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

Expression of a valine-resistant acetolactate synthase activity mediated by the ilv O and ilv G genes of Escherichia coli K-12

A strain carrying the ilv0603 mutation has been isolated in E. coli K-12 and its characteristics were found to be very similar to those previously reported by Ramakrishnan and Adelberg (1965a) for other ilv0 mutants. The strain carrying the ilv0603 mutation is resistant to valine inhibition (Valr) and we show that this resistance depends on the expression of a newly recognized gene, ilvG, which is located at min 75, between ilvE and ilvD on the E. coli K-12 map. The ilvG gene causes the expression of a Valr acetolactate synthase, which is detectable only when the ilv0603 mutation is also present in cis on the same chromosome. Under these conditions the Valr acetolactate synthase activity is eluted, on a hydroxylapatite column, at an ionic strength slightly lower than that required for elution of the remaining acetolactate synthase activity (sensitive to valine inhibition). The Valr peak is missing in a strain carrying an ilvG (amber) mutation.

Inhibition of leucyl-transfer ribonucleic acid synthetasymol

The bacteriostatic effect of low concentrations of the antibiotic granaticin on Bacillus subtilis is relieved by the addition leucine to the growth medium. In cells treated with granaticin, aminoacylation of leucine tRNA is specifically decreased, but the content of free leucine is not. It is concluded that granaticin interferes with the charging process of leucine tRNA in B. subtilis leading to leucine auxotrophy.

Transcriptional control of the isoleucine-valine messenger RNA's in E. coli K-12

Hybridization of messenger ribonucleic acid (mRNA) isolated from Escherichia Coli K-12 to deoxyribonucleic acid (DNA) from lambdaCI857st68h80dilv was used to detect isoleucine-valine (ilv) specific mRNA. A number of strains partially constitutive for the isoleucine-valine enzymes had levels of ilv mRNA 2 to 3-fold higher than the parent strain. Starvation for any of the branched-chain amino acids resulted in a 20 to 23-fold increase in ilv mRNA as compared to repressed levels. These differences were not due to altered growth rates or to changes in the stability of ilv mRNA. These data indicate that regulation of the isoleucine-valine enzymes by multivalent repression occurs mainly at the level of transcription. Kinetics of elongation of ilv mRNA after repression are consistent with the assumption that the mechanism of multivalent repression involves the prevention of further initiations by RNA polymerase.

Structural genes for a newly recognized acetolactate synthase in Escherichia coli K-12

Evidence is reported that shows the presence in Escherichia coli K-12 of a newly found acetolactate synthase. This enzyme is the product of two genes, ilvH and ilvI, both located very close to leu. Amber mutations have been found in both genes and therefore their products are polypeptides. Mutations in the ilvH gene cause the appearance of an acetolactate synthase activity which is relatively resistant to valine inhibition and can be separated by adsorption on hydroxylapatite from another activity present in the extract and more sensitive to valine inhibition than the former. A mutant altered in the ilvI gene was isolated among the revertants sensitive to valine inhibition of an ilvH mutant. Such a mutant lacks the resistant acetolactate synthase. A temperature-sensitive revertant of the ilvI mutant contained a temperature-sensitive acetolactate synthase. Thus ilvI is the structural gene for a specific acetolactate synthase. The activity of the ilvH gene product has been measured by adding an extract containing it to a purified ilvI acetolactate synthase, which, upon incubation, became more sensitive to valine inhibition. Conversely, a valine-sensitive acetolactate synthase (the product of the ilvH and the ilvI genes) became more resistant to valine inhibition upon incubation with an extract of a strain containing a missense ilvH gene product.

