Molecular events in the growth inhibition of Bacillus subtilis by D-tyrosine

The transformable strain of Bacillus subtilis strain 168 is extremely susceptible to growth inhibition by d-tyrosine. The molecular events associated with the inhibition of growth by d-tyrosine in this strain include the false feedback inhibition and probably the false repression of prephenate dehydrogenase. These effects were found to contribute to the formation of d-tyrosine-containing proteins by decreasing the intracellular concentration of l-tyrosine. Accordingly, growth inhibition of strain 168 by the d isomer of tyrosine was shown to be progressive, enduring, and delayed by prior growth on l-tyrosine. The synthesis of cellular macromolecules and viable cell count were progressively diminished in d-tyrosine-inhibited cultures. Several different enzyme activities were reduced after growth in the presence of d-tyrosine. Isotopic d-tyrosine was incorporated into cellular proteins without change of optical configuration. Long chains of cells with completed septa were observed microscopically, and therefore some cell wall effect may also be implicated.

Metabolic influences on tyrosine excretion in Bacillus subtilis

The biosynthetic pathway for tyrosine synthesis is regulated both by repression of enzyme synthesis and by feedback inhibition of enzyme activity in Bacillus subtilis. Nevertheless, wild-type cells produce significantly more tyrosine than is required for protein synthesis, and part of this is excreted into the medium. Alteration of nutritional and other environmental conditions of cultivation strongly influenced the amount of tyrosine excretion. The excretion of tyrosine by wild-type cells was compared with that of a regulatory mutant having a feedback-insensitive prephenate dehydrogenase. Tyrosine excretion varied directly with the in vitro activity of prephenate dehydrogenase and inversely with temperature in the two strains. The regulatory mutant excreted five times as much tyrosine as the wild type at all growth temperatures examined. The carbon source used for growth significantly influenced the level of tyrosine excretion. The specific activity of prephenate dehydrogenase was also affected by the carbon source. Incorporation studies with isotopic tyrosine and fluorometric determinations of tyrosine concentrations extractable in hot water were used to measure operationally the tyrosine pools in the mutant and wild-type strains. The effects of various environmental conditions on the synthesis and excretion of tyrosine led to the conclusion that metabolic controls governing end product contrations exist which are completely independent of regulation by feedback inhibition and repression.

Taxonomic implications of temperature dependence of the allosteric inhibition of 3-deoxy-D-arabino-heptulosonate 7-phosphate synthetase in Bacillus

The qualitative pattern of control for 3-deoxy-d-arabino-heptulosonate 7-phosphate (DAHP) synthetase is a stable and strongly conserved trait of related bacteria and constitutes a reliable generic character. In Bacillus, the generic control pattern for DAHP synthetase is sequential feedback inhibition, a regulatory pattern in which branch-point metabolites are feedback inhibitors. Member species of this genus have DAHP synthetases which vary quantitatively in the effect of temperature upon the sensitivity of the enzyme to feedback inhibition by prephenate. The magnitude of this temperature effect has been expressed quantitatively as the allosteric temperature ratio. The species clusters definable by allosteric temperature ratios correlate exceedingly well with subgroups previously distinguished on the basis of sporangial structure. Hence, two independently derived arrangements of Bacillus subgroups, depending upon very different methodologies, matched for all but 3 of the 24 species considered. The taxonomic position of these subgroups of the genus Bacillus is discussed.

Comparative allostery of 3-deoxy-D-arabino-heptulosonate-7-phosphate synthetase as a molecular basis for classification

The allosteric pattern of control for 3-deoxy-d-arabino-heptulosonate 7-phosphate (DAHP) synthetase has previously been shown to be strongly conserved among the member species of a given genus in bacteria. The implications of this finding as a procedural tool of bacterial phylogeny were pursued by a study of two organisms, Sporosarcina ureae and Aeromonas formicans, the taxonomic positions of which have been historically controversial. S. ureae has characteristics of both Bacillaceae and Micrococcaceae, and A. formicans has characteristics of both Enterobacteriaceae and pseudomonads. Since the patterns of control for DAHP synthetase in all four of these microbial groups are different from one another but internally homogeneous within each group, the results obtained from the two test organisms were unambiguous. It was concluded that S. ureae is properly classified within Bacillaceae, probably deserving generic rank, and that A. formicans belongs with the family Enterobacteriaceae.

Electrophysiological actions of diphenylhydantoin on rabbit atria. Dependence on stimulation frequency, potassium, and sodium

Isolated rabbit left atrial preparations were perfused with Tyrode's solutions containing 1 to 10 µg/ml (4 x 10 -6 -4 x 10 -5 M) diphenylhydantoin (DPH), 2.6-5.6 mM K + , and 154-308 mM Na + . Steady-state transmembrane resting and action potentials were recorded from these preparations with glass microelectrodes at stimulation rates ranging from 0.2 to 3/sec. DPH had little or no effect on the relationship between extracellular [K + ] and membrane resting potential. Action potential overshoot was generally decreased by 5 and 10 µg/ml DPH and increased by 1 µg/ml DPH at stimulation rates of 2 and 3/sec in the presence of increased [K + ]. DPH and increased [K + ] acted synergistically to shorten action potential duration (measured at 50% repolarization). The effect of DPH on phase 0 of the action potential (measured as action potential rise time between 10 and 50% and 50 and 90% depolarization) was markedly dependent upon drug concentration, extracellular [K + ] and stimulation rate. The lowest concentration of DPH (1 µg/ml) usually shortened action potential rise time, particularly when it had been prolonged by increasing extracellular [K + ]. Conversely, the highest concentration of DPH (10 µg/ml) and increased [K + ] acted synergistically to prolong action potential rise time (i.e., decrease depolarization rate). When present, the depressant effect of DPH on membrane depolarization was rapidly antagonized by increasing extracellular [Na + ]. It is proposed that DPH may either enhance or depress (like quinidine) membrane activity in atrial tissue, and that both the direction and magnitude of effect are strongly dependent upon drug concentration, ionic milieu, and heart rate.

