobic growth. Single mutants lacking manganese SOD (MnSOD) but possessing iron SOD (FeSOD) have a smaller increase, and single mutants lacking FeSOD but possessing MnSOD do not show such an increase. The enhancement of mutagenesis is completely dependent on the presence of oxygen, and treatments that increase the flux of superoxide radicals produce even higher levels of mutagenesis. The presence of a plasmid overproducing either form of SOD reduces the level of mutagenesis to that of wild type, showing that the O2-dependent enhancement results from a lack of SOD. The enhancement of mutagenesis is RecA-independent, and a complete lack of SOD does not induce the SOS response during aerobic growth. However, the enhanced mutagenesis in aerobically grown sodA sodB mutants is largely dependent on functional exonuclease III, suggesting that the increased flux of superoxide radicals results in DNA lesions that can be acted on by this enzyme, leading to mutations.

Hydrogen peroxide-inducible proteins in Salmonella typhimurium overlap with heat shock and other stress proteins

Hydrogen peroxide treatment induces the synthesis of 30 proteins in Salmonella typhimurium. Five of these proteins are also induced by heat shock, including the highly conserved DnaK protein. The induction of one of these five proteins by heat shock is dependent on oxyR, a positive regulator of hydrogen peroxide-inducible genes, while the induction of the other four by heat shock is oxyR independent. Five of the 30 hydrogen peroxide-inducible proteins have been identified, and their structural genes have been mapped. Other stresses such as nalidixic acid, ethanol, or cumene hydroperoxide treatment also induce subsets of the 30 hydrogen peroxide-inducible proteins as well as additional proteins. Hydrogen peroxide-inducible proteins are shown to be largely different from those proteins induced by aerobiosis. In addition, the expression of the katG (catalase) gene is shown to be regulated by oxyR at the level of mRNA.

Escherichia coli DnaK protein possesses a 5'-nucleotidase activity that is inhibited by AppppA

AppppA and the DnaK protein have both been hypothesized to function in regulating the heat shock response of Escherichia coli. The proposals are that AppppA serves as a signal (alarmone) to turn on the heat shock response, whereas the DnaK protein is necessary to turn off the heat shock response. A simple model would be that the DnaK protein turns off the response by degrading AppppA. We disproved this model by demonstrating that the DnaK protein possesses a 5'-nucleotidase activity capable of degrading many cellular nucleotides but not AppppA. Although AppppA was not a substrate, it did inhibit the 5'-nucleotidase activity of the DnaK protein. This inhibition may be specific and have biological function since the mutant DnaK756 protein, which is defective in turning off the heat shock response, is partially desensitized to AppppA inhibition. These findings led us to consider other possible mechanisms for AppppA and the DnaK protein in heat shock regulation.

Accumulation of dinucleoside polyphosphates in Saccharomyces cerevisiae under stress conditions. High levels are associated with cell death

Adenosine tetraphosphonucleosides (Ap4X) were measured in Saccharomyces cerevisiae by a coupled phosphodiesterase-luciferase assay. After exposure of the cells to cadmium or to hyperthermic treatment (46 degrees C) a marked increase of the cellular pool from 0.08 microM (base level) to 4 microM or higher was observed. The accumulation of Ap4X to high levels is associated with irreversible processes leading to cell death.

[Adaptation of cell metabolism to environmental changes: regulation of gene expression of transfer RNA and unusual nucleic acid building-blocks]

In the living cell transfer ribonucleic acids (tRNAs) serve for the transfer of information from genes to proteins. In this article evidence will be presented showing that changes of particular tRNA modifications cause alterations in gene expression, when an organism is exposed to a metabolic stress, e.g. limitation of oxygen or nutrients. tRNA modifications seem to be important to adapt cells to environmental changes. These mechanisms of adaptation are considered to have developed as survival strategies in microorganisms especially when oxygen accumulated in the atmosphere.

