Studies on the mechanism of repression of arginine biosynthesis in Escherichia coli. 3. Repression of enzymes of arginine biosynthesis in arginyl-tRNA synthetase mutants

Biosynthesis of Arginine and Polyamines

Early investigations on arginine biosynthesis brought to light basic features of metabolic regulation. The most significant advances of the last 10 to 15 years concern the arginine repressor, its structure and mode of action in both E. coli and Salmonella typhimurium, the sequence analysis of all arg structural genes in E. coli and Salmonella typhimurium, the resulting evolutionary inferences, and the dual regulation of the carAB operon. This review provides an overall picture of the pathways, their interconnections, the regulatory circuits involved, and the resulting interferences between arginine and polyamine biosynthesis. Carbamoylphosphate is a precursor common to arginine and the pyrimidines. In both Escherichia coli and Salmonella enterica serovar Typhimurium, it is produced by a single synthetase, carbamoylphosphate synthetase (CPSase), with glutamine as the physiological amino group donor. This situation contrasts with the existence of separate enzymes specific for arginine and pyrimidine biosynthesis in Bacillus subtilis and fungi. Polyamine biosynthesis has been particularly well studied in E. coli, and the cognate genes have been identified in the Salmonella genome as well, including those involved in transport functions. The review summarizes what is known about the enzymes involved in the arginine pathway of E. coli and S. enterica serovar Typhimurium; homologous genes were identified in both organisms, except argF (encoding a supplementary OTCase), which is lacking in Salmonella. Several examples of putative enzyme recruitment (homologous enzymes performing analogous functions) are also presented.

Linkage map of Escherichia coli K-12, edition 10: the traditional map

This map is an update of the edition 9 map by Berlyn et al. (M. K. B. Berlyn, K. B. Low, and K. E. Rudd, p. 1715-1902, in F. C. Neidhardt et al., ed., Escherichia coli and Salmonella: cellular and molecular biology, 2nd ed., vol. 2, 1996). It uses coordinates established by the completed sequence, expressed as 100 minutes for the entire circular map, and adds new genes discovered and established since 1996 and eliminates those shown to correspond to other known genes. The latter are included as synonyms. An alphabetical list of genes showing map location, synonyms, the protein or RNA product of the gene, phenotypes of mutants, and reference citations is provided. In addition to genes known to correspond to gene sequences, other genes, often older, that are described by phenotype and older mapping techniques and that have not been correlated with sequences are included.

The arginine repressor of Escherichia coli

This review tells the story of the arginine repressor of Escherichia coli from the time of its discovery in the 1950s until the present. It describes how the research progressed through physiological, genetic, and biochemical phases and how the nature of the repressor and its interaction with its target sites were unraveled. The studies of the repression of arginine biosynthesis revealed unique features at every level of the investigations. In the early phase of the work they showed that the genes controlled by the arginine repressor were scattered over the linkage map and were not united, as in other cases, in a single operon. This led to the concept of the regulon as a physiological unit of regulation. It was also shown that different alleles of the arginine repressor could result in either inhibition of enzyme formation, as in E. coli K-12, or in stimulation of enzyme formation, as in E. coli B. Later it was shown that the arginine repressor is a hexamer, whereas other repressors of biosynthetic pathways are dimers. As a consequence the arginine repressor binds to two palindromic sites rather than to one. It was found that the arginine repressor not only acts in the repression of enzyme synthesis but also is required for the resolution of plasmid multimers to monomers, a completely unrelated function. Finally, the arginine repressor does not possess characteristic structural features seen in other prokaryotic repressors, such as a helix-turn-helix motif or an antiparallel beta-sheet motif. The unique features have sustained continuous interest in the arginine repressor and have made it a challenging subject of investigation.

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.

