Dual-control of the tryptophan operon is mediated by both tryptophanyl-tRNA synthetase and the repressor

Molecular basis of Tropheryma whipplei doxycycline susceptibility examined by transcriptional profiling

OBJECTIVES AND METHODS

Tropheryma whipplei is a poorly studied bacterium responsible for Whipple's disease. In this study, its susceptibility to doxycycline was investigated at a transcriptional level using a whole-genome DNA microarray.

RESULTS

Exposure of T. whipplei to the MIC of doxycycline (0.5 mg/L) induced antibiotic-specific primary expression profiles, while indirect effects were detected at 10 x MIC. In contrast to what was observed for several microorganisms exposed to antibiotics, the heat-shock proteins were not affected. Consistent with the mode of action of this translation inhibitor, genes encoding for ribosomal proteins and translation factors were differentially transcribed. This analysis also evidenced the regulation of genes that should account for cell growth arrest. Long-term survival of non-replicating bacteria is likely to be ensured by an increased level of ppGpp, the nucleotide effector of the stringent response. The gene expression profile observed with 10 x MIC was mainly characterized by the up-regulation of ABC transporters that possibly form efflux and detoxification systems, through which T. whipplei may limit the effects of this bacteriostatic compound. Obtained microarray data showed good agreement with real-time quantitative PCR (R2 = 0.969).

CONCLUSIONS

This work represents the first comprehensive genomic approach providing insights into the expression signature triggered by the exposure of T. whipplei to antibiotics.

Increased transcription rates correlate with increased reversion rates in leuB and argH Escherichia coli auxotrophs

Escherichia coli auxotrophs of leuB and argH were examined to determine if higher rates of transcription in derepressed genes were correlated with increased reversion rates. Rates of leuB and argH mRNA synthesis were determined using half-lives and concentrations, during exponential growth and at several time points during 30 min of amino acid starvation. Changes in mRNA concentration were primarily due to increased mRNA synthesis and not to increased stability. Four strains of E. coli amino acid auxotrophs, isogenic except for relA and argR, were examined. In both the leuB and argH genes, rates of transcription and mutation were compared. In general, strains able to activate transcription with guanosine tetraphosphate (ppGpp) had higher rates of mRNA synthesis and mutation than those lacking ppGpp (relA2 mutants). argR knockout strains were constructed in relA(+) and relA mutant strains, and rates of both argH reversion and mRNA synthesis were significantly higher in the argR knockouts than in the regulated strains. A statistically significant linear correlation between increased rates of transcription and mutation was found for data from both genes. In general, changes in mRNA half-lives were less than threefold, whereas changes in rates of mRNA synthesis were often two orders of magnitude. The results suggest that specific starvation conditions target the biosynthetic genes for derepression and increased rates of transcription and mutation.

Mechanism of regulation of transcription initiation by ppGpp. I. Effects of ppGpp on transcription initiation in vivo and in vitro

To determine the role of ppGpp in both negative and positive regulation of transcription initiation during exponential growth in Escherichia coli, we examined transcription in vivo and in vitro from the growth-rate-dependent rRNA promoter rrnB P1 and from the inversely growth-rate-dependent amino acid biosynthesis/transport promoters PargI, PhisG, PlysC, PpheA, PthrABC, and PlivJ. rrnB P1 promoter activity was slightly higher at all growth-rates in strains unable to synthesize ppGpp (deltarelAdeltaspoT) than in wild-type strains. Consistent with this observation and with the large decrease in rRNA transcription during the stringent response (when ppGpp levels are much higher), ppGpp inhibited transcription from rrnB P1 in vitro. In contrast, amino acid promoter activity was considerably lower in deltarelAdeltaspoT strains than in wild-type strains, but ppGpp had no effect on amino acid promoter activity in vitro. Detailed kinetic analysis in vitro indicated that open complexes at amino acid promoters formed much more slowly and were much longer-lived than rrnB P1 open complexes. ppGpp did not increase the rates of association with, or escape from, amino acid promoters in vitro, consistent with its failure to stimulate transcription directly. In contrast, ppGpp decreased the half-lives of open complexes at all promoters, whether the half-life was seconds (rrnB P1) or hours (amino acid promoters). The results described here and in the accompanying paper indicate that ppGpp directly inhibits transcription, but only from promoters like rrnB P1 that make short-lived open complexes. The results indicate that stimulation of amino acid promoters occurs indirectly. The accompanying paper evaluates potential models for positive control of amino acid promoters by ppGpp that might explain the requirement of ppGpp for amino acid prototrophy.

