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

An important role for the multienzyme aminoacyl-tRNA synthetase complex in mammalian translation and cell growth

In mammalian cells, aminoacyl-tRNA synthetases (aaRSs) are organized into a high-molecular-weight multisynthetase complex whose cellular function has remained a mystery. In this study, we have taken advantage of the fact that mammalian cells contain two forms of ArgRS, both products of the same gene, to investigate the complex's physiological role. The data indicate that the high-molecular-weight form of ArgRS, which is present exclusively as an integral component of the multisynthetase complex, is essential for normal protein synthesis and growth of CHO cells even when low-molecular-weight, free ArgRS is present and Arg-tRNA continues to be synthesized at close to wild-type levels. Based on these observations, we conclude that Arg-tRNA generated by the synthetase complex is a more efficient precursor for protein synthesis than Arg-tRNA generated by free ArgRS, exactly as would be predicted by the channeling model for mammalian translation.

1-Methylguanosine deficiency of tRNA influences cognate codon interaction and metabolism in Salmonella typhimurium

1-Methylguanosine (m1G) is present next to the 3' end of the anticodon (position 37) in tRNA(1,2,3,Leu), tRNA(1,2,3,Pro), and tRNA(3Arg). A mutant of Salmonella typhimurium lacks m1G in these seven tRNAs when grown at or above 37 degrees C, as a result of a mutation (trmD3) in the structural gene (trmD) for the tRNA(m1G37)methyltransferase. The m1G deficiency induced 24 and 26% reductions in the growth rate and polypeptide chain elongation rate, respectively, in morpholinepropanesulfonic acid (MOPS)-glucose minimal medium at 37 degrees C. The expression of the leuABCD operon is controlled by the rate with which tRNA(2Leu) and tRNA(3Leu) read four leucine codons in the leu-leader mRNA. Lack of m1G in these tRNAs did not influence the expression of this operon, suggesting that m1G did not influence the efficiency of tRNA(2,3Leu). Since the average step time of the m1G-deficient tRNAs was increased 3.3-fold, the results suggest that the impact of m1G in decoding cognate codons may be tRNA dependent. The trmD3 mutation rendered the cell more resistant or sensitive to several amino acid analogs. 3-Nitro-L-tyrosine (NT), to which the trmD3 mutant is sensitive, was shown to be transported by the tryptophan-specific permease, and mutations in this gene (mtr) render the cell resistant to NT. Since the trmD3 mutation did not affect the activity of the permease, some internal metabolic step(s), but not the uptake of the analog per se, is affected. We suggest that the trmD3-mediated NT sensitivity is by an abnormal translation of some mRNA(s) whose product(s) is involved in the metabolic reactions affected by the analog. Our results also suggest that tRNA modification may be a regulatory device for gene expression.

Isolation of the gene (miaE) encoding the hydroxylase involved in the synthesis of 2-methylthio-cis-ribozeatin in tRNA of Salmonella typhimurium and characterization of mutants

The modified nucleoside 2-methylthio-N-6-isopentenyl adenosine (ms2i6A) is present at position 37 (3' of the anticodon) of tRNAs that read codons beginning with U except tRNA(I,V Ser) in Escherichia coli. Salmonella typhimurium 2-methylthio-cis-ribozeatin (ms2io6A) is found in tRNA, probably in the corresponding species that have ms2i6A in E. coli. The gene (miaE) for the tRNA(ms2io6A)hydroxylase of S. typhimurium was isolated by complementation in E. coli. The miaE gene was localized close to the argI gene at min 99 of the S. typhimurium chromosomal map. Its DNA sequence and transcription pattern together with complementation studies revealed that the miaE gene is the second gene of a dicistronic operon. Southern blot analysis showed that the miaE gene is absent in E. coli, a finding consistent with the absence of the hydroxylated derivative of ms2i6A in this species. Mutants of S. typhimurium which have MudJ inserted in the miaE gene and which, consequently, are blocked in the ms2i6A hydroxylation reaction were isolated. Unexpectedly, such mutants cannot utilize the citric acid cycle intermediates malate, fumarate, and succinate as carbon sources.

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.