Canavanine-resistant variants of human lymphoblasts

Variants resistant to canavanine, an arginine analogue, have been isolated from two long-term human lymphoblastoid cell lines. They are 20-fold more resistant to canavanine than the parental lines and this phenotype is stable in the absence of canavanine for more than 100 generations. The specific activity of argininosuccinate synthetase, the first of two enzymes necessary for the conversion of citrulline to arginine, is elevated in variants from both cell lines. Furthermore, this enzyme activity is refractory to the repression caused by arginine in normal lymphoblasts. The specific activity of argininosuccinate lyase, the second enzyme in the pathway from citrulline to arginine, is not appreciably changed. Arginine uptake appears normal in the variants since they grow as well as the parental lines in media containing a wide range of arginine concentrations. Arginyl-tRNA synthetase activity is also unchanged. Thus the canavanine-resistant variants have altered control of at least one urea cycle enzyme and appear to be regulatory mutants of human cells.

Properties of an Escherichia coli K-12 mutant defective in the transport of arginine and ornithine

A canavanine-resistant mutant strain, defective in the transport of arginine and ornithine, was isolated and characterized. Experiments presented show that both the kinetics of influx and the steady state of accumulation of arginine and ornithine are affected by the mutation, whereas the activity of other related transport systems remains unchanged. On the basis of competitive studies, it is concluded that L-canavanine can inhibit efficiently the arginine-specific uptake system. D-Arginine appears to be a moderate inhibitor. None of the basic amino acid-binding proteins of the mutant strain showed detectable alterations in terms of quantity, physical properties, or affinity constants. Studies on the relationship between the number of transport carriers and the steady state of accumulation of arginine suggested the presence of a reduced number of membrane carriers in the mutant strain. It is proposed that the mutation affects a regulatory gene concerned with controlling the amount of membrane carriers produced, which are components of the arginine- and ornithine-specific uptake systems. The mutation maps at min 62 on the recalibrated linkage map of Escherichia coli K-12, in a locus closely linked or identical to argP.

Arginyl-tRNA synthetase from Escherichia coli. Influence of arginine biosynthetic precursors on the charging of arginine-acceptor tRNA with [14C]arginine

The behaviour of arginyl-tRNA synthetase (EC 6.1.1.19) in the presence of the arginine biosynthetic precursors, argininosuccinate, ornithine and citrulline, was studied in several Escherichia coli K12 strains and in E. coli W. The results of kinetic measurements with partially purified extracts indicate that the arginyl-tRNA synthetase of E. coli is not inhibited by the arginine precursors. The apparent affinity constant Km for arginine of the K12 enzyme is about 3.4 muM in the absence and in the presence of these precursors, whereas the W enzyme an apparently slightly lowered Km and a decreased [14C]arginyl-tRNA equilibrium level in the presence of argininosuccinate. This however was shown to be due to isotopic dilution of [14C]arginine by non-radioactive amino acid formed from argininosuccinate by argininosuccinate lyase (EC 4.3.2.1) contaminating the synthetase preparation. This finding emphasizes the necessity of using pure arginyl-tRNA synthetase in order to study the possible regulatory involvement of this enzyme in the control of the arginine regulon in vitro.

Dual regulation by arginine of the expression of the Escherichia coli argECBH operon