Hypermutation in derepressed operons of Escherichia coli K12

This article presents evidence that starvation for leucine in an Escherichia coli auxotroph triggers metabolic activities that specifically target the leu operon for derepression, increased rates of transcription, and mutation. Derepression of the leu operon was a prerequisite for its activation by the signal nucleotide, guanosine tetraphosphate, which accumulates in response to nutritional stress (the stringent response). A quantitative correlation was established between leuB mRNA abundance and leuB- reversion rates. To further demonstrate that derepression increased mutation rates, the chromosomal leu operon was placed under the control of the inducible tac promoter. When the leu operon was induced by isopropyl-D-thiogalactoside, both leuB mRNA abundance and leuB- reversion rates increased. These investigations suggest that guanosine tetraphosphate may contribute as much as attenuation in regulating leu operon expression and that higher rates of mutation are specifically associated with the derepressed leu operon.

Analysis of regulation of the ilvGMEDA operon by using leader-attenuator-galK gene fusions

Five of the genes for the biosynthesis of isoleucine and valine form the ilvGMEDA operon of Escherichia coli K-12. Expression of the operon responds to changes in the availability of isoleucine, leucine, and valine (ILV). Addition of an excess of all three amino acids results in reduced expression of the operon, whereas limitation for one of the three amino acids causes an increase in expression. The operon is preceded by a leader-attenuator which clearly regulates the increased expression that occurs due to reduced aminoacylation of tRNA. To assess the factors that result in the reduced expression of this operon upon the addition of ILV, a series of plasmids were constructed in which the ilv regulatory region was fused to galK. In response to addition of the amino acids, expression of the galK gene fused to the leader-attenuator decreased five- to sevenfold, instead of the twofold observed for the chromosomal operon. A deletion analysis with these plasmids indicated that the ILV-specific decrease in expression required an intact leader-attenuator but not ilvGp2 or the DNA that precedes this promoter. This conclusion was supported by both S1 nuclease analysis of transcription initiation and determination of galK mRNA levels by RNA-RNA hybridization.

Regulation of in vitro expression of the Escherichia coli frd operon: alanine and Fnr represent positive and negative control elements

The frdABCD operon of Escherichia coli encodes the anaerobically expressed terminal electron transport enzyme, fumarate reductase. Two mutually exclusive hairpin loop structures can occur in frdmRNA just downstream of the start of the frdA cistron. The mRNA sequence involved encodes a stretch of sequence rich in Ala and uses all four of the codons for this amino acid. In vitro expression of the frdABCD operon showed that as the level of plasmid DNA was increased from 150 fmol to 225 fmol, transcription of mRNA was suddenly elevated 6.5-fold, consistent with the concept of titrating out a repressor protein. Further studies showed that the concomitant 10.9-fold increase in translation of protein was heavily biased towards the proximal end of the operon, with little or no expression of FrdC or FrdD and a ratio of FrdA:FrdB of 2.6:1. Addition of Ala to the S-30 extract caused a 6.1-fold amplification of frd messenger transcription, a 17.6-fold increase in Frd protein translation, and a balancing of the subunit ratios to 1:1:1:1. The expression of the bla gene carried on the plasmid was not affected by DNA titration or the addition of Ala. When fnr DNA was added in equimolar ratio to frdDNA the amplification of fumarate reductase expression by Ala was abolished and the ratio of subunits produced showed a high degree of polarity with or without Ala.