D-Tyrosine as a metabolic inhibitor of Bacillus subtilis

The d-isomer of tyrosine is a potent inhibitor of growth in transformable strain 168 of Bacillus subtilis. A d-tyrosine-resistant mutant of the inhibited strain was isolated which excreted l-tyrosine, had a diminished growth rate, and required l-phenylalanine to attain the growth rate of the wild-type parent. Mapping by deoxyribonucleate transformation located this resistance in the gene coding for prephenate dehydrogenase. This enzyme in the d-tyrosine-resistant mutant was insensitive to the usual feedback inhibition exerted by l-tyrosine in extracts of strain 168. In contrast, the growth of poorly transformable strain 23 of B. subtilis, as well as that of several other Bacillus species, was not affected by the analogue. Transformation mapping demonstrated no linkage of this latter "natural resistance" to several different aromatic markers. Prephenate dehydrogenase in extracts from strain 23 was as sensitive as that from strain 168 to feedback inhibition by l-tyrosine in vitro. The relationships of the latter results to the regulation of tyrosine biosynthesis and the possible nature of strain differences in d-tyrosine sensitivity are discussed.

Antimetabolite action of 5-methyltryptophan in Bacillus subtilis

The roles of anthranilate and histidine in the regulation of tryptophan synthesis were implicated by observations of antagonistic relationships with 5-methyltryptophan in Bacillus subtilis.

Regulation of enzyme synthesis in the aromatic amino acid pathway of Bacillus subtilus

The control of the synthesis of certain key enzymes of aromatic amino acid biosynthesis was studied. Tyrosine represses the first enzyme of the 3-deoxy-d-arabino heptulosonic acid 7-phosphate pathway, DAHP synthetase, as well as shikimate kinase and chorismate mutase about fivefold in cultures grown under conditions limiting the synthesis of the aromatic amino acids. A mixture of tyrosine and phenylalanine represses twofold further. Tryptophan does not appear to be involved in the control of these enzymes. The specific activity of at least one early enzyme, dehydroquinase, remains essentially constant under a variety of nutritional supplementations. Two enzymes in the terminal branches are repressed by the amino acids they help to synthesize: prephenate dehydrogenase can be repressed fourfold by tyrosine, and anthranilate synthetase can be repressed over 200-fold by tryptophan. There is no evidence that phenylalanine represses prephenate dehydratase. Regulatory mutants have been isolated in which various enzymes of the pathway are no longer repressible. One class is derepressed for several of the prechorismate enzymes, as well as chorismate mutase and prephenate dehydrogenase. In another mutant, several enzymes of tryptophan biosynthesis are no longer repressible. Thus, the rate of synthesis of enzymes at every stage of the pathway is under control of various aromatic amino acids. Tyrosine and phenylalanine control the synthesis of enzymes involved in the synthesis of the three aromatic amino acids. Each terminal branch is under the control of its end product.

Comparative regulation of isoenzymic 3-deoxy-D-arabino-heptulosonate 7-phosphate synthetases in microorganisms

The independent control of regulatory isoenzymes by different metabolites constitutes one well-known pattern of control in branched metabolic pathways. This pattern was previously found to be widely distributed in the aromatic amino acid pathway of microorganisms in the case of the first enzyme of the sequence, 3-deoxy-d-arabino-heptulosonate 7-phosphate (DAHP) synthetase. The comparative stability of the isoenzymes as well as the effect of aromatic amino acids in the growth medium upon the levels of the individual isoenzymes were shown for Salmonella typhimurium. Several lines of evidence are discussed to demonstrate the strong reliance of Escherichia coli upon the phenylalanine-sensitive isoenzyme for the ordinary biosynthetic needs of wild-type strains. The frequent occurrence of "dominant" isoenzyme species which resist repressive effects of the inhibitory end products was noted. The lack of an obligatory correlation of the level of an isoenzyme activity and the synthesis of the end product which specifically controls its activity is used to discount the possibility that each isoenzyme might feed a unique and separate metabolic pool of end-product precursor. An isoenzymic DAHP synthetase sensitive to feedback inhibition by low levels of tryptophan was fractionated from tyrosine- and phenylalanine-sensitive isoenzymes in cell-free extracts of Neurospora crassa.

Comparative control of a branch-point enzyme in microorganisms

Thirty-two genera of microorganisms were identified with one of six distinctive control patterns for the enzyme 3-deoxy-d-arabino-heptulosonate-7-phosphate synthetase. These patterns included sequential feedback inhibition, isoenzyme feedback inhibition, cumulative feedback inhibition, and three (apparent) simple one-effector patterns. Documentation is provided of an overwhelming tendency for control patterns to be strongly conserved among the member species of the various genera that were examined.