The correlation between the level of messenger ribonucleic acid (mRNA) specific for the argECBH gene cluster (argECBH mRNA) measured by ribonucleic acid-deoxyribonucleic acid (RNA-DNA) hybridization and the rates of synthesis of N-acetylornithine deacetylase (argE enzyme) and of argininosuccinate lyase (argH enzyme) of Escherichia coli strain K-12 were determined for steady-state growth with and without added L-arginine and during the transition periods between these two states. During the transient period after arginine removal (transient derepression), the synthesis of enzymes argE and argH was initially three to five times greater than the steady-state derepressed rate finally reached 50 min later. The level of argECHB mRNA correlated well both quantitatively and temporally with the rates of enzyme synthesis during this transition. The level of in vivo charged arginyl-transfer RNA (tRNAarg), monitored simultaneously, was initially only 5 to 10% and gradually increased to a final level of 80% after 45 min. During the transient period after arginine addition (transient repression), the rates of synthesis of enzymes argE and argH decreased to almost zero and gradually reached steady-state repressed rates after about 180 min. The argECBH mRNA level remained constant at the steady-state repressed level throughout transient repression, revealing a discontinuity between the level of this mRNA and rates of enzyme synthesis. A similar discrepancy was noted during the transition after ornithine addition. In vivo charged tRNAarg remained constant at 80% during this transition. After removal of arginine, the zero-level transient enzyme synthesis developed after only 7.5 min of arginine deprivation and was maximum after 30 min. The results suggest an accumulation of a molecule regulated by arginine that plays a role in transient repression. Our data indicate that arginyl-tRNA synthetase is not this molecule since its synthesis was unaffected by arginine. The ratios of steady-state argE and argH enzyme synthesis without arginine to that with arginine were 12 and 20, respectively, whereas the similar ratio for argECBH mRNA was 2 to 3. The repressed level of argECBH mRNA was not affected by attempts to repress or derepress the ppc+ gene (carried on the DNA used for hybridization), and the repressed level of argECBH mRNA was lowered about 50% in cells carrying an internal argBH deletion. These data taken together indicate the presence of an excess of untranslated argECBH mRNA during both transient and steady-state repression by arginine. Thus, a second regulatory mechanism, not yet defined, appears to play an important role in arginine regulation of enzyme synthesis.

In vitro synthesis and repression of argininosuccinase in Escherichia coli K12; partial purification of the arginine repressor

Phi80dargECBH DNA has been used to direct cell-free synthesis of argininosuccinase, the argH gene product in Escherichia coli K12. In vitro enzyme synthesis is sensitive to repression by partially purified preparations from an argR+ strain but not by corresponding preparations from an argR- strain. Using DNA-cellulose chromatography, approximately seventyfold purification of repressor has been obtained. The partially purified preparation represses argininosuccinase synthesis but has no effect on beta-galactosidase synthesis.

Accumulation of arginine precursors in Escherichia coli: effects on growth, enzyme repression, and application to the forward selection of arginine auxotrophs

The accumulation or ornithine, citrulline, and possibly acetylornithine by Escherichia coli K-12 arginineless mutants provided with acetylarginine as source of arginine causes severe growth inhibition. This occurs under conditions where comparable derivatives of E. coli W (Bollon and Vogel, 1973) show little or no growth inhibition. The same conditions, which have been reported to cause noncorrelative synthesis of acetylornithinase and argininosuccinase in E. coli W (Bollon and Vogel, 1973), do not alter the correlative pattern of enzyme synthesis observed in E. coli K-12. Moreover, previously reported effects of ornithine and citrulline on repression of the arginine regulon in E. coli W are not observed in the K-12 strains examined. The bearing of these observations on possible differences between the mechanism of enzyme repression operating in the two types of strains cannot yet be fully evaluated; it is, however, clear that considerable care should be exercised before extrapolating the results obtained with one type of strain to the other one. The particularly strong inhibition of acetylarginine utilization exerted by ornithine in E. coli K-12 allows the forward selection of several classes of arginine auxotrophs from strains deficient in carbamoylphosphate biosynthesis and thus capable of ornithine accumulation. Possible applications of this technique to the genetic analysis of the bipolar argECBH operon are discussed.

Mutant of Escherichia coli K-12 defective in the transport of basic amino acids

Escherichia coli K-12 possesses two active transport systems for arginine, two for ornithine, and two for lysine. In each case there is a low- and a high-affinity transport system. They have been characterized kinetically and by response to competitive inhibition by arginine, lysine, ornithine and other structurally related amino acids. Competitors inhibit the high-affinity systems of the three amino acids, whereas the low-affinity systems are not inhibited. On the basis of kinetic evidence and competition studies, it is concluded that there is a common high-affinity transport system for arginine, ornithine, and lysine, and three low-affinity specific ones. Repression studies have shown that arginine and ornithine repress each other's specific transport systems in addition to the repression of their own specific systems, whereas lysine represses its own specific transport system. The common transport system was found to be repressible only by lysine. A mutant was studied in which the uptake of arginine, ornithine, and lysine is reduced. The mutation was found to affect both the common and the specific transport systems.