Regulation of the pool size of valine in Escherichia coli K-12

Three mutations (ilvH611, ilvH612, and ilvH613) are described which make Escherichia coli K-12 resistant to valine inhibition and are located near leu. The expression of the ilv genes appears to be normal in these mutants since the isoleucine-valine biosynthetic enzymes are not derepressed relative to the wild type. The intracellular concentration of valine is, however, higher in the mutants than in the isogenic ilvH(+) strain. These mutants also excrete valine, probably because of the high intracellular concentration of this amino acid. The pool size of valine is regulated independently from that of isoleucine and leucine. The increased intracellular concentration of valine is due to a decreased feedback inhibition that valine exerts on its own biosynthetic pathway. In fact, acetolactate synthase activity assayed in extracts of ilvH612 and ilvH613 mutants is more resistant to valine inhibition than the activity assayed in the ilvH(+) isogenic strain. Two forms of acetolactate synthase activity can be separated from these extracts by adsorption and elution on hydroxylapatite. One of them is as sensitive to valine inhibition as that of the wild type, the other is more resistant to valine inhibition.

Deoxyribonucleic acid-directed in vitro synthesis of ilv-specific messenger ribonucleic acid by extracts of Escherichia coli K-12

The synthesis of ilv-specific messenger ribonucleic acid (mRNA) by extracts of Escherichia coli K-12 has been demonstrated in a deoxyribonucleic acid (DNA)-dependent, coupled transcription-translation system. ilv-Specific mRNA was determined by hybridization either to double-stranded lambdacI857St68h80dilv DNA (lambdah80dilv DNA) immobilized on nitrocellulose filters or to its separate l and r strands in liquid. During conditions optimal for protein synthesis, slightly more than 6% of the total [(3)H]RNA synthesized by S-30 extracts of the threonine deaminase-negative strain CU5136 was ilv-specific. Of this RNA, nearly 30% was complementary to the l (correct) strand. Total ilv-specific mRNA synthesis in vitro was not affected by omission of valine or all 20 amino acids from the reaction mixture. Hybridization of ilv-specific mRNA made in vitro to the l strand of lambdah80dilv DNA was effectively reduced in the presence of unlabeled RNA extracted from an ilv derepressed strain but not from an ilv deletion strain. In a purified transcription system, employing commercial RNA polymerase, twofold more ilv-specific mRNA was synthesized than in the coupled system, but this increase was entirely due to greater transcription of the r (incorrect) strand. An S-30 extract prepared from a strain isogenic to strain CU5136 but derepressed for ilvA gene expression synthesized twofold more ilv-specific mRNA in the coupled system. The significance of these findings is discussed.

Defects of two temperature-sensitive lysyl-transfer ribonucleic acid synthetase mutants of Bacillus subtilis

Two temperature-sensitive mutants (lysS1 and lysS2) of the lysyl-transfer ribonucleic acid synthetase (l-lysine:tRNA ligase [adenosine 5'-monophosphate], EC 6.1.1.6) of Bacillus subtilis have been isolated. Although protein synthesis is inhibited in both mutants at the restrictive temperature (42 to 45 C), the mutants remain viable in a minimal medium. In comparison with the wild-type lysyl-tRNA synthetase, the l-lysine-dependent exchange of [(32)P]pyrophosphate with adenosine 5'-triphosphate (ATP) for both mutant enzymes is decreased. The lysS1 enzyme is completely defective in the ATP-dependent attachment of l-lysine to tRNA, whereas the lysS2 enzyme has 3- to 10-fold reduced levels of this activity. Temperature-resistant transformants have wild-type enzyme levels, whereas partial revertants to temperature resistance have varied levels of enzyme activity. The attachment and exchange activities of the lysS2 enzyme are more heat labile in vitro than the wild-type enzyme, as is the attachment activity of a partial revertant of the lysS1 mutant. The lysS1 and the lysS2 lysyl-tRNA synthetases have higher apparent K(m) values for lysine and ATP, in both the activation and the attachment reactions. The lysS2 enzyme has a V(max) for tRNA(lys) one-third that of the wild-type enzyme. Molecular weights of approximately 150,000 for the wild-type and lysS2 enzymes and approximately 76,000 for the lysS1 enzyme were estimated from sedimentation positions in sucrose density gradients assayed by the ATP-pyrophosphate exchange activity. We propose that the two mutations (lysS1 and lysS2) directly affect the sites for exchange activity, but indirectly alter attachment activity as a consequence of defective subunit association.