Hydrolysis of diadenosine 5',5''-P',P''-triphosphate (Ap3A) by porcine aortic endothelial cells

Diadenosine triphosphate is present in platelet-dense granules and released quantitatively on platelet aggregation. We have found that intact porcine aortic endothelial cells can efficiently hydrolyze extracellular diadenosine triphosphate. The products of diadenosine triphosphate hydrolysis are adenosine monophosphate and adenosine diphosphate. Adenosine diphosphate is a potent stimulus of platelet aggregation. Since platelet-dense granules contain high concentrations of adenosine triphosphate and adenosine diphosphate, we examined endothelial cell hydrolysis of a mixture of diadenosine triphosphate and adenosine triphosphate. We find that the presence of adenosine triphosphate severely inhibits the hydrolysis of diadenosine triphosphate. Thus, although endothelial cells can rapidly clear extracellular diadenosine triphosphate, during platelet aggregation the hydrolysis of diadenosine triphosphate may be slow due to the presence of high concentrations of other adenine nucleotides. This phenomenon may be important physiologically if, as current evidence implies, diadenosine triphosphate is involved in the maintenance of hemostasis.

Changes in intracellular levels of Ap3A and Ap4A in cysts and larvae of Artemia do not correlate with changes in protein synthesis after heat-shock

Artemia larvae respond to a brief heat-shock between 28 degrees and 40 degrees C with an increase in the synthesis of two groups of proteins of Mr 68,000 and 89,000. At 40 degrees C synthesis of all other proteins is strongly repressed. Cysts, which are naturally thermotolerant, synthesise both heat-shock proteins at temperatures up to 47 degrees C but maintain normal protein synthesis. During pre-emergence development, Ap3A is present in cysts at a concentration twice that of Ap4A. The maximum level of 7.6 pmol/10(6) cells is reached shortly before hatching of the larvae. After hatching, the levels of both nucleotides decline. A 40 degrees C heat-shock produces a 1.8-fold increase in both nucleotides within 20 min in cysts and larvae. A 2.8-fold increase results from a 47 degrees C heat-shock to cysts. The rates of increase parallel but do not precede the increases in the heat-shock proteins. Since non-heat-shocked cysts possess higher levels of Ap3A and Ap4A than do heat-shocked larvae, the observed heat-induced changes in gene expression cannot be explained simply in terms of the intracellular concentrations of these nucleotides.

Three diadenosine 5',5''-P1,P4-tetraphosphate hydrolytic enzymes from Physarum polycephalum with differential effects by calcium: a specific dinucleoside polyphosphate pyrophosphohydrolase, a nucleotide pyrophosphatase, and a phosphodiesterase

Two new enzymes that hydrolyze diadenosine tetraphosphate (Ap4A) have been isolated from the acellular slime mold Physarum polycephalum. Both enzymes are different from the Physarum Ap4A symmetrical pyrophosphohydrolase previously described on the basis of their substrate specificities, reaction products, molecular weights, and divalent cation requirements. One enzyme is a nucleotide pyrophosphatase that asymmetrically hydrolyzes Ap4A to AMP and ATP. This enzyme hydrolyzes several mono- and dinucleotides with the corresponding nucleotide monophosphate as one of the products. The percentage hydrolysis of NAD+, Ap4A, and Ap4G, each at 10 microM, was 100, 56, and 51, respectively. A divalent cation is required for activity, with Ca2+ yielding 20-30 times greater activity than Mg2+ or Mn2+. Values of Km for Ap4A and Vmax are similar to the corresponding values for Ap4A symmetrical pyrophosphohydrolase. The second enzyme is a phosphodiesterase I with broad substrate reactivity. This enzyme also asymmetrically hydrolyzes Ap4A, but it does not hydrolyze NAD+. Activity of the phosphodiesterase I is stimulated by divalent cations, with Ca2+ being 50-60 times more stimulatory than Mg2+ or Mn2+. The apparent molecular weights of the nucleotide pyrophosphatase and phosphodiesterase are 184,000 and 45,000, respectively. In contrast, the Ap4A pyrophosphohydrolase hydrolyzes Ap4A to ADP, is inhibited by Ca2+ and other divalent cations, and has an apparent molecular weight of 26,000 as previously reported.