Characterization of a cold-sensitive hisW mutant of Salmonella typhimurium

A cold-sensitive mutant of Salmonella typhimurium LT2 that grows at 37 C but not at 20 C has altered repression regulation in at least two amino acid biosynthetic pathways (histidine and isoleucine). The lesion conferring cold sensitivity that is linked with hisW is recessive. Assays for the acceptance of some amino acids by transfer ribonucleic acid (tRNA) reveal a decreased ability of the mutant tRNA to accept arginine, phenylalanine, and histidine. A mutation in a gene for tRNA maturation is a likely possibility for the mutation producing these effects on growth, regulation, and amino acid acceptance.

Control of arginine biosynthesis in Escherichia coli: role of arginyl-transfer ribonucleic acid synthetase in repression

The physiological role of arginyl-transfer ribonucleic acid (Arg-tRNA) synthetase (E.C. 6.1.1.13, arginine: RNA ligase adenosine monophosphate) in repression of arginine biosynthetic enzymes was examined. Mutants with nonrepressible synthesis of arginine biosynthetic enzymes were isolated from various strains of Escherichia coli by resistance to growth inhibition by canavanine, an arginine analogue. These mutants possessed reduced Arg-tRNA synthetase activities which were qualitatively different from the synthetase activity of the wild type. The mutant enzymes exhibited turnover in vivo and were less stable in vitro than the wild type at both 4 C and 40 C; they possessed different affinities for both arginine and canavanine as measured by the three common assay systems for aminoacyl-tRNA synthetases. Furthermore, in one case it was shown that (i) the mutant possesed unaltered uptake of arginine, and (ii) that the mutant possessed diminished ability to incorporate canavanine into proteins and to attach canavanine to tRNA. These observations suggested that the mutation to canavanine resistance involved a structural change in Arg-tRNA synthetase. Likewise, the results of genetic experiments suggested that the mutants differed from the wild-type strain at only one locus, and that this lies in the region of the chromosomes that includes a structural gene for Arg-tRNA synthetase. It appears that Arg-tRNA synthetase may be involved in some way in repression by arginine of its own biosynthetic enzymes.

Repression of enzymes of arginine biosynthesis by L-canavanine in arginyl-transfer ribonucleic acid synthetase mutants of Escherichia coli

We show that the arginine analogue, l-canavanine, repressed the accumulation of translatable messenger ribonucleic acid (RNA) for three arginine biosynthetic enzymes in Escherichia coli. The method used to determine the level of translatable messenger RNA depended upon measurement of a burst of enzyme synthesis as described previously. E. coli strains with defective arginyltransfer ribonucleic acid (tRNA) synthetase (argS mutants) were insensitive to canavanine repression. When deprived of leucine, a leu argS strain regained normal sensitivity to canavanine repression. The level of in vivo canavanyl-tRNA(arg) was determined for a normal strain and an argS mutant. After 20 min of growth with canavanine only 9% of tRNA(arg) from the argS strain was protected from periodate oxidation, while 42% of the tRNA(arg) from an argS(+) strain was charged. When deprived of leucine, leu argS or leu argS(+) strains grown with canavanine contained more than 60% charged tRNA(arg). Reverse phase column chromatography of periodate-oxidized tRNA from canavanine-grown argS and argS(+) strains showed no preferential charging of any isoaccepting species of tRNA(arg). Therefore, we failed to detect a specific arginyl-tRNA species that might be involved in repression by canavanine. However, the data suggest that canavanine repression of the arginine pathway occurs only when high levels of canavanyl-tRNA are present, and thus support the notion that arginyl-tRNA synthetase plays a role in generating a repression signal.

