Correlation between the serine sensitivity and the derepressibility of the ilv genes in Escherichia coli relA- mutants

Toxic accumulation of alpha-ketobutyrate caused by inhibition of the branched-chain amino acid biosynthetic enzyme acetolactate synthase in Salmonella typhimurium

Biochemical and genetic analyses of the bacterium Salmonella typhimurium suggest that accumulation of alpha-ketobutyrate partially mediates the herbicidal activity of acetolactate synthase inhibitors. Growth inhibition of wild-type bacteria by the herbicide sulfometuron methyl was prevented by supplementing the medium with isoleucine, an allosteric inhibitor of threonine deaminase-catalyzed synthesis of alpha-ketobutyrate. In contrast, isoleucine did not rescue the growth of a mutant containing a threonine deaminase unresponsive to isoleucine. Moreover, the hypersensitivity of seven Tn10 insertion mutants to growth inhibition by sulfometuron methyl and alpha-ketobutyrate correlated with their inability to convert alpha-ketobutyrate to less noxious metabolites. We propose that alpha-ketobutyrate accumulation is an important component of sulfonylurea and imidazolinone herbicide action.

Mutations in the spoT gene of Salmonella typhimurium: effects on his operon expression

The spoT gene of Salmonella typhimurium has been identified. Mutations in spoT map between gltC and pyrE at 79 min. The spoT1 mutant has elevated levels of guanosine 5'-diphosphate-3'-diphosphate (ppGpp) during steady-state growth and exhibits a slower than normal decay of ppGpp after reversal of amino acid starvation. The spoT1 mutation elevates his operon expression but is distinct from known his regulatory mutations. Elevated his operon expression in spoT mutants causes resistance to the histidine analogs, 1,2,4-triazole-3-alanine and 3-amino-1,2,4-triazole. These properties of spoT mutants allowed us to identify and characterize additional spoT mutants. Approximately 40% of these mutants are temperature sensitive for growth on minimal medium, suggesting that the spoT function is essential or that excessive accumulation of ppGpp is lethal.

Evidence for cAMP-mediated control of isoleucyl-tRNA synthetase formation in Escherichia coli K-12

The ability of cAMP to inhibit isoleucyl-tRNA synthetase (IRS) formation has been demonstrated in wild type K-12 Escherichia coli and two adenyl-cyclase (cya) mutants. cAMP appeared not to have any effect on either the valyl- or arginyl-tRNA synthetase (VRS and ARS respectively). Addition of cAMP led to a reduction in rate of IRS synthesis but not VRS or ARS. Furthermore, derepression of IRS and VRS by isoleucine limitation was completely prevented by cAMP.

Role of 2-ketobutyrate as an alarmone in E. coli K12: inhibition of adenylate cyclase activity mediated by the phosphoenolpyruvate: glycose phosphotransferase transport system

2-ketobutyrate and its analogues were found to inhibit strongly and transiently the rate of beta-galactosidase synthesis in Escherichia coli K12. This effect was ascribed to a strong and transient inhibition of the adenylate cyclase activity. By using pts mutants, we showed, in agreement with our previous results (Daniel et al. 1983), that the likely target of 2-ketobutyrate and its analogues is the phosphoenolpyruvate: glycose phosphotransferase transport system (PTS). Furthermore, evidence for such a cascade effect caused by 2-ketobutyrate and its analogues allowed us to corroborate our previous proposal (Daniel et al. 1983) that 2-ketobutyrate, a precursor of isoleucine, acts as an E. coli alarmone monitoring the passage from anaerobic to aerobic growth conditions.

Metabolic alterations mediated by 2-ketobutyrate in Escherichia coli K12

We have previously proposed that 2-ketobutyrate is an alarmone in Escherichia coli. Circumstantial evidence suggested that the target of 2-ketobutyrate was the phosphoenol pyruvate: glycose phosphotransferase system (PTS). We demonstrate here that the phosphorylated metabolites of the glycolytic pathway experience a dramatic downshift upon addition of 2-ketobutyrate (or its analogues). In particular, fructose-1,6-diphosphate, glucose-6-phosphate, fructose-6-phosphate and acetyl-CoA concentrations drop by a factor of 10, 3, 4, and 5 respectively. This result is consistent with (i) an inhibition of the PTS by 2-ketobutyrate, (ii) a control of metabolism by fructose-1,6-diphosphate. Since fructose-1,6-diphosphate is an activator of phosphoenol pyruvate carboxylase and of pyruvate kinase, the concentration of their common substrate, phosphoenol pyruvate, does not decrease in parallel.