Diadenosine tetraphosphate activates cytosol 5'-nucleotidase

The rate of hydrolysis of IMP (0.5 mM) by cytosol 5'-nucleotidase from Artemia embryos was increased up to 7-fold by concentrations of around 10 microM diadenosine tetraphosphate (Ap4A). Half maximal activation of the enzyme was accomplished with 5 microM Ap4A. The Km (S 0.5) values of the nucleotidase for IMP, GMP, AMP, XMP and CMP decreased about 10 fold in the presence of 10 microM Ap4A. Maximum velocity of the enzyme was not affected by Ap4A. ATP had been previously described as an activator of the enzyme. However, comparatively with Ap4A, concentrations of ATP two orders of magnitude higher are needed to elicit similar effects on the enzyme. Preliminary results indicate that Ap4A is also an activator of the cytosol 5'-nucleotidase from rat liver.

In vitro effect of the Escherichia coli heat shock regulatory protein on expression of heat shock genes

In Escherichia coli, the ability to elicit a heat shock response depends on the htpR gene product. Previous work has shown that the HtpR protein serves as a sigma factor (sigma 32) for RNA polymerase that specifically recognizes heat shock promoters (A.D. Grossman, J.W. Erickson, and C.A. Gross Cell 38:383-390, 1984). In the present study we showed that sigma 32 synthesized in vitro could stimulate the expression of heat shock genes. The in vitro-synthesized sigma 32 was found to be associated with RNA polymerase. In vivo-synthesized sigma 32 was also associated with RNA polymerase, and this polymerase (E sigma 32) could be isolated free of the standard polymerase (E sigma 70). E sigma 32 was more active than E sigma 70 with heat shock genes; however, non-heat-shock genes were not transcribed by E sigma 32. The in vitro expression of the htpR gene required E sigma 70 but did not require E sigma 32.

In vivo levels of diadenosine tetraphosphate and adenosine tetraphospho-guanosine in Physarum polycephalum during the cell cycle and oxidative stress

Cellular levels of diadenosine tetraphosphate (Ap4A) and adenosine tetraphospho-guanosine (Ap4G) were specifically measured during the cell cycle of Physarum polycephalum by a high-pressure liquid chromatographic method. Ap4A was also measured indirectly by a coupled phosphodiesterase-luciferase assay. No cell cycle-specific changes in either Ap4A or Ap4G were detected in experiments involving different methods of assay, different strains of P. polycephalum, or different methods of fixation of macroplasmodia. Our results on Ap4A are in contrast with those reported previously (C. Weinmann-Dorsch, G. Pierron, R. Wick, H. Sauer, and F. Grummt, Exp. Cell Res. 155:171-177, 1984). Weinmann-Dorsch et al. reported an 8- to 30-fold increase in Ap4A in early S phase in P. polycephalum, as measured by the phosphodiesterase-luciferase assay. We also measured levels of Ap4A, Ap4G, and ATP in macroplasmodia treated with 0.1 mM dinitrophenol. Ap4A and Ap4G transiently increased three- to sevenfold after 1 h and then decreased concomitantly with an 80% decrease in the level of ATP after 2 h in the presence of dinitrophenol. These results do not support the hypothesis that Ap4A is a positive pleiotypic activator that modulates DNA replication, but they are consistent with the hypothesis proposed for procaryotes that Ap4A and Ap4G are signal nucleotides or alarmones of oxidative stress (B.R. Bochner, P.C. Lee, S.W. Wilson, C.W. Cutler, and B.N. Ames, Cell 37:225-232, 1984).

Alteration of adenyl dinucleotide metabolism by environmental stress

Exposure of cultured mammalian cells to a variety of conditions that induce the synthesis of stress proteins, including hyperthermia, ethanol, cadmium, and arsenite resulted in an increased cellular content of adenyl dinucleotides including diadenosine tetraphosphate (Ap4A). Exposure to other agents that cause metabolic perturbations not known to induce the synthesis of stress proteins, such as cyclohexamide, cytosine arabinoside, hydroxyurea, and ultraviolet irradiation did not alter the content of these nucleotides. It is proposed that these unique nucleotides may mediate adaptive responses of mammalian cells to environmental stress.