Unusual valyl-transfer ribonucleic acid synthetase mutant of Escherichia coli

Escherichia coli strain NP2907 was isolated as a spontaneous mutant of strain NP29, which possesses a thermolabile valyl-transfer ribonucleic acid (tRNA) synthetase. The valyl-tRNA synthetase of the new mutant, unlike that of its immediate parent, retains enzymatic activity in vitro but differs from the wild-type enzyme in stability and apparent K(m) for adenosine triphosphate. The new mutant locus, valS-102, cotransduces with pyrB at the same frequency as does the parental locus, valS-1. Cultures of strain NP29 cease growth immediately in any medium when shifted from 30 to 40 C. The new mutant grows normally at 30 C, and upon a shift to 40 C growth quickly accelerates exactly as for normal cells. Exponential growth, however, cannot be sustained at 40 C. At a point characteristic for each medium, growth becomes linear with time. This transition occurs almost immediately in rich media and after 1.5 generations in glucose minimal medium. Net synthesis of valyl-tRNA synthetase ceases in the new mutant as soon as the temperature is raised to 40 C, irrespective of the growth medium. We conclude that it is the amount of valyl-tRNA synthetase activity that limits the rate of growth in the linear phase at 40 C. This property of the mutant makes it possible to evaluate the in vivo efficiency of this enzyme at different growth rates and thereby to determine the concentration that is necessary for a given rate of protein synthesis. The results of our measurements indicate that cells of E. coli growing in minimal medium normally possess a functional excess of valyl-tRNA synthetase with respect to protein synthesis and to repression of threonine deaminase.

Substrate specificity of a mutant alanyl-transfer ribonucleic acid synthetase of Escherichia coli

The correlation between the in vivo functioning and the in vitro behavior of the thermolabile alanyl-transfer ribonucleic acid (tRNA) synthetase (ARS) of Escherichia coli strain BM113 is presented. As a measure for the ARS activity inside the cell, the amount of acylated tRNA(ala) in vivo was determined. The rapid drop of the per cent tRNA(ala) charged which was observed upon shifting a culture of BM113 to the nonpermissive temperature indicates that in vivo acylation of tRNA(ala) might be the growth-limiting step at high temperature. Since neither growth nor the in vivo charging level of tRNA(ala) was affected by the addition of high l-alanine concentrations to the medium, one may infer that impaired functioning of the mutant enzyme at 40 C seems not to be due to reduced affinity of the enzyme for the amino acid. Separation of bulk tRNA of E. coli and of yeast on benzoylated diethylaminoethyl cellulose and charging of the fractions of the column by wild-type and mutant ARS reveal that only those tRNA species aminoacylated by the wild-type enzyme are also charged by the mutant ARS. Determination of the K(m) values of wild-type and mutant ARS for the three isoaccepting tRNA(ala) species of E. coli shows a ca. 10-fold increase of the apparent K(m) values of the mutant enzyme for all three species. Thus, the mutation proportionally reduces the apparent affinity for tRNA(ala) without causing any detectable recognition errors. Investigation of heat inactivation kinetics of wild-type and mutant ARS without and in the presence of substrates provides further evidence that only the transfer site of the ARS is altered by the mutation. Moreover, whereas both enzymes possess the same pH optimum of the relative maximal velocity, their pH dependence of the K(m) values for tRNA is different. The K(m) of the wild-type enzyme decreases at pH values below 7.0 and that of the mutant enzyme shows the inverse tendency; this again indicates an alteration of the tRNA binding site.

Production of L-arginine by arginine hydroxamate-resistant mutants of Bacillus subtilis

l-Arginine hydroxamate inhibited the growth of various bacteria, and the inhibition was readily reversed by arginine. l-Arginine hydroxamate (10(-3)m) completely inhibited the growth of Bacillus subtilis. This inhibitory effect was prevented by 2.5 x 10(-4)ml-arginine, which was the most effective of all the natural amino acids in reversing the inhibition. l-Arginine hydroxamate-resistant mutants of Bacillus subtilis were isolated and found to excrete l-arginine in relatively high yields. One of the mutants, strain AHr-5, produced 4.5 mg of l-arginine per ml in shaken culture in 3 days.