Complementation between different mutations in the ilvA gene of Escherichia coli K-12

An ilvA mutation carried by a ø80i(lambda)dilv transducing phage complemented some ilvA mutations and did not complement others. Complementation was accompanied by appearance of threonine deaminase activity in vivo. These results divided the ilvA mutations into two sets which formerly appeared to define two cistrons.

Pleiotropy of hisT mutants blocked in pseudouridine synthesis in tRNA: leucine and isoleucine-valine operons

The hisT gene codes for an enzyme responsible for the conversion of uridine to pseudouridine (Psi) in the anticodon region of many tRNA species in Salmonella typhimurium. We have previously shown that a hisT mutant has tRNA(His) which lacks pseudouridine in this region and as a consequence has an altered chromatographic behavior. We show here a similar alteration in chromatographic behavior of all tRNA(Leu) and one tRNA(Ile) species from a hisT mutant. By contrast, tRNA(Val), which contains no pseudouridine except for the one in the TPsiCG sequence, is chromatographically unaltered in a hisT mutant. The absence of pseudouridine in the anticodon region of tRNA in hisT mutants has been previously shown to cause derepression of the histidine operon. We show here that in hisT mutants the regulation of the leucine and the isoleucine and valine operons is also affected: the enzymes of these operons are refractory to repression by the branched chain amino acids. However, there is no difference between hisT and wild type in the pattern of derepression caused by isoleucine or valine limitation and only a slight difference in the enzyme levels in cells grown on minimal medium. The alteration in the regulation of branched chain amino acid operons may also explain why hisT mutants are resistant to inhibition of growth by the amino acid analogues 5,5,5-trifluoroleucine, beta-hydroxyleucine, and norleucine and by the oligopeptides glycylglycylnorleucine and norleucylnorleucine.

Derepressed levels of the isoleucine-valine and leucine enzymes in his T 1504, a strain of Salmonella typhimurium with altered leucine transfer ribonucleic acid

A hisT mutant of Salmonella typhimurium was found to have altered regulation of the isoleucine-valine and leucine enzymes. These enzymes in the hisT strain were derepressed two- to eightfold over those of the parent wild-type strain when grown in minimal medium or under repressing conditions. The amount of tRNA(Leu) and the cellular concentration of charged tRNA(Leu) was about the same in the hisT strain and in the wild type. However, leucyl-tRNA from the mutant was chromatographically different from that of wild type, confirming previous reports that hisT strains have altered tRNA(Leu). These results suggest strongly that tRNA(Leu) is involved in repression of the isoleucine-valine and leucine enzymes in S. typhimurium.

Mutations affecting the different transport systems for isoleucine, leucine, and valine in Escherichia coli K-12