Effect of elevated temperature on catalase and superoxide dismutase during maize development

Seeds of the inbred maize lines, W64A, R6-67, and D10, were germinated and grown at 25 degrees, 35 degrees, or 40 degrees C for up to 10 days. The catalase activity in scutella of W64A seedlings grown at 40 degrees C was slightly lower than that in seedlings grown at 25 degrees C. The total superoxide dismutase activity in scutella was lower in seedlings grown at 40 degrees C than in those grown at 25 degrees C during the first 3 days of germination, but thereafter was not significantly different at these temperatures. The high-catalase mutant lines, R6-67 and D10, grown at 40 degrees C exhibited a developmental pattern of catalase activity that was severalfold lower than that seen in seedlings grown at 25 degrees C. The decrease in catalase activity in R6-67 seedlings grown at 40 degrees C was correlated with lower amounts of CAT-2 protein, which is normally present at significantly high levels in this line. The application of a catalase synthesis inhibitor revealed that the low levels of CAT-2 in R6-67 grown at 40 degrees C were due to slightly higher degradation rates and a significant drop in the rate of catalase protein synthesis.

Changes in plant gene expression during stress

Changes in gene expression which occur during periods of environmentally induced stress provide models for the study of gene regulation. Several types of stress have been shown to elicit a specific and reproducible pattern of gene expression in various plant species. These stress factors include heat shock, anaerobiosis, plant pathogens, oxygen free radicals, heavy metals, water stress, and chilling. In some cases, changes in specific genes have been identified, such as increases in the expression of the gene encoding the phytoalexin-synthesizing enzyme in pathogen elicitor-treated cells. However, in most cases, the functional identity of stress-induced genes is unknown. The alterations in gene expression during stress usually are rapid and repeatable, making these genetic systems ideal for examination of factors and mechanisms involved in gene regulation.

Hydrogen peroxide toxicity may be enhanced by heat shock gene induction in Drosophila

Recent evidence suggests that low dose exposure of cells to hydrogen peroxide and/or induction of heat shock protein (HSP) synthesis will render cells resistant to the lethal effects of a subsequent high dose hydrogen peroxide stress. We explored this possibility in the Drosophila melanogaster Schneider tissue culture line 2. It was found that chronic low dose exposure (1 mM H2O2 for 3 days) resulted in marked potentiation of the toxic effects of a subsequent high dose exposure (50 mM H2O2 for 1 h), as assessed by impairment of uridine incorporation and cell proliferation. Cells preexposed to low dose H2O2 exhibited enhanced heat shock gene transcription upon exposure to high dose H2O2, as compared to cells that did not receive low dose preexposure. Transcriptional induction of the heat shock genes by a mild non-toxic heat shock resulted in marked enhancement of the anti-proliferative effects of a subsequent H2O2 exposure. Thus, low dose hydrogen peroxide exposure or mild heating results in subsequent enhancement of high dose hydrogen peroxide toxicity; this effect correlates with enhanced heat shock gene expression. Possible mechanisms are discussed.

Inhibition of aminoacyl-tRNA synthetases by the mycotoxin patulin

The effect of patulin on tRNA aminoacylation has been determined. This mycotoxin inhibits the aminoacylation process by irreversibly inactivating aminoacyl-tRNA synthetases. At neutral and alkaline pH-values, the inactivation occurs mainly by modification of essential thiol groups of the protein, whereas at acidic pH, where the effect is the most pronounced, the modification of other amino acid residues cannot be excluded.

Toxicity and mutagenicity of plumbagin and the induction of a possible new DNA repair pathway in Escherichia coli

Actively growing Escherichia coli cells exposed to plumbagin, a redox cycling quinone that increases the flux of O2- radicals in the cell, were mutagenized or killed by this treatment. The toxicity of plumbagin was not found to be mediated by membrane damage. Cells pretreated with plumbagin could partially reactivate lambda phage damaged by exposure to riboflavin plus light, a treatment that produces active oxygen species. The result suggested the induction of a DNA repair response. Lambda phage damaged by H2O2 treatment were not reactivated in plumbagin-pretreated cells, nor did H2O2-pretreated cells reactivate lambda damaged by treatment with riboflavin plus light. Plumbagin treatment did not induce lambda phage in a lysogen, nor did it cause an increase in beta-galactosidase production in a dinD::Mu d(lac Ap) promoter fusion strain. Cells pretreated with nonlethal doses of plumbagin showed enhanced survival upon exposure to high concentrations of plumbagin, but were unchanged in their susceptibility to far-UV irradiation. polA and recA mutants were not significantly more sensitive than wild type to killing by plumbagin. However, xth-1 mutants were partially resistant to plumbagin toxicity. It is proposed that E. coli has an inducible DNA repair response specific for the type of oxidative damage generated during incubation with plumbagin. Furthermore, this response appears to be qualitatively distinct from the SOS response and the repair response induced by H2O2.