Isolation and partial characterization of temperature-sensitive Escherichia coli mutants with altered leucyl- and seryl-transfer ribonucleic acid synthetases

Two temperature-sensitive mutants of Escherichia coli have been found in which the conditional growth is a result of a thermosensitive leucyl-transfer ribonucleic acid (tRNA) synthetase and seryl-tRNA synthetase, respectively. The corresponding genetic loci, leuS and serS, cotransduce with lip and serC, respectively. As a result of the mutationally altered leucyl-tRNA synthetase, some leucine-, valine-, and isoleucine-forming enzymes were derepressed. Thus, leucyl-tRNA synthetase is involved in the repression of the enzymes needed for the synthesis of branched-chain amino acids.

Inhibition of arginyl-transfer ribonucleic acid synthetase activity of Escherichia coli by arginine biosynthetic precursors

The arginine biosynthetic precursors, ornithine, citrulline, and argininosuccinate, inhibit arginyl-transfer ribonucleic acid (tRNA) synthetase (EC 6.1.1.13, arginine: soluble RNA ligase, adenosine monophosphate) activity in the in vitro attachment assay system. Ornithine is the most potent, argininosuccinate is next, and citrulline is least effective. The implications of these results are discussed in relation to arginyl-tRNA synthetase activity and the level of the arginine biosynthetic enzymes during conditions of restricted and unrestricted supply of arginine to cells.

Methionine-mediated repression in Saccharomyces cerevisiae: a pleiotropic regulatory system involving methionyl transfer ribonucleic acid and the product of gene eth2

Detailed study of methionine-mediated repression of enzymes involved in methionine biosynthesis in Saccharomyces cerevisiae led to classification of these enzymes into two distinct regulatory groups. Group I comprises four enzymes specifically involved in different parts of methionine biosynthesis, namely, homoserine-O-transacetylase, homocysteine synthetase, adenosine triphosphate sulfurylase, and sulfite reductase. Repressibility of these enzymes is greatly decreased in strains carrying a genetically impaired methionyl-transfer ribonucleic acid (tRNA) synthetase (mutation ts(-) 296). Conditions leading to absence of repression in the mutant strain have been correlated with a sharp decrease in bulk tRNA(met) charging, whereas conditions which restore repressibility of group I enzymes also restore tRNA(met) charging. These findings implicate methionyl-tRNA in the regulatory process. However, the absence of a correlation in the wild type between methionyl-tRNA charging and the levels of methionine group I enzymes suggests that only a minor iso accepting species of tRNA(met) may be devoted with a regulatory function. Repressibility of the same four enzymes (group I) was also decreased in strains carrying the regulatory mutation eth2(r). Although structural genes coding for two of these enzymes, as well as mutations ts(-) 296 and eth2(r) segregate independently to each other, synthesis of group I enzymes is coordinated. The pleiotropic regulatory system involved seems then to comprise beside a "regulatory methionyl tRNA(met)," another element, product of gene eth2, which might correspond either to an aporepressor protein or to the "regulatory tRNA(met)" itself. Regulation of group II enzymes is defined by response to exogenous methionine, absence of response to either mutations ts(-) 296 and eth2(r), and absence of coordinacy with group I enzymes. However, the two enzymes which belong to this group and are both involved in threonine and methionine biosynthesis undergo distinct regulatory patterns. One, aspartokinase, is subject to a bivalent repression exerted by threonine and methionine, and the other, homoserine dehydrogenase, is subject only to methionine-mediated repression. Participation of at least another aporepressor and another corepressor, different from the ones involved in regulation of group I enzymes, is discussed.

Mutants of Salmonella typhimurium with an altered leucyl-transfer ribonucleic acid synthetase

Two trifluoroleucine-resistant mutants of Salmonella typhimurium, strains CV69 and CV117, had an altered leucyl-transfer ribonucleic acid (tRNA) synthetase. The mutant enzymes had higher apparent K(m) values for leucine (ca. 10-fold) and lower specific activities (ca. twofold) than the parent enzyme when tested in crude extracts. Preparations of synthetase purified ca. 60-fold from the parent and strain CV117 differed sixfold in their leucine K(m) values. In addition, the mutant enzyme was inactivated faster than the parent enzyme at 50 C. The growth rates of strains CV69 and CV117 at 37 C were not significantly different from that of the parent, whereas at 42 C strain CV69 grew more slowly than the parent. Leucine-, valine-, and isoleucine-forming enzymes were partially derepressed when the mutants were grown in minimal medium; the addition of leucine repressed these enzymes to wild-type levels. During growth in minimal medium, the proportion of leucine tRNA that was charged in the mutants was about 75% of that in the parent. The properties of strain CV117 were shown to result from a single mutation located near gal at minute 18 on the genetic map. These studies suggest that leucyl-tRNA synthetase is involved in repression of the enzymes required for the synthesis of branched-chain amino acids.

