Modified methods for the determination of carbamyl aspartate

Pyrimidine biosynthetic pathway of Baccillus subtilis

Biochemical and genetic data were obtained from a series of 51 Pyr- strains of Bacillus subtilis. The observed enzymatic deficiencies allowed the mutants to be placed into 12 clases, some of which represent defects in more than one of the six known pyrimidine biosynthetic enzymes. Mapping analysis by transformation has shown that all the Pyr- mutations are located in a single small area of the B. subtilis genome. A correlation of the biochemical defects and the genetic data has been made. Those mutations conferring similar enzymatic deficiencies were found to be contiguous on the B. subtilis map. Regulatory aspects of the pyrimidine pathway have also been investigated and are compared to previously reported results from other organisms. Evidence is presented which bears upon the possible physical association of the first three enzymes and the association of at least some of the enzymes of this pathway with particulate elements of the cell. A model for the organization of the enzymes is presented with dihydroorotate dehydrogenase as the central enzyme in a proposed aggregate.

Stimulation of derepressed enzyme synthesis in bacteria by growth on sublethal concentrations of chloramphenicol

Culturing of Salmonella typhimurium or Escherichia coli cells in the presence of low concentrations (</=1 mug/ml) of chloramphenicol (CAP) permitted exponential growth, but at doubling times up to twice those of controls. When such cultures were subsequently starved for uracil or arginine, derepression of aspartate transcarbamylase (ATCase) or ornithine transcarbamylase, respectively, was enhanced three- to 10-fold as compared to cultures not exposed to CAP. Enhancement of beta-galactosidase synthesis by prior exposure to CAP was also observed in uracil-starved E. coli cultures. Stimulation of enzyme synthesis appeared to be a specific effect of CAP; low levels of erythromycin, puromycin, sparsomycin, tetracycline, and rifampin did not show such effects. Derepression of ATCase synthesis in exponentially growing cells in the presence of CAP did not result in stimulation of enzyme synthesis by CAP. A prior history of growth of a culture in the presence of CAP was shown to be necessary for enhancement of enzyme synthesis by CAP; furthermore, continued presence of CAP in the medium during starvation was not necessary for enhanced enzyme synthesis and inhibited it in some instances. Enhanced enzyme synthesis in starving, CAP-treated cultures could be blocked by rifampin, which suggested that CAP treatment allows prolonged or more extensive messenger ribonucleic acid synthesis.

A procedure for the in vivo determination of enzyme activity in higher plant tissue

Rapid freezing of higher plant tissue in liquid nitrogen renders the cells permeable to a wide range of substrate molecules. Tissue permeabilized by repeated freeze-thaw treatment can be used for the measurement of several enzymes. With most of the tissues examined maximum in vivo activities were obtained using a combination of freeze-thaw treatment followed by vacuum infiltration. The activities and properties of enzymes determined with this procedure are very comparable with those obtained for enzymes prepared by conventional extraction procedures.

Inactivation of aspartic transcarbamylase in sporulating Bacillus subtilis: demonstration of a requirement for metabolic energy

The aspartic transcarbamylase (ATCase) activity of Bacillus subtilis cells disappears rapidly from stationary-phase cells prior to sporulation. ATCase activity does not appear in the culture fluid during the stationary phase; hence the enzyme appears to be inactivated in the cells. The enzyme is inactivated normally in two different mutants lacking proteases; the activity is very stable in crude extracts of cells or in the culture fluid. These results suggest that ATCase is not inactivated by the general proteolysis that occurs in sporulating bacteria. The inactivation of ATCase can be completely inhibited after it has begun by oxygen starvation or addition of fluoroacetate. Inhibitors of oxidative phosphorylation and electron transport also interrupt the inactivation of ATCase. The inactivation of ATCase is very slow in two mutant strains that are deficient in enzymes of tricarboxylic acid cycle. Addition of gluconate to stationary cultures of the mutant strains, which is known to restore depleted adenosine 5'-triphosphate pools in these bacteria, also restores inactivation of ATCase. These experiments support the conclusion that the generation of metabolic energy is necessary for the inactivation of ATCase in stationary cells. ATCase activity is stable in growing cells in which ATCase synthesis is repressed by addition of uracil; the enzyme is inactivated normally, however, when such cells cease growing.

Wheat-germ aspartate transcarbamoylase. The effects of ligands on the inactivation of the enzyme by trypsin and denaturing agents

In the absence of added ligands aspartate transcarbamoylase (EC 2.1.3.2) from wheat germ is inactivated fairly rapidly by trypsin, by heat (60 degrees C), by highly alkaline conditions (pH11.3) and by sodium dodecyl sulphate. Addition of UMP alone, at low concentrations, decreases the rate of inactivation by each of these agents significantly. Carbamoyl phosphate alone does not alter the rate of inactivation by trypsin and by the detergent, but it antagonizes the effect of UMP in protecting the enzyme against these agents. These results have been interpreted to mean that two conformational states are reversibly accessible to the enzyme, namely an easily inactivated state favoured in the presence of carbamoyl phosphate and a more resistant state favoured in the presence of UMP. In the absence of ligands the enzyme is in the easily inactivated conformation. At very high concentrations l-aspartate also protects the enzyme but to a smaller extent than UMP. Some implications of these results are discussed.