Transcription attenuation in Salmonella typhimurium: the significance of rare leucine codons in the leu leader

The leucine operon of Salmonella typhimurium is controlled by a transcription attenuation mechanism. Four adjacent leucine codons within a 160-nucleotide leu leader RNA are thought to play a central role in this mechanism. Three of the four codons are CUA, a rarely used leucine codon within enteric bacteria. To determine whether the nature of the leucine codon affects the regulation of the leucine operon, we used oligonucleotide-directed mutagenesis to first convert one CUA of the leader to CUG and then convert all three CUA codons to CUG. CUG is the most frequently used leucine codon in enteric bacteria. A mutant having (CUA)2CUGCUC in place of (CUA)3CUC has an altered response to leucine limitation, requiring a slightly higher degree of limitation to effect derepression. Changing (CUA)3CUC to (CUG)3CUC has more dramatic effects upon operon expression. First, the basal level of expression is lowered to the point that the mutant grows more slowly than the parent in a minimal medium lacking leucine. Second, the response of the mutant to a leucine limitation is dramatically altered such that even a strong limitation elicits only a modest degree of derepression. If the mutant is grown under conditions of leucyl-tRNA limitation rather than leucine limitation, complete derepression can be achieved, but only at a much higher degree of limitation than for the wild-type operon. These results provide a clear-cut example of codon usage having a dramatic effect upon gene expression.

Escherichia coli B/r leuK mutant lacking pseudouridine synthase I activity

Escherichia coli B/r strain EB146 containing mutation leuK16 has elevated levels of enzymes involved in the synthesis of leucine, valine, isoleucine, histidine, and tryptophan (Brown et al., J. Bacteriol. 135:542-550, 1978). We show here that strain EB146 (leuK16) has properties that are similar to those of E. coli and Salmonella typhimurium hisT strains. In tRNA1Leu from both hisT and leuK strains, positions 39 and 41 are uridine residues rather than pseudouridine residues. Furthermore, in tRNA3Leu and tRNA4Leu from a leuK strain, uridine residues at positions 39 and 40, respectively, are unmodified. Pseudouridine synthase I activity is missing in extracts of strain EB146 (leuK16), and extracts of strain EB146 (leuK16) and of a hisT strain do not complement one another in vitro. Four phenotypes of strain EB146 (leuK16), leucine excretion, wrinkled colony morphology, and elevated levels of leu and his enzymes, are complemented by a plasmid having a 1.65-kilobase DNA fragment containing the E. coli K-12 hisT locus. These results indicate that either leuK codes for pseudouridine synthase I (and is thus a hisT locus in reality) or, less likely, it codes for a product that affects the synthesis or activity of pseudouridine synthase I.

Starvation for ilvB operon leader amino acids other than leucine or valine does not increase acetohydroxy acid synthase activity in Escherichia coli

Eleven different amino acids are encoded in the ilvB leader mRNA. Starvation for leucine or valine, but not for any of the other nine amino acids, resulted in high levels of acetohydroxy acid synthase I. These results are discussed in terms of a report (C.A. Hauser and G.W. Hatfield, Proc. Natl. Acad. Sci. U.S.A. 81:76-79, 1984) which suggests that threonine and alanine, in addition to leucine and valine, are involved in the regulation of the ilvB operon.

Cloning and expression of the ilvB gene of Escherichia coli K-12

A plasmid containing the ilvB operon, which codes for acetohydroxy acid synthase I of Escherichia coli K-12, was isolated using a ligated mixture of DNA from plasmid pBR322 and FilvB4 treated with endonuclease SalI. A shortened derivative of this plasmid was isolated by cloning a 3.4 kb bacterial fragment into plasmid pKEN005 to yield plasmid pTCN12. The orientation of the ilvB operon relative to plasmid genes was determined by restriction enzyme mapping. Measurement of the level of the product of the ilvB gene, acetohydroxy acid synthase I, indicated that plasmid pTCN12 contained a functional ilvB promoter and control region. The DNA frm this plasmid was used as a probe to show that the rate of synthesis of ilvB mRNA was proportional to the levels of acetohydroxy acid synthase I.

Altered regulation of isoleucine-valine biosynthesis in a hisW mutant of Salmonella typhimurium

Control of isoleucine-valine biosynthesis was examined in the cold-sensitive hisW3333 mutant strain of Salmonella typhimurium. During growth at the permissive temperature (37 degrees C), the isoleucine-valine (ilv) biosynthetic enzyme levels of the hisW mutant were two- to fourfold below these levels in an isogenic hisW+ strain. Upon a reduction in growth temperature to partially permissive (30 degrees C), the synthesis of these enzymes in the hisW mutant was further reduced. However, synthesis of the ilv enzymes was responsive to the repression signal(s) caused by the addition of excess amounts of isoleucine, valine, and leucine to the hisW mutants. Such a "super-repressed" phenotype as that observed in this hisW mutant is similar to that previously shown for the hisU1820 mutant, but was different from the regulatory response of the hisT1504 mutant strain. Moreover, by the use of growth-rate-limiting amounts of the branched-chain amino acids, it was shown that this hisW mutant generally did not increase the synthesis of the ilv enzymes as did the hisW+ strain. Overall, these results are in agreement with the hypothesis that the hisW mutant is less responsive to ilv specific attenuation control than is the hisW+ strain and suggest that this limited regulatory response is due to an alteration in the amount or structure of an element essential to attenuation control of the ilv operons.