Isoleucine auxotrophy as a consequence of a mutationally altered isoleucyl-transfer ribonucleic acid synthetase

Among mutants which require isoleucine, but not valine, for growth, we have found two distinguishable classes. One is defective in the biosynthetic enzyme threonine deaminase (l-threonine hydro-lyase, deaminating, EC 4.2.1.16) and the other has an altered isoleucyl transfer ribonucleic acid (tRNA) synthetase [l-isoleucine: soluble RNA ligase (adenosine monophosphate), EC 6.1.1.5]. The mutation which affects ileS, the structural gene for isoleucyl-tRNA synthetase, is located between thr and pyrA at 0 min on the map of the Escherichia coli chromosome. This mutationally altered isoleucyl-tRNA synthetase has an apparent K(m) for isoleucine ( approximately 1 mm) 300-fold higher than that of the enzyme from wild type; on the other hand, the apparent V(max) is altered only slightly. When the mutationally altered ileS allele was introduced into a strain which overproduces isoleucine, the resulting strain could grow without addition of isoleucine. We conclude that the normal intracellular isoleucine level is not high enough to allow efficient charging to tRNA(Ile) by the mutant enzyme because of the K(m) defect. A consequence of the alteration in isoleucyl-tRNA synthetase was a fourfold derepression of the enzymes responsible for isoleucine biosynthesis. Thus, a functional isoleucyl-tRNA synthetase is needed for isoleucine to act as a regulator of its own biosynthesis.

Depression kinetics of ornithine transcarbamylase in Escherichia coli

A mathematical model for the derepression of ornithine transcarbamylase (OTC) in Escherichia coli strain W was derived from a set of 14 assumptions concerning the arginine regulon. The model assumes that active repressor for the arginine regulon is unstable and is only formed when the level of arginyl-tRNA is in excess of the level necessary to maintain protein synthesis for a given cell doubling time. The presence of active repressor was assumed to inhibit the synthesis of messenger RNA coding for the synthesis of the enzymes of the arginine biosynthetic pathway. Numerical estimates of the model's parameters were made and, by simulation on a digital computer, the model was shown to fit kinetic data for derepression of OTC in E. coli W cells in minimal medium growing in flask culture with a doubling time of 60 min and growing in a chemostat with a generation time of 460 min for an assumed OTC-specific mRNA half-life (t(1/2)) of 9 min. The model was also shown to predict the increase in the size of bursts of OTC synthesis elicited by addition of arginine to cultures of derepressing E. coli cells with the increase in the delay time before arginine addition. Approximate analytical solutions to the model were obtained for the early phase of derepression and for repression of OTC. These were used to derive graphical methods for determining t(1/2) from repression and derepression transient changes in the OTC level.

Biosynthesis of the branched-chain amino acids in yeast: a trifluoroleucine-resistant mutant with altered regulation of leucine uptake

A trifluoroleucine-resistant mutant of yeast has been isolated that exhibits reduced incorporation of the analogue into protein (15%) of that in the wild type. In the mutant, uptake of the analogue and leucine into the expandable (water-extractable) pool is enhanced, passage from the expandable to the conversion (nonwater-, ethanol-extractable) pool is unaffected, and endogenous synthesis of leucine is normally regulated. Although the leucyl transfer ribonucluic acid (tRNA) synthetase appears normal, and the tRNA(leu) has wild-type acceptor activities in vitro and in vivo, the level of the mutant trifluoroleucyl tRNA pool is only 2 to 3% of that in the wild type. The data support the idea of a mutation affecting passage between the conversion pool and the site of charging of the analogue. The mutation is dominant and exhibits pleiotropic effects: the first leucine biosynthetic enzyme appears nonrepressible, and the leucine, valine, and tyrosine uptake systems are constitutively elevated (three- to fourfold) in the absence of exogenous amino acids.