Defective regulation of the phenylalanine biosynthetic operon in mutants of the phenylalanyl-tRNA synthetase operon

Among mutants of Escherichia coli resistant to p-fluorophenylalanine (PFP) were some with constitutive expression of the phenylalanine biosynthetic operon (the pheA operon). This operon is repressed in the wild type by phenylalanine. The mutation in three of these mutants mapped in the aroH-aroD region of the E. coli chromosome at 37 min. A plasmid bearing wild-type DNA from this region restored p-fluorophenylalanine sensitivity and wild-type repression of the pheA operon. Analysis of subclones of this plasmid and comparison of its restriction map with published maps indicated that the mutations affecting regulation of the pheA operon lie in the structural genes for phenylalanyl-tRNA synthetase, pheST, probably in pheS. Thus, the pheST operon has a role in the regulation of phenylalanine biosynthesis, the most likely being that wild-type phenylalanyl-tRNA synthetase maintains a sufficient intracellular concentration of Phe-tRNA(Phe) for attenuation of the pheA operon in the presence of phenylalanine. A revised gene order for the 37-min region of the chromosome is reported. Read clockwise, the order is aroD, aroH, pheT, and pheS.

Theoretical evaluation of transcriptional pausing effect on the attenuation in trp leader sequence

The effect of transcriptional pausing on attenuation is investigated theoretically on the basis of the attenuation control mechanism presented by Oxender et al. (Oxender, D. L., G. Zurawski, and C. Yanofsky, 1979, Proc. Natl. Acad. Sci. USA. 76:5524-5528). An extended stochastic model including the RNA polymerase pausing in the leader region is developed to calculate the probability of relative position between the RNA polymerase transcribing the trp leader sequence and the ribosome translating the transcript. The present study results in a new rationale that the transcriptional pausing site in the leader sequence makes the attenuation control both more sensitive as an on/off switch and less sensitive to variations in the concentration of cellular metabolites not connected with the need for expressing, or not expressing, the particular operon. It is also proposed that the transcriptional pausing diminishes the dependence of attenuation control characteristics on the number of nucleotides in the leader sequence. This result may be useful for understanding the attenuation control efficiencies of other amino acid leader sequences with different lengths of nucleotides.

Overproduction of tryptophanyl-tRNA synthetase relieves transcription termination at the Escherichia coli tryptophan operon attenuator

Overproduction of tryptophanyl-tRNA synthetase increased trp operon expression by reducing transcription termination at the trp attenuator. The total cellular level of charged tRNATrp was not affected by increased levels of the synthetase. We propose that excess synthetase binds charged tRNATrp and reduces the concentration available for translation.

Modulation of Escherichia coli tryptophan (trp) attenuation by the UGA readthrough process

We constructed plasmid pAtrp46 in which lacZ gene expression is regulated by the attenuator of the Escherichia coli tryptophan (trp) operon. The attenuation of trp, which occurs in the presence of an excess of tryptophan, is reflected by a decrease in the expression of the lacZ gene of pAtrp46 in a trpR- strain. Experiments with pAtrp46 further support our previous results (Engelberg-Kulka et al. 1982b) that suppression of a UGA termination codon by normal charged tRNATrp, a process called UGA readthrough, is a necessary mechanism in trp attenuation. Our experiments also suggest that plasmid pAtrp46 is useful for studies of other aspects of trp attenuation.