Uptake of isoleucine, leucine, and valine in Escherichia coli K-12 is due to several transport processes for which kinetic evidence has been reported elsewhere. A very-high-affinity transport process, a high-affinity transport process, and three different low-affinity transport processes were described. In this paper the existence of these transport processes is confirmed by the isolation and preliminary characterization of mutants altered in one or more of them. The very-high-affinity transport process is missing either in strains carrying the brnR6(am) mutation or in strains carrying the brn-8 mutation. This appears to be a pleiotropic effect since other transport systems are also missing. Mutant analysis shows that more than one transport system with high affinity is present. One of them, high-affinity 1, which needs the activity of a protein produced by the brnQ gene, transports isoleucine, leucine, and valine and is unaffected by threonine. The other, high-affinity 2, which needs the activity of a protein produced by the brnS gene, transports isoleucine, leucine, and valine; this uptake is inhibited by threonine which probably is a substrate. Another protein, produced by the brnR gene, is required for uptake through both high-affinity 1 and high-affinity 2 transport systems. The two systems therefore appear to work in parallel, brnR being a branching point. The brnQ gene is located close to phoA at 9.5 min on the chromosome of E. coli, the brnR gene is located close to lac at 9.0 min, and the brnS gene is close to pdxA at 1 min. A mutant lacking the low-affinity transport system for isoleucine was isolated from a strain in which the high-affinity system was missing because of a brnR mutation. This strain also required isoleucine for growth because of an ilvA mutation. The mutant lacking the low-affinity transport system was unable to grow on isoleucine but could grow on glycylisoleucine. This mutant had lost the low-affinity transport for isoleucine, whereas those for leucine and valine were unaffected. A pleiotropic consequence of this mutation (brn-8) was a complete absence of the very-high-affinity transport system due either to the alteration of a common gene product or to any kind of secondary interference which inhibits it. Mutants altered in isoleucine-leucine-valine transport were isolated by taking advantage of the inhibition that valine exerts on the K-12 strain of E. coli. Mutants resistant both to valine inhibition (Val(r)) and to glycylvaline inhibition are regulatory mutants. Val(r) mutants that are sensitive to glycylvaline inhibition are transport mutants. When the very-high-affinity transport process is repressed (for example by methionine) the frequency of transport mutants among Val(r) mutants is higher, and it is even higher if the high-affinity transport process is partially inhibited by leucine.

Role for free isoleucine of glycyl-leucine in the repression of threonine deaminase in Escherichia coli

In Escherichia coli, the three branched-chain amino acid activating enzymes appear to be essential for multivalent repression of the isoleucine- and valine-forming enzymes. The results of experiments with a mutant, strain CU18, having an altered threonine deaminase, indicate that free isoleucine and some form of threonine deaminase (the product of the ilvA gene) are also involved in multivalent repression. This strain exhibits abnormally high derepressibility but normal repressibility of its ilv gene products, and its threonine deaminase is inhibited only by high concentrations of isoleucine. In strain CU18, the isoleucine analogue, thiaisoleucine, is incapable of replacing isoleucine in the multivalent repression of the ilv genes, whereas the analogue can fully replace the natural amino acid in repression in other strains examined. The dipeptide, glycyl-leucine, which, like isoleucine, is a heterotropic negative effector of threonine deaminase but is not a substrate for isoleucyl-transfer ribonucleic acid synthetase, can completely prevent the accumulation of threonine deaminase-forming potential during isoleucine starvation in strains with normal threonine deaminases. It can not, however, prevent such accumulation in strain CU18 or in other strains in which threonine deaminase is insensitive to any concentration of isoleucine.

Participation of branched-chain amino acid analogues in multivalent repression

Two isoleucine analogues and two leucine analogues were examined for their ability to replace the natural amino acid preventing the accumulation of threonine deaminase-forming potential. The procedure used to study repression by the analogues distinguishes between true repression and the formation of inactive enzyme by the analogue in question. The leucine analogue 4-azaleucine was found to replace leucine in multivalent repression of threonine deaminase-forming potential in Escherichia coli but not in Salmonella typhimurium. Another leucine analogue, trifluoroleucine, was only partially effective in causing repression in either organism. The isoleucine analogue 4-azaisoleucine was ineffective in replacing isoleucine in repression. In contrast, 4-thiaisoleucine effectively replaced isoleucine in the repression of threonine deaminase-forming potential in S. typhimurium and E. coli.

Autoregulation: a role for a biosynthetic enzyme in the control of gene expression

It was previously proposed, primarily on the basis of evidence in vitro, that L-threonine deaminase, the ilvA gene product, is required for repression of its own synthesis and for repression of the other genes in the ilv-ADE operon. In this communication, evidence in vivo is presented that supports this autoregulatory model. Further evidence is presented that suggests that L-threonine deaminase is also required for induction of the ilvC gene product. The autoregulatory model is presented in an expanded form to include recent evidence that L-threonine deaminase (EC 4.2.1.16) is a central element for repression of the ilvADE and ilvB operons, and for induction of the ilvC operon.