Mutants of Escherichia coli K-12 with an altered glutamyl-transfer ribonucleic acid synthetase

Three streptomycin-suppressible lethal mutants of Escherichia coli K-12 have been shown to possess structurally altered glutamyl-transfer ribonucleic acid (tRNA) synthetases. Each mutant synthetase displays a K(m) value for glutamate which is 10-fold higher than the parental value, and the mutations reside in two widely separate loci on the genetic map. Mixing of the mutant extracts in pairs gave no indication of in vitro complementation. All three enzymes charge the minor tRNA(glu) fraction identically, but one (EM 120) charges the major fraction at a twofold lower rate than do the other two (EM 102 and EM 111). Possible explanations for the existence of the two synthetase loci are presented.

Isolation and partial characterization of Escherichia coli mutants with altered glycyl transfer ribonucleic acid synthetases

Isolates with mutations in glyS, the structural gene for glycyl-transfer ribonucleic acid (tRNA) synthetase (GRS) in Escherichia coli, are frequently found among glycine auxotrophs. Extracts of glyS mutants have altered GRS activities. The mutants grow with normal growth rates in minimal media when high levels of glycine are provided. No other metabolite of a variety tested is capable of restoring normal growth. The glyS mutants fail to make ribonucleic acid (RNA) when depleted of exogenous glycine in strains which are RC(str) but do so when the cells are RC(rel). In contrast, biosynthetic mutants which are unable to synthesize glycine (glyA mutants) do not make RNA when deprived of glycine even if they are RC(rel); in this case, RNA is synthesized upon glycine deprivation only when the nucleic acid precursors made from glycine are provided in the medium. The level of serine transhydroxymethylase is unaltered in extracts of any of the glyS mutants, even though the level of charged tRNA(Gly) is at least 20-fold lower than that found in a prototrophic parent; this indicates that, if there is control over the synthesis of serine transhydroxymethylase, it is not modified by reduced levels of charging of the major species of tRNA(Gly).

Isolation and characterization of a mutant of Escherichia coli blocked in the synthesis of putrescine

A mutant of Escherichia coli is described which is defective in the conversion of arginine to putrescine. The activity of the enzyme agmatine ureohydrolase is greatly reduced, whereas the activity of the other two enzymes of the pathway, the constitutive arginine decarboxylase and the inducible arginine decarboxylase, are within the normal range. The growth behavior of the mutant reflects the enzymatic block. It grows well in the absence of arginine, but only poorly in the presence of arginine. Under the former conditions, putrescine can be formed from ornithine as well as arginine, whereas under the latter conditions, because of feedback control, it can be formed only from arginine.

Mutants of Escherichia coli with an altered tyrosyl-transfer ribonucleic acid synthetase

We have isolated several mutants defective in the gene for tyrosyl-transfer ribonucleic acid (tRNA) synthetase (tyrS). One of these mutants is described in detail. It was isolated as a tyrosine auxotroph with defects both in the tyrosyl-tRNA synthetase and in the tyrosine biosynthetic enzyme, prephenate dehydrogenase. It also had derepressed levels of the tyrosine-specific 3-deoxy-d-arabinoheptulosonic acid-7-phosphate (DAHP) synthetase. The latter finding suggested that a wild-type tyrS gene was required for repression of the tyrosine biosynthetic enzymes. The following results demonstrated that this hypothesis was not correct. (i) When the defective tyrS gene was transferred to another strain, the tyrosine-specific DAHP synthetase in that strain was not derepressed, and (ii) two other mutants with defective tyrosyl-tRNA synthetases had repressed levels of the tyrosine biosynthetic enzymes. The tyrS gene was located near minute 32 on the Escherichia coli chromosome by interrupted mating experiments.