Studies on the involvement of the UGA readthrough process in the mechanism of attenuation of the tryptophan operon of Escherichia coli

We asked if UGA suppression by charged tRNATrp, a process called UGA readthrough, is involved in the mechanism of attenuation of the tryptophan (trp) operon in Escherichia coli. For this purpose we used two mutations: strA(LD1) which causes restriction of UGA readthrough, and revA which partially overcomes the restriction of UGA readthrough caused by strA(LD1)(Engelberg-Kulka et al. 1982). trp attenuation was monitored by the regulation of the synthesis of the trp operon enzyme anthranilate synthetase (ASase) in trpR strains. We showed that the strA(LD1) mutation causes a significant increase in the level of synthesis of ASase in the presence of an excess of tryptophan, while the revA mutation reverses this effect, indicating that transcription termination at the trp attenuator site is relieved by restriction of UGA readthrough. Based on our results and the sequence data of the trp leader RNA of E. coli (Oxender et al. 1979), we offer a model for the involvement of the UGA readthrough process in trp attenuation. We suggest that the UGA readthrough process permits trp attenuation to respond to slight changes in the cellular concentration of charged tRNATrp.

Regulation of tryptophanyl-tRNA synthetase formation

A previously constructed trp-S-lacZ fusion encoding a hybrid protein with beta-galactosidase activity was subcloned from a multicopy plasmid onto a lambda vector. Single-copy lysogens of lambda trpS-lacZ were used to determine whether trpS was regulated in a manner similar to that of other aminoacyl-tRNA synthetases. trpS regulation was found to resemble that of the majority of synthetases, in that expression of the lysogen-encoded hybrid beta-galactosidase varied with growth rate; beta-galactosidase activity increased 2.5-fold as the generation time decreased from 150 to 37 min. This regulatory response was confirmed by DNA/RNA hybridization experiments, which also suggested that this form of metabolic regulation occurred at the transcriptional level. No alteration in the level of hybrid beta-galactosidase was observed, however, when cells were starved for tryptophan.

The regulation of the ammonia assimilatory enzymes in Rel+ and Rel- strains of Salmonella typhimurium

The influence of the relA1 mutation on the regulation of the ammonia assimilatory enzymes, glutamate dehydrogenase (EC 1.4.1.4), glutamine synthetase (EC 6.3.1.2), and glutamate synthase (EC 1.4.1.3), was examined. When cells grown in rich media (either Luria broth or glucose-ammonia plus casamino acids) were transferred to a glucose-ammonia medium, the relA mutant failed to resume growth and did not have the same increase in any of the assimilatory enzyme activities as the rel+ strain. This effect was particularly dramatic for glutamate dehydrogenase, which increased 6-fold in the rel+ strain. Measurements of the guanosine nucleotide concentrations showed that the rel+ strain had a ppGpp concentration about 9 times that of the relA mutant 5 min after the shift to minimal medium. These results are consistent with those for other biosynthetic enzymes and show that the ammonia assimilatory enzymes require a relA product for their synthesis during shift from rich to minimal media. In addition, we examined the response of these strains to a change in nitrogen source. The relA mutant again failed to resume growth after a shift from glucose-ammonia to glucose-arginine medium. Even though the ppGpp concentration did not increase, the rel+ strain grew and increased glutamine synthetase activities about 2-fold. These changes the absence of increased ppGpp levels suggest that some other relA-mediated function is important during this change in nitrogen source.

An aminoacyl tRNA synthetase binds to a specific DNA sequence and regulates its gene transcription

Alanine tRNA synthetase represses transcription of its own gene by binding specifically to a palindromic sequence which flanks the gene's transcription start site. Transcription repression is greatly enhanced by elevated concentrations of the cognate amino acid. The amino acid effect is caused by direct association of the ligand with the synthetase, which in turn mediates tighter binding to the DNA.

Attenuation in the control of expression of bacterial operons

Bacterial operons concerned with the biosynthesis of amino acids are often controlled by a process of attenuation. The translation product of the initial segment of the transcript of each operon is a peptide rich in the amino acid that the particular operon controls. If the amino acid is in short supply translation is stalled at the relevant codons of the transcript long enough for the succeeding segment of the transcript to form secondary structures that allow the transcribing RNA polymerase molecule to proceed through a site that otherwise dictates termination of transcription. This site is the attenuator; the process is attenuation.