Proteolysis of poly(ADPribose) polymerase by a pyrophosphate- and nucleotide-stimulated system dependent on two different classes of proteinase

We have identified a system in human lymphocytes which proteolytically cleaves poly(ADPribose) polymerase to specific fragments of molecular weight 96 000, 79 000 and 62 000-60 000. This proteolytic processing is dependent on two different classes of proteinase. One of these proteinases is a serine proteinase, since the processing is inhibited by phenylmethylsulfonyl fluoride, antipain, soybean trypsin inhibitor and diisopropylfluorophosphate, the other is a cathepsin D-like proteinase, since processing is also inhibited by pepstatin A. The processing that occurs in permeabilized cells can be simulated in vitro by treating purified poly(ADPribose) polymerase with trypsin, but not by treating the polymerase with cathepsin D. Since processing at the cellular level is blocked by inhibitors of either of the two proteinases, but only trypsin could cleave the purified polymerase, this suggests that in the cell the action of the cathepsin D-like proteinase is a prerequisite for cleavage of poly(ADPribose) polymerase by the serine proteinase. Thus, a pathway involving sequential action of these proteinases may exist. Proteolysis in permeabilized human lymphocytes is stimulated by nucleotides containing a pyrophosphate group, such as 5',5'''-P1,P4-tetraphosphate and ATP, or by pyrophosphate itself. In contrast, nucleotides containing only a single phosphate, such as AMP and cyclic AMP, or inorganic sodium phosphate, do not show this stimulation of proteolysis. These results suggest that a pyrophosphate linkage is the minimum molecular requirement for stimulation of proteolytic processing of poly(ADPribose) polymerase. Proteolytic processing of poly(ADPribose) polymerase is independent of ADPribosylation. Following proteolysis, specific fragments of the polymerase, particularly the 62 000-60 000 molecular weight fragment(s), are still capable of being ADPribosylated.

Molecular cloning of the Escherichia coli gene for diadenosine 5',5'''-P1,P4-tetraphosphate pyrophosphohydrolase

A clone overproducing diadenosine tetraphosphatase (diadenosine 5', 5'''-P1, P4-tetraphosphate pyrophosphohydrolase) activity was isolated from an Escherichia coli cosmid library. Localization of the DNA region responsible for stimulation of this activity was achieved by deletion mapping and subcloning in various vectors. Maxicell experiments and immunological assays demonstrated that a 3.5-kilobase-pair DNA fragment carried the structural gene apaH encoding the E. coli diadenosine tetraphosphatase. The DNA coding strand was determined by cloning this fragment in both orientations in pUC plasmids. It was also shown that the overproduction of diadenosine tetraphosphatase decreased the dinucleoside tetraphosphate concentration in E. coli by a factor of 10.

Positive control of a regulon for defenses against oxidative stress and some heat-shock proteins in Salmonella typhimurium

S. typhimurium become resistant to killing by hydrogen peroxide and other oxidants when pretreated with nonlethal levels of hydrogen peroxide. During adaptation to hydrogen peroxide, 30 proteins are induced. Nine are constitutively overexpressed in dominant hydrogen peroxide-resistant oxyR mutants. Mutant oxyR1 is resistant to a variety of oxidizing agents and overexpresses at least five enzyme activities involved in defenses against oxidative damage. Deletions of oxyR are recessive and uninducible by hydrogen peroxide for the nine proteins overexpressed in oxyR1, demonstrating that oxyR is a positive regulatory element. The oxyR1 mutant is also more resistant than the wild-type parent to killing by heat, and it constitutively overexpresses three heat-shock proteins. The oxyR regulatory network is a previously uncharacterized global regulatory system in enteric bacteria.