Aspartate transcarbamoylase from Phaseolus aureus. Partial purification and properties

1. Aspartate transcarbamoylase from 4-day-old radicles of Phaseolus aureus was purified 190-fold by (NH(4))(2)SO(4) fractionation, DEAE-cellulose and DEAE-Sephadex chromatography and Sephadex-gel filtration. The partially purified enzyme, which required P(i) for maximum stability, had an apparent molecular weight of 83000+/-5000. 2. Uridine nucleotides were found to inhibit the activity; UMP was the most potent inhibitor, followed by UDP and UTP. No other nucleotide was found to affect the enzyme, nor could UMP inhibition be overcome by adding another nucleotide. Aspartate gives a hyperbolic substrate-saturation curve, both with and without UMP. The nucleotide inhibitor is non-competitive with respect to this substrate. Carbamoyl phosphate also yields a hyperbolic substrate-saturation curve in the absence of feedback inhibitor, but when UMP is added a sigmoidal pattern results, and the inhibition is competitive with carbamoyl phosphate. 3. The degree of inhibition by UMP is not affected by p-chloromercuribenzoate, urea, mild heat pretreatment or change in pH over the range 8.5-10.5, but is affected by temperature. 4. The aspartate analogue, succinate, both activates and inhibits the reaction, depending on the concentrations of aspartate and succinate used. 5. Kinetic studies with the partially purified enzyme showed that the K(m) for carbamoyl phosphate (0.091 mm) is much lower than that for aspartate (1.7mm). A sequential reaction mechanism was inferred from product-inhibition kinetics, with carbamoyl phosphate binding to the enzyme before aspartate, and the product, carbamoylaspartate, being released ahead of P(i). Initial-velocity studies gave a set of parallel reciprocal plots, compatible with an essentially irreversible step occurring before the binding of aspartate.

Pyrimidine nucleotide biosynthesis in Phaseolus aureus. Enzymic aspects of the control of carbamoyl phosphate synthesis and utilization

1. Carbamoyl phosphate synthetase activity of Phaseolus aureus extracts was assayed by coupling it to the catalytic subunit of Escherichia coli aspartate transcarbamoylase and determining the [(14)C]carbamoylaspartate so formed. The stability of the activity was improved by the addition of ornithine and dimethyl sulphoxide to the extraction medium. 2. The synthetase activity was found to utilize either glutamine or ammonia as amino donor, the Michaelis constants being 0.17+/-0.03mm and 6.1+/-1.0mm respectively. N-Acetylglutamate did not significantly alter the rate with either substrate, and azaserine inhibited the reaction with both amino donors to the same extent. 3. Ornithine was shown to stimulate the activity, and to counteract inhibition by UMP. The purine nucleotides IMP and GMP enhanced carbamoyl phosphate formation, whereas AMP had an inhibitory effect. 4. The Michaelis constant for carbamoyl phosphate was determined in concentrated extracts for both aspartate transcarbamoylase and ornithine transcarbamoylase activities, and was 0.13+/-0.03mm and 1.58+/-0.16mm respectively. The ratio of the activities of these two enzymes, determined at near-saturating substrate concentrations, was 1:3 (aspartate transcarbamoylase/ornithine transcarbamoylase). 5. It is concluded that in this plant tissue there is one enzyme, carbamoyl phosphate synthetase, supplying carbamoyl phosphate to both the pyrimidine and arginine pathways, that the pyrimidine pathway claims most of the available carbamoyl phosphate (depending on the concentration of the nucleotide effectors) when this intermediate is present at low concentrations; and that when the carbamoyl phosphate concentration is increased, possibly by ornithine stimulation, a larger proportion can be taken up by the arginine pathway.