In vivo role of the relA+ gene in regulation of the lac operon

Under conditions of amino acid limitation, beta-galactosidase was produced at a 70-fold higher rate in a relA+ strain than in an isogenic relA strain of Escherichia coli K-12. Under identical conditions with the relA+ and relA strains carrying various lac promoter mutations, rates of beta-galactosidase synthesis in relA+ (relative to relA) ranged from 26-fold higher (promoter mutant Pr 13) to only 5-fold higher (promoter mutant PrL8uv5). This promoter specificity was independent of strain background and the means of eliciting amino acid limitation. Addition of cyclic AMP to the growth medium altered the relA+/relA difference for beta-galactosidase synthesis from the wild-type lac promoter. The experiments suggest that the relA+/relA difference in lac expression arises primarily at the point of transcription initiation. The results are discussed in relation to recent in vitro data showing a promoter-specific guanosine 5'-diphosphate 3'-diphosphate stimulation of lac transcription (P. Primakoff and S. W. Artz, Proc. Natl. Acad. Sci. U.S.A. 76:1726-1730).

Stringent control and protein synthesis in bacteria

Most bacteria have evolved a number of regulatory mechanisms which allow them to maintain a balanced and rather constant cellular composition in response to nutritional variations. In particular, when the availability of any aminoacyl-tRNA species becomes limiting (namely through amino acid starvation or inactivation of an aminoacyl-tRNA synthetase), several biochemically distinct physiological processes are significantly modified. This coordinate adjustment of cellular activity is termed the "stringent response". Under such conditions of aminoacyl-tRNA limitation, protein synthesis still proceeds, but various quantitative as well as qualitative changes in polypeptide metabolism can be observed. In this review, after a brief recall of the main characteristics of the stringent response, several aspects concerning protein synthesis in deprived bacteria have been presented. First, the rates of residual protein formation, peptide chain growth and protein degradation, and the molecular weight distribution of proteins newly synthesized have been analyzed. Then, the data relative to the biosynthetic regulation of non-ribosomal and ribosomal proteins have been summarized and compared to the results obtained from in vitro experiments using transcription-translation coupled systems. Finally, the problem of translational fidelity during deprivation has been discussed in connection with the metabolic behavior of polysomal structures which are still maintained in cells. The stringent dependence of cellular activity on aminoacyl-tRNA supply is known to be abolished by single-site mutations which confer to bacteria a phenotype referred to as "relaxed". These mutant strains provide an useful analytical tool in the scope of understanding the stringency phenomenon. Therefore, their proteosynthetic activity under aminoacyl-tRNA deprivation has also been studied here, in comparison to that of normal wild-type strains.

[Regulation of the biosynthesis of branched aminoacyl tRNA synthetases in Bacillus cereus T]

The regulation of the biosynthesis of isoleucyl-, valyl-, and leucyl-tRNA synthetases was studied with an isoleucine-valine requiring mutant of Bacillus cereus T. It was shown that valyl-tRNA synthetase regulation is under multivalent control involving both valine and isoleucine. The isoleucyl- and leucyl-tRNA synthetases are repressed by the respective cognate amino acid. When two amino acids were removed from the culture medium, derepression of the two corresponding aminoacyl tRNA synthetases was expected, but only one appears. With a threonine deaminase constitutive mutant, it was demonstrated that the derepression mecanism of the synthetase was correlated with the intracellular level of the ilv A gene product. These results are in good agreement with the model proposed by several authors. In this model, threonine deaminase, or some form of this enzyme, is involved in a positive control of the regulation of the branched-chain aminoacyltransfer ribonucleic acid synthetases.

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.