Inactivation of Escherichia coli glutamine synthetase by xanthine oxidase, nicotinate hydroxylase, horseradish peroxidase, or glucose oxidase: effects of ferredoxin, putidaredoxin, and menadione

Previous studies have shown that several mixed-function oxidation (MFO) systems are capable of catalyzing the inactivation of glutamine synthetase (GS) [R.L. Levine, C. N. Oliver, R. M. Fulks, and E. R. Stadtman (1978) Proc. Natl. Acad. Sci. USA 78, 2120-2124] and a number of the other enzymes [L. Fucci, C. N. Oliver, M. J. Coon, and E. R. Stadtman (1983) Proc. Natl. Acad. Sci. USA 80, 1521-1525]. It has now been found that in the presence of Fe(III), O2, and an appropriate electron donor (hypoxanthine or NADPH, respectively) glutamine synthetase is also inactivated by either milk xanthine oxidase or Clostridial nicotinate hydroxylase. Inactivation of glutamine synthetase by either of these flavoproteins is greatly stimulated by the presence of electron carrier proteins possessing nonheme-iron-sulfur (NHIS) clusters (i.e., ferredoxin or putidaredoxin) or by the presence of menadione. The inactivation reactions are partially inhibited by free radical scavengers, superoxide dismutase, (SOD), histidine, mannitol, dimethyl sulfoxide, and dimethylthiourea, and are inhibited completely by either Mn(II), EDTA, or catalase. The sensitivity to SOD inhibition is greatly suppressed when the xanthine oxidase system is supplemented with either ferredoxin or redoxin. In the presence of the latter NHIS-proteins (and only when they are present), MFO systems, comprised of either horseradish peroxidase and H2O2 or glucose oxidase, O2, and glucose, can also catalyze the inactivation of GS. The ability of ferredoxin and putidaredoxin to promote oxidation modification of GS by any one of these MFO systems suggests that proteins with NHIS centers may mediate the generation (or stabilization) of highly reactive radical intermediates.

Catabolism of bis(5'-nucleosidyl) oligophosphates in Escherichia coli: metal requirements and substrate specificity of homogeneous diadenosine-5',5'''-P1,P4-tetraphosphate pyrophosphohydrolase

Diadenosine-5',5'''-P1,P4-tetraphosphate pyrophosphohydrolase (diadenosinetetraphosphatase) from Escherichia coli strain EM20031 has been purified 5000-fold from 4 kg of wet cells. It produces 2.4 mg of homogeneous enzyme with a yield of 3.1%. The enzyme activity in the reaction of ADP production from Ap4A is 250 s-1 [37 degrees C, 50 mM tris(hydroxymethyl)aminomethane, pH 7.8, 50 microM Ap4A, 0.5 microM ethylenediaminetetraacetic acid (EDTA), and 50 microM CoCl2]. The enzyme is a single polypeptide chain of Mr 33K, as determined by sodium dodecyl sulfate-polyacrylamide gel electrophoresis analysis and high-performance gel permeation chromatography. Dinucleoside polyphosphates are substrates provided they contain more than two phosphates (Ap4A, Ap4G, Ap4C, Gp4G, Ap3A, Ap3G, Ap3C, Gp3G, Gp3C, Ap5A, Ap6A, and dAp4dA are substrates; Ap2A, NAD, and NADP are not). Among the products, a nucleoside diphosphate is always formed. ATP, GTP, CTP, UTP, dATP, dGTP, dCTP, and dTTP are not substrates; Ap4 is. Addition of Co2+ (50 microM) to the reaction buffer containing 0.5 microM EDTA strongly stimulates Ap4A hydrolysis (stimulation 2500-fold). With 50 microM MnCl2, the stimulation is 900-fold. Ca2+, Fe2+, and Mg2+ have no effect. The Km for Ap4A is 22 microM with Co2+ and 12 microM with Mn2+. The added metals have similar effects on the hydrolysis of Ap3A into ADP + AMP. However, in the latter case, the stimulation by Co2+ is small, and the maximum stimulation brought by Mn2+ is 9 times that brought by Co2+. Exposure of the enzyme to Zn2+ (5 microM), prior to the assay or within the reaction mixture containing Co2+, causes a marked inhibition of Ap4A hydrolysis.(ABSTRACT TRUNCATED AT 250 WORDS)

5',5'''-P1, P4 diadenosine tetraphosphate (Ap4A): a putative initiator of DNA replication