Histidyl-transfer ribonucleic acid synthetase mutants requiring a high internal pool of histidine for growth

Mutants that require histidine due to an altered structural gene for the histidyl-transfer ribonucleic acid synthetase (hisS) have been isolated by a general selection for histidine-requiring strains in which the mutation producing histidine auxotrophy is unlinked to the histidine operon. One of the mutants has been shown to require an abnormally high internal histidine pool for growth owing to an altered synthetase that is unstable at low histidine concentrations. It is difficult to determine accurately the K(m) for histidine of the synthetase enzyme from the mutant because of the instability of the enzyme at limiting histidine concentrations; however, a histidine K(m) value has been estimated that is approximately 100 times higher than the histidine K(m) of the wild-type enzyme. For the mutant strains to achieve the high internal pool of histidine required for growth, all the systems that transport histidine from the growth medium must be functioning to capacity. Amino acids that interfere with histidine transport strongly inhibit the growth of the mutants. The mutants have been useful in providing a selective genetic marker for transductional mapping in the hisS region. The mutants are discussed as representative of a general class of curable mutants that have an altered enzyme with poor affinity for a substrate or coenzyme.

Close linkage of the genes serC (for phosphohydroxy pyruvate transaminase) and serS (for seryl-transfer ribonucleic acid synthetase) in Escherichia coli K-12

Escherichia coli strain K28, isolated after nitrosoguanidine mutagenesis, was found to be auxotrophic for serine. It was also temperature sensitive for growth as a result of producing an altered seryl-transfer ribonucleic acid (tRNA) synthetase (EC 6.1.1.11, l-serine: tRNA ligase [AMP]). The auxotrophy was caused by a mutation in the structural gene for phosphohydroxy-pyruvate transaminase (serC), which was distinct from, but closely linked to, the structural gene for seryl-tRNA synthetase (serS). We conclude that the relevant genes are in the order gal-serS-serC-aroA.

Regulation of the tyrosine biosynthetic enzymes in Salmonella typhimurium: analysis of the involvement of tyrosyl-transfer ribonucleic acid and tyrosyl-transfer ribonucleic acid synthetase

Mutants of Salmonella typhimurium were isolated that require tyrosine for growth because of an altered tyrosyl-transfer ribonucleic acid (tRNA) synthetase. Extracts of one strain (JK10) contain a labile enzyme with decreased ability to transfer tyrosine to tRNA(Tyr) and a higher K(m) for tyrosine than the wild-type enzyme. Strain JK10 maintains repressed levels of the tyrosine biosynthetic enzymes when the growth rate is restricted due to limitation of charged tRNA(Tyr). Several second-site revertants of strain JK10 exhibit temperature-sensitive growth due to partially repaired, heat-labile tyrosyl-tRNA synthetase. The tyrosine biosynthetic enzymes are not derepressed in thermosensitive strains grown at the restrictive temperature. A class of tyrosine regulatory mutants, designated tyrR, contains normal levels of tyrosyl-tRNA synthetase and tRNA(Tyr). These results suggest that charging of tRNA(Tyr) is not necessary for repression. This conclusion is substantiated by the finding that 4-aminophenylalanine, a tyrosine analogue which causes repression of the tyrosine biosynthetic enzymes, is not attached to tRNA(Tyr) in vivo, nor does it inhibit the attachment reaction in vitro. A combined regulatory effect due to the simultaneous presence of tyrS and tyrR mutations in the same strain was detected. The possibility of direct participation of tyrosyl-tRNA synthetase in tyrosine regulation is discussed.

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.