Regulation of aromatic amino acid biosynthesis in Escherichia coli K-12: control of the aroF-tyrA operon in the absence of repression control

Evidence was found which indicated that a mutation in gene trpS affected the rate of synthesis of tyrosine-repressible 3-deoxy-D-arabinoheptulosonic acid-7-phosphate (DAHP) synthetase. The effect was found to occur independently of repression mediated by the tyrR gene product, and it was not due to a change in growth rate, nor was it a manifestation of the stringent response. It is proposed that in the proximal region of the aroF-tyrA operon there is an attenuator site controlled by the level of charged tryptophanyl-transfer RNA. In addition, it was demonstrated that starvation for certain amino acids led to degradation of tyrosine-repressible DAHP synthetase, but not phenylalanine-repressible DAHP synthetase, and supplementation with the missing amino acid led to an increased rate of synthesis of tyrosine-repressible DAHP synthetase during subsequent growth.

Comparative studies on the regulation of tryptophan synthesis

In vitro DNA recombination techniques have revolutionized the study of genetic control of biosynthetic pathways. Using examples drawn from the pathway of tryptophan synthesis, approaches to the deciphering of regulatory signals and response mechanisms through transposition of DNA segments and DNA sequence analysis will be presented. After reviewing the known chromosomal arrangements and regulatory patterns of trp genes in the bacterial groups studied so far, and describing the results of transferring all or part of the pathway's genes from one organism to a distantly related one, the use of this technique to analyze new organisms will be described. Along with some advantages over the conventional methods there are some pitfalls. Finally, since it is likely that events analogous to recombinant DNA experiments take place readily in nature, their consequences in studies of bacterial evolution will be conjectured.

The relA locus specifies a positive effector in branched-chain amino acid transport regulation

The regulation of branched-chain amino acid transport and periplasmic binding proteins was studied in Escherichia coli strains which were isogenic except for the relA locus, the gene for the "stringent factor," which is responsible for guanosine tetraphosphate synthesis. The strain containing the relA mutation could not be derepressed for the synthesis of leucine transport or binding proteins when shifted from a medium containing all 20 amino acids in excess to one in which leucine was limiting. The relA+ strain showed normal derepression under these conditions.

Concerted repression of the synthesis of the arginine biosynthetic enzymes by aminoacids: a comparison between the regulatory mechanisms controlling aminoacid biosyntheses in bacteria and in yeast

It has been shown that in bacteria, besides specific regulatory mechanisms, the synthesis of aminoacid biosynthetic enzymes is also controlled by the endogenous aminoacid pool. The latter regulates the intracellular level of ppGpp, a positive effector of RNA messenger transcription. A similar regulatory control exists in yeast but does not appear to involve the same general effector. This was established by the observation that derepression of the enzymes belonging to several aminoacid biosynthetic pathways follows aminoacid starvation or tRNA discharging. We now report the repression of the arginine pathway by the total aminoacid pool. New mutations affecting the repressibility of the arginine enzymes as well as enzymes belonging to other aminoacid biosyntheses, when cells are grown in the presence of an excess of aminoacids, were identified.

The regulatory region of the trp operon of Serratia marcescens

The nucleotide sequence of the regulatory region of the trp operon of Serratia marcescens is presented. It contains a transcription termination control site in the transcribed leader region preceding the first structural gene of the operon as well as a regulated promoter/operator transcription initiation region. The structural organisation of the leader region differs substantially from that of Escherichia coli.

A site of action for tRNA mediated regulation of the ilvOEDA operon of Escherichia coli K12

Transfer RNA (tRNA), rho factor threonine deaminase and the ilvO locus are molecular participants in the regulation of isoleucine-valine (ilv) biosynthesis. Isogenic strains have been constructed with the hisT76 mutation in pairwise combination with ilvO mutations, the rho221 mutation and the ilvDAC115 deletion mutation. The role of the altered tRNA of the hisT76 mutation was found to be independent of the sites of action of the ilvO- mutation, rho factor, and threonine deaminase. The expression of the ilvOEDA operon is stimulated 2-fold when the hisT76 mutation is present in strains containing either ilvO- or rho221 mutations. The expression of the ilvOEDA operon remains nonrepressed in a hisT76 strain deleted for threonine deaminase. These results indicate that the hisT76 undermodified tRNAs are influencing the initiation of transcription of the ilvOEDA operon.