Wheat-germ aspartate transcarbamoylase. Kinetic behaviour suggesting an allosteric mechanism of regulation

1. Some kinetic properties of aspartate transcarbamoylase (EC 2.1.3.2), that had been purified approx. 20-fold from wheat germ, were studied. 2. A plot of enzyme activity against pH showed a low maximum at pH8.4 and a second, higher, maximum at pH10.5. A plot of percentage inhibition by 0.2mm-UMP against pH was approximately parallel to the plot of activity against pH, except that between pH6.5 and 7.5 the enzyme was insensitive to 0.2mm-UMP. 3. Kinetics were studied in detail at pH10.0 and 25 degrees C. In the absence of UMP, initial-rate plots were hyperbolic when the concentration of either substrate was varied. UMP decreased both V(max.) and K(m) in plots of initial rate against l-aspartate concentration, but the plots remained hyperbolic. However, UMP converted plots of initial rate against carbamoyl phosphate concentration into a sigmoidal shape, without significantly affecting V(max.). Plots of initial rate against UMP concentration were also sigmoidal. 4. The theoretical model proposed by Monod et al. (1965) gave a partial explanation of these results. When quasi-equilibrium conditions were assumed analysis in terms of this model suggested a trimeric enzyme binding the allosteric ligands, carbamoyl phosphate and UMP, nearly exclusively to the R and T conformational states respectively, and existing predominantly in the R state when ligands were absent. However, the values of the Hill coefficients for the co-operativity of each allosteric ligand were somewhat less than those predicted by the theory. 5. Some of the implications of these results are discussed, and the enzyme is contrasted with the well-known aspartate transcarbamoylase of Escherichia coli.

Arginine control of transcription of argECBH messenger ribonucleic acid in Escherichia coli

The level of messenger ribonucleic acid specific for the argECBH gene cluster (arg-mRNA) of Escherichia coli was measured by deoxyribonucleic acid-ribonucleic acid hybridization in a number of strains. During the first 10 min after removal of arginine (derepression), the rate of arg-mRNA accumulation was six to ten times greater than that found in arginine-repressed argR(+) cells. In the absence of arginine, l-canavanine (200 mug/ml) repressed arg-mRNA synthesis to a level only 20 to 30% lower than that found after arginine deprivation. High levels of arg-mRNA were produced by argR(-) strains with or without added arginine. Within about 2 min after arginine addition to argR(+) cells, the rate of synthesis of arg-mRNA reached the repressed level. Likewise, 2.5 min after rifampin addition, all transcription of arg-mRNA was completed. These data are consistent with the view that arginine signals repression by inhibiting the initiation of transcription of arg-mRNA mediated in some way by the argR gene. The kinetics of arg-mRNA accumulation and the kinetics of completion of transcription together with the profile of hybridizable arg-mRNA in sucrose density gradients (major component 16S) suggest that the argECBH gene cluster is transcribed in short pieces rather than as a single unit.

Regulation and mechanism of phosphoribosylpyrophosphate synthetase: repression by end products

Phosphoribosylpyrophosphate (PRPP) synthetase participates in the biosynthesis in bacteria of purine nucleotides, pyrimidine nucleotides, tryptophan, and histidine. The regulation of the synthesis of PRPP synthetase in Salmonella typhimurium was studied. Addition of end products to the growth medium, singly or in combination, resulted in small decreases in the specific activity of PRPP synthetase, but levels of the enzyme were never decreased to less than half of those found when the bacteria were grown on minimal medium. Growth of the bacteria on several different carbon sources or starvation for phosphate had little effect on the specific activity of PRPP synthetase. Over-production of histidine in a histidine regulatory mutant, which would be expected to result in a depletion of intracellular PRPP pools, did not alter PRPP synthetase specific activity. PRPP synthetase levels were examined in auxotrophic strains of S. typhimurium that had been starved for the end products of PRPP. In each case derepression of an enzyme in the biosynthetic pathway for the limiting end product was demonstrated. However, only alterations in the levels of pyrimidine bases in the culture medium brought about derepression and repression of PRPP synthetase. Excess pyrimidines do not completely repress the enzyme. Deprivation of exponentially growing cells for pyrimidines by growth of an auxotrophic mutant on media containing orotic acid, which enters the cells slowly, resulted in a 10-fold derepression of PRPP synthetase. Derepression of PRPP synthetase during uracil starvation was prevented by chloramphenicol. The PRPP synthetase activities of extracts from repressed and derepressed cells responded in identical fashion to heat inactivation, cellulose acetate electrophoresis at several pH values, and in kinetic experiments.

Translational repression in the arginine system of Escherichia coli

Translation of bacterial mRNA, divorced from transcription, has been obtained for enzymes of arginine synthesis; evidence has been acquired for repression by arginine at the level of translation. mRNAs for acetylornithinase and ornithine transcarbamylase were accumulated by arginine starvation of argR(+) and argR(-) arginine auxotrophs derived from Escherichia coli K12. Further transcription was inhibited with rifampicin or miracil D, and enzyme formation was measured in the presence of either an excess of, or a restricted supply of, arginine. For the argR(+) strain 961, little mRNA was found without starvation; for the argR(-) strain 977, a considerable amount of mRNA was demonstrated even without starvation. There was relatively little translation for the argR(+) strain, but not for the argR(-) strain, in the presence of excess arginine, apparently due to an accelerated degradation of mRNA in the argR(+) strain under repressive conditions.