Isolation of transducing bacteriophages for the histidine and isoleucine-valine operons in Escherichia coli K-12

In vitro studies have been of great value in elucidating the mechanism of the regulation of several bacterial operons. To obtain a deoxyribonucleic acid preparation enriched for the histidine (his) and for the isoleucine-valine (ilv) operons, we have isolated bacteriophages carrying the his and the ilv regions of the Escherichia coli chromosome. Transposition of the his operon to a site close to the att80 region of the E. coli chromosome has been carried out selecting for integration of a temperature-sensitive F'his(+) in the tonB locus. This transposed strain has been lysogenized with phi80i(lambda). Upon induction of the lysogen, His(+) transductants have been isolated, which, on further induction give rise to HFT (high frequency of transduction) lysates. Preliminary characterization of the transducing phage is reported. The ilv operon, carried on an F' particle, has been fused to an episome carrying the att80 region. The fused episome has been lysogenized with phi80i lambdat68. Upon induction of the lysogen, Ilv(+) transductants have been isolated which on further induction give rise to HFT lysates.

New amino acid regulatory locus having unusual properties in heterozygous merodiploids

Spontaneous mutants of Escherichia coli B/r resistant to 5',5',5',-trifluoro-dl-leucine contain defects in a gene which maps to the left of the threonine region. Low-level constitutive expression of the isoleucine-valine and leucine operons is caused by this mutation in haploid strains. This is in contrast to extremely high levels of gene expression in the heterozygous merodiploids (F' wild type/mutant allele). The properties of these mutants define a new locus and suggest that it encodes a subunit protein which is involved in the repression of the structural genes for the branched-chain amino acid pathways.

Escherichia coli K-12 mutants altered in the transport of branched-chain amino acids

Two mutants of Escherichia coli K-12 are described which are resistant to the inhibition that valine exerts on the growth of E. coli. These mutants have lesions at two different loci on the chromosome. One of them, brnP, is linked to leu (87% cotransduction) and is located between leu and azi represented on the map at 1 min; the other, brnQ, is linked to phoA (96% cotransduction), probably between proC and phoA and represented at 10 min. These mutants are resistant to valine inhibition but are sensitive to dipeptides containing valine. Since it is known that dipeptides are taken up by E. coli through a transport system(s) different from those used by amino acids, this sensitivity to the peptides suggests an alteration in the active transport of valine. The mutants are resistant to valine only if leucine is present in the growth medium; the uptake of valine is less in both mutants than it is in wild-type E. coli, and it is reduced even further if leucine is present. Under these conditions the total uptake of valine is almost completely abolished in the brnQ mutant. The brnP mutant takes up about 60% as much valine as does the wild type, but no exogenous valine is incorporated into proteins. The apparent K(m) and V(max) of isoleucine, leucine, and valine for the transport system are reported; the brnP mutant, when compared to the wild type, has a sevenfold higher K(m) for isoleucine and a 17-fold lower K(m) for leucine; the V(max) for the three amino acids is reduced in the brnQ mutant, up to 20-fold for valine. The transport of arginine, aspartic acid, glycine, histidine, and threonine is not altered in the brnQ mutant under conditions in which that of the branched amino acids is. Evidence is reported that O-methyl-threonine enters E. coli through the transport system for branched amino acids, and that thiaisoleucine does not.

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.

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.

Biochemical characterization of a mutant isoleucyl-transfer ribonucleic acid synthetase from Escherichia coli K-12

Isoleucyl-transfer ribonucleic acid (tRNA) synthetase [l-isoleucine: soluble RNA ligase (adenosine monophosphate), EC 6.1.1.5; IRS] was partially purified from Escherichia coli K-12 and from an ileS mutant that appears to be altered in IRS. The half-life of wild-type IRS, incubated at 60.5 C, is 69 min, whereas that of mutant IRS is 8 min. Mutant IRS shows about a 100-fold lower affinity than wild-type IRS for isoleucine, dl-valine, thiaisoleucine, and O-methyl-dl-threonine, both in the pyrophosphate exchange assay and in the assay of isoleucyl-tRNA formation. The affinity of the mutant enzyme for adenosine triphosphate in the assay of isoleucyl-tRNA formation is 15-fold lower than that of the wild-type enzyme. The affinity of mutant IRS for tRNA is not changed as compared with wild-type IRS. These data show that mutant IRS has an altered structure and clearly confirm that ileS is the structural gene for IRS.