The proposal that Ap4A acts as an inducer of DNA replication is based primarily on two pieces of evidence (7). The intracellular levels of Ap4A increase ten- to 1000-fold as cells progress into S phase and the introduction of Ap4A into nonproliferating cells stimulated DNA synthesis. There is also some additional suggestive evidence such as the binding of Ap4A to a protein that is associated with multiprotein forms of the replicative DNA polymerase alpha and the ability of this enzyme to use Ap4A as a primer for DNA synthesis in vitro with single-stranded DNA templates. These observations have stimulated interest in the cellular metabolism of Ap4A. This is well since there is a great need for additional experimentation in order to clearly establish Ap4A as an inducer of DNA replication. Microinjection experiments of Ap4A into quiescent cells are needed in order to ascertain if Ap4A will stimulate DNA replication and possibly cell division in intact cells. Studies of the effects of nonhydrolyzable analogs of Ap4A on DNA replication in intact quiescent cells could also prove valuable. Although Ap4A can function as a primer for in vitro DNA synthesis by DNA polymerase alpha this may not be relevant in regard to its in vivo role in DNA replication. Ap4A in vivo could interact with key protein(s) in DNA replication and in this way act as an effector molecule in the initiation of DNA replication. In this regard the interaction of Ap4A with a protein associated with a multiprotein form of DNA polymerase alpha isolated from S-phase cells is of interest. More experiments are required to determine if there is a specific target protein(s) for Ap4A in vivo and what its role in DNA replication is. The cofractionation of tryptophanyl-tRNA synthetase with the replicative DNA polymerase alpha from animal and plant cells is of interest. The DNA polymerase alpha from synchronized animal cells also interacted with Ap4A. Although the plant cell alpha-like DNA polymerase did not interact with Ap4A this DNA polymerase was not a multiprotein form of polymerase alpha and the synchrony of the wheat germ embryos was not known. A possible tie between protein-synthesizing systems and the regulation of proteins involved in DNA replication may exist. The requirement of protein synthesis for the initiation of DNA replication has long been known. Also, it is well established that many temperature-sensitive mutants for tRNA synthetases are also DNA-synthesizing mutants. More investigation in this area may be warranted.(ABSTRACT TRUNCATED AT 400 WORDS)

Is Ap4A an activator of eukaryotic DNA replication?

The most well established fact concerning Ap4A metabolism is that the concentration of this compound is cell cycle and cell proliferation dependent. An additional intriguing fact is that Ap4A can stimulate DNA synthesis in cell extracts, and when injected into living cells. In view of these facts, it is not surprising that Ap4A has been postulated to regulate the initiation of DNA replication. However, in our opinion, experimental efforts designed to test this hypothesis do not conclusively link Ap4A to DNA replication. Work on the mechanism of stimulation of DNA synthesis in vitro indicates that Ap4A and a variety of adenylated nucleotides increase DNA synthetic rates by acting as primers. Thus far there is no evidence that this primer function plays a role in the initiation of normal DNA replication in vivo, or that Ap4A is unique in this capacity to stimulate initiation processes. Additional experiments have shown an association of partially purified DNA alpha polymerase with both tryptophanyl-tRNA synthetase and a protein capable of binding Ap4A. The Ap4A-binding protein appears to be necessary for Ap4A to assume the correct conformation for priming, since physiological levels of Ap4A are not stimulatory for highly purified DNA alpha polymerase. The relevance of tRNA synthetases to the regulation hypothesis is their ability to produce Ap4A. Ironically, mammalian tryptophanyl-tRNA synthetase does not appear to have this capacity. Furthermore, the association of alpha polymerase with either Ap4A-binding protein or tryptophanyl-tRNA synthetase in vivo has not been conclusively demonstrated. Although Ap4A has been postulated to regulate many phenomena in eukaryotes and bacteria, such as entry into S phase and the response to oxygen deprivation, the links between Ap4A and these processes are still only circumstantial. It is tempting to extrapolate from the alarmone and stringent responses of bacteria to other systems, but these phenomena are not known to occur in eukaryotic cells. Similar deprivation and inhibition experiments in mammalian cells have been shown to stop growth at a synchronous position in cell cycle, and the Ap4A concentration has been found simply to vary accordingly. The addition or depletion of Ap4A from intracellular pools has not been shown to alter cell cycle. Therefore, while the speculation concerning the role of Ap4A in vivo is a good source of future experiments, at this point its role as an important regulatory compound is far from demonstrated.