Promoter selectivity of Escherichia coli RNA polymerase. II: Altered promoter selection by mutant holoenzymes

Using the in vitro mixed transcription system (Kajitani and Ishihama (1983a, 1983b), we examined selective transcription of truncated DNA templates carrying lac(UV5), rrnE or rpsA promoters by RNA polymerase holoenzymes from pairs of wild-type parents and mutants with a mutation in one or more RNA polymerase subunit genes. The promoter selectivity of RNA polymerases from two sigma-subunit mutants carrying either rpoD2 or rpoD285 differed markedly from that of the respective wild-type enzymes. Both the parental RNA polymerases, however, exhibited abnormal promoter selectivity compared with holoenzymes from various wild-type E. coli strains. On the other hand, all the RNA polymerases from rpoB and/or rpoC mutants and the respective wild-type parents were similar, if not identical, in promoter selection at low temperature. At high temperature, however, RNA polymerases from mutants carrying rpoB2B7 and rpoC4, affecting the beta and beta' subunits, respectively, showed decreased transcription from the high-affinity slow-transcribable promoter rrnEp2 whereas the rpoC92 and rpoB906 X rpoC907 mutant enzymes both lost transcription activity from the strong promoter lacP(UV5). Taking all these observations together we conclude that not only the sigma subunit but also the beta and beta' subunits are involved in the recognition of promoters.

Escherichia coli K12 mutants defective in the glycine cleavage enzyme system

Two routes of one-carbon biosynthesis have been described in Escherichia coli K12. One is from serine via the serine hydroxymethyltransferase (SHMT) reaction, and the other is from glycine via the glycine cleavage (GCV) enzyme system. To isolate mutants deficient in the GCV pathway, we used a selection procedure that is based on the assumption that loss of this enzyme system in strains blocked in serine biosynthesis results in their inability to use glycine as a serine source. Mutants were accordingly isolated that grow with a serine supplement, but not with a glycine supplement. Enzyme assays demonstrated that three independently isolated mutants have no detectable GCV enzyme activity. The absence of a functional GCV pathway results in the excretion of glycine, but has no affect on the cell's primary source of one-carbon units, the SHMT reaction. The new mutations, designated gcv, were mapped between the serA and lysA genes on the E. coli chromosome.

2-Ketobutyrate: a putative alarmone of Escherichia coli

2-ketobutyrate is synthesized from threonine by threonine deaminase (dehydratase) in E. coli. The effects of 2-ketobutyrate as a regulatory metabolite were studied in vivo. 2-ketobutyrate was shown to inhibit the phosphoenolpyruvate (PEP): sugar phosphotransferase system resulting in aspartate starvation, elevation of ppGpp endogenous pools, and cessation of growth in E. coli grown in glucose and related carbon sources. Accordingly, we propose that 2-ketobutyrate might serve as an alarmone whose concentration precisely governs the shift from anaerobic growth to aerobic growth in E. coli. Such shifts are common phenomena among the Enterobacteriaceae.

Reversal of growth inhibition in Escherichia coli by bisulfite

Bisulfite inhibits growth, protein synthesis and RNA synthesis in micro-organisms. These inhibitory effects, in Escherichia coli K12, were diminished when certain amino acids were added to the minimal growth medium. The effects of individual amino acids were additive, synergistic and independent of their concentration. At 10 or 5 mM bisulfite, the presence of tyrosine had the greatest importance, while phenylalanine, glycine and methionine played a lesser role. E. coli cultures resume growth subsequent to bisulfite inhibition, after a lag whose duration is proportional to the initial concentration of added bisulfite. This resumption of growth was shown not to result from the depletion of bisulfite (by air oxidation or metabolism), but rather to adaptation to the presence of bisulfite concentrations that previously were inhibitory. The inhibitory effects of bisulfite were duplicated by lesser concentrations of cysteine. Both substances inhibited RNA synthesis by a mechanism that was independent of the stringent control system. A sulfite reductase deficient mutant was more resistant to inhibition by both bisulfite and cysteine. The various results indicate that growth inhibition by bisulfite is not due to covalent reactions with cellular target molecules, but to conversion of bisulfite to other substances, probably including cysteine, which in turn inhibit bacterial growth.