Effect of tryptophan analogs on derepression of the Escherichia coli tryptophan operon by indole-3-propionic acid

The abilities of 14 tryptophan analogs to repress the tryptophan (trp) operon have been studied in Escherichia coli cells derepressed by incubation with 0.25 mM indole-3-propionic acid (IPA). trp operon expression was monitored by measuring the specific activities of anthranilate synthase (EC 4.1.3.27) and the tryptophan synthase (EC 4.2.1.20) beta subunit. Analogs characterized by modification or removal of the alpha-amino group or the alpha-carboxyl group did not repress the trp operon. The only analogs among this group that appeared to interact with the trp aporepressor were IPA, which derepressed the trp operon, and d-tryptophan. Analogs with modifications of the indole ring repressed the trp operon to various degrees. 7-Methyl-tryptophan inhibited anthranilate synthase activity and consequently derepressed the trp operon. Additionally, 7-methyltryptophan prevented IPA-mediated derepression but, unlike tryptophan, did so in a non-coordinate manner, with the later enzymes of the operon being relatively more repressed than the early enzymes. The effect of 7-methyltryptophan on IPA-mediated derepression was likely not due to the interaction of IPA with the allosteric site of anthranilate synthase, even though feedback-resistant mutants of anthranilate synthase were partially resistant to derepression by IPA. The effect of 7-methyltryptophan on derepression by IPA was probably due to the effect of the analog-aporepressor complex on trp operon expression.

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

Upon addition of excess one carbon metabolites (including serine)bacteria stop growing because of isoleucine starvation. After such treatment stringent bacteria rapidly resume normal growth whereas relaxed mutants remain unable for some time to grow. We show here that this is due to a lack of derepressibility of ilv genes after the starvation period. Results are also presented which show that RNA polymerase structural mutants may be selected among the clones resistant to a mixture of serine, methionine and glycine, in relA- strains. Finally circumstancial evidence suggests that the one carbon metabolism may be involved in a process controlling isoleucine metabolism.

A new locus (leuK) affecting the regulation of branched-chain amino acid, histidine, and tryptophan biosynthetic enzymes

A locus (leuK) affecting regulation of the leucine operon was selected by isolating a spontaneous Ara+ derivative of an Escherichia coli B/r strain carrying an ara-leu fusion in which the arabinose operon is under leucine control. Genetic analyses by P1 transduction demonstrated that the lesion is located to the right of the galactose operon. Regulation of the biosynthetic enzymes for leucine, isoleucine-valine, histidine, and tryptophan was altered in a strain carrying leuK16. High-level gene expression in the heterozygous merodiploid strain F' leuK+/leuK16) demonstrated the dominance of the mutant allele to the wild-type allele. No apparent effect was observed in the mutant on N-acetylornithinase, a biosynthetic enzyme in the arginine pathway, nor on any of the 18 aminoacyl-tRNA synthetases examined. However, compared with that of the parent strain, the extent of the charging of leucyl-, isoleucyl-, valyl-, histidyl-, and arginyl-tRNA was decreased in the mutant.

Effect of mutations affecting lysyl-tRNAlys on the regulation of lysine biosynthesis in Escherichia coli

When studying mutants affecting lysyl-tRNA synthetase or tRNAlys (hisT, hisW), a lack of correlation is clearly observed between the amount of lysyl-tRNA and the level of derepression of several lysine biosynthetic enzymes. This exlcudes the possible role of lysyl-tRNA as the specific corepressor of the lysine regulon. However, the level of derepression of DAP-decarboxylase, the last enzyme of the lysine pathway, is very low in the hisT mutant; this indicates that tRNAlys is a secondary effector involved in the regulation of the synthesis of this enzyme.