Cysteine and growth inhibition of Escherichia coli: threonine deaminase as the target enzyme

Cysteine has been shown to inhibit growth in Escherichia coli strains C6 and HfrH 72, but not M108A. Growth inhibition was overcome by inclusion of isoleucine, leucine, and valine in the medium. Isoleucine biosynthesis was apparently affected, since addition of this amino acid alone could alter the inhibitory effects of cysteine. Homocysteine, mercaptoethylamine, and mercaptoethanol inhibited growth to varying degrees in some strains, these effects also being prevented by addition of branched-chain amino acids. Cysteine, mercaptoethylamine, and homocysteine were inhibitors of threonine deaminase but not transaminase B, two enzymes of the ilvEDA operon. Cysteine inhibition of threonine deaminase was reversed by threonine, although the pattern of inhibition was mixed. These results suggest a relationship between the growth-inhibitory effects of cysteine and other sulfur compounds and the inhibition of isoleucine synthesis at the level of threonine deaminase.

The relA locus and the regulation of lysine biosynthesis in Escherichia coli

The allelic state of relA influences the phenotype of Escherichia coli strains carrying the lysA22 mutation:lysA22 relA strains are Lys- where lysA22 relA+ strains grow (slowly) in the absence of lysine. This physiological effect has been related to an effect of the expression of the relA locus on the regulation of lysine biosynthesis. The fully derepressed levels of some lysine enzymes (aspartokinase III, aspartic semialdehyde dehydrogenase, didhydrodipicolinate reductase) are observed under lysine limitation only in rel+ strains. And the induction of DAP-decarboxylase by DAP is much higher in rel+ than in rel- strains when an amino acid limitation of growth is also realised. These results are in agreement with the hypothesis of Stephens et al. (1975) on a possible role of the stringent regulation as a general signal for amino acid deficiency.

Serine sensitivity of Escherichia coli K 12: partial characterization of a serine resistnat mutant that is extremely sensitive to 2-ketobutyrate

E. coli wild type bacteria display sensitivity towards serine. A selection medium is described which allows selection of serine resistant mutants. One such mutant is described which presents pleiotropic alterations: it exhibits a thermosensitive growth pattern, alteration in the metabolism of the pppGpp and ppGpp nucleotides, cAMP intracellular level alteration, extreme sensitivity to 2-ketobutyric acid and a defect in the phosphotransferases permeation system. A conjecture explaining these apparently unrelated defects supposes that serine metabolism interferes via phosphoenol pyruvate with a cytoplasmic control of membrane activity (the mutant would be defective in the coupling between membrane and the protein responsible for its cytoplasmic control) and that 2-ketobutyrate is an effector of this activity.

Involvement of cyclic AMP and its receptor protein in the sensitivity of Escherichia coli K 12 toward serine: excretion of 2-ketobutyrate, a precursor of isoleucine

A relationship between serine-induced growth sensitivity and the cAMP-CAP complex is established. Mutants of Escherichia coli K 12 deficient either in the cya or crp gene function exhibit a resistant phenotype on serine media although they harbor a relA allele normally leading to sensitivity toward serine. The presence of a crp allele in a cya delta relA background restores the sensitivity phenotype, while the analysis of serine resistant mutants selected from a crp cya delta relA strain shows that the mutation leading to resistance is located at, or very near, the crp gene, giving a more or less Crp- phenotype. In addition crp cya delta relA strains excrete large quantities of 2-ketobutyrate when grown on glucose M63 medium. This excretion is unambiguously linked to the presence of the crp allele and is correlated with an enhanced threonine deaminase activity. Besides, the complex regulation exerted on the acetolactate synthase activities is discussed.

Is a metabolic control for the doubling of the macromolecule synthesizing machinery possible

The usual models for cell multiplication only involve macromolecular events and ignore the influence of metabolism. The conjecture presented here shows how metabolism could be involved. Separate pathways might cooperate for the synthesis of a small molecule, acting as positive control of several macromolecular syntheses, including stable RNA synthesis. A detailed analysis of the available data showing an influence of metabolites on macromolecular syntheses suggests that two main pathways might cooperate, one involving serine and folic acid metabolism and the second involving threonine and the branched chain aminoacids. Experiments are suggested to challenge the conjecture.