Mutations in genes cpxA and cpxB of Escherichia coli K-12 cause a defect in acetohydroxyacid synthase I function in vivo

Mutations in Escherichia coli genes cpxA and cpxB together cause a temperature-sensitive defect in isoleucine and valine syntheses that is related specifically to acetohydroxyacid synthase I. This enzyme catalyzes the first pair of homologous reactions required for the synthesis of these two amino acids. At both permissive and nonpermissive temperatures, mutant cells containing ilvB (the structural gene for acetohydroxyacid synthase I) cloned in a derivative of plasmid pBR322 synthesized comparable amounts of ilvB mRNA and contained several times the enzyme activity normally required to sustain exponential growth, yet these cells remained temperature sensitive for growth in the absence of isoleucine and valine. These observations suggest that the primary effect of the cpx mutations is to block enzyme function in vivo. The enzyme was unstable in mutant cells at growth temperatures above 37 degrees C, but this instability appeared to be a secondary effect on the cpx mutations.

Valine-resistant Escherichia coli K-12 strains with mutations in the ilvB operon

Escherichia coli K-12 mutants resistant to growth inhibition by valine were isolated. These strains contained mutations in the ilvB operon effecting either the regulation of acetohydroxy acid synthase I or the sensitivity of the enzyme to end product inhibition by valine.

Regulation of cyclic AMP of the ilvB-encoded biosynthetic acetohydroxy acid synthase in Escherichia coli K-12

The biosynthetic acetohydroxy acid synthase activities of E. coli K 12 are encoded by three genetic loci namely, ilvB (acetohydroxy acid synthase I), ilvG (acetohydroxy acid synthase II) and ilvHI (acetohydroxy acid synthase III). The previously reported involvement of cyclic AMP in the regulation of the biosynthetic acetohydroxy acid synthase isozymes in E. coli K-12 was found to be due to the effect of this nucleotide on the expression of ilvB. Cyclic AMP had no effect on acetohydroxy acid synthase activity in strains lacking wild-type ilvB activity but containing the remaining isozymes. Very little activity of acetohydroxy acid synthase coded for by ilvV was found when ppGpp and cyclic AMP were severely limited. Addition of cyclic AMP under these conditions increased ilvB expression 24-fold. The data suggest that in addition to multivalent repression and ppGpp, cyclic AMP plays a major role in the regulation of the ilvB biosynthetic operon.

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.

Synthesis and activities of branched-chain aminoacyl-tRNA synthetases in threonine deaminase mutants of Escherichia coli

Valyl-, isoleucyl-, and leucyl-tRNA synthetase activities were examined in an Escherichia coli K-12 strain that possessed a deletion of three genes of the ilv gene cluster, ilvD, A, and C, and in a strain with the same deletion that also carried the lambdadilvCB bacteriophage. It was observed that the branched-chain tRNA synthetase activities of both strains were considerably less than those of the normal strain during growth in unrestricted medium. Furthermore, during an isoleucine limitation, there was a further reduction in isoleucyl-tRNA synthetase activity and an absence of the isoleucine-mediated derepression of valyl-tRNA synthetase formation in both of these mutants, as compared with the normal strain. In addition, it was observed that these branched-chain synthetase activities were reduced in steady-state cultures of several ilvA point mutants. However, upon the introduction of the ilv operon to these ilvA mutants by use of lambda bacteriophage, there was a specific increase in the branched-chain synthetase activities to levels comparable to those of the normal strain. These results support our previous findings that the stability and repression control of synthesis of these synthetases require some product(s) missing in the ilvDAC deletion strain and strongly suggest this component is some form of the ilvA gene product, threonine deaminase.

Genetically determined differences in concentrations of isoaccepting tRNAs in Escherichia coli

Two examples of genetically determined altered concentrations of isoaccepting tRNAs are presented. The concentrations of isoaccepting tRNAsThr are selectively changed by a mutation causing a fourfold overproduction of the cognate aminoacyl-tRNA-synthetase, the threonyl-tRNA synthetase, whereas the distribution of isoaccepting tRNAs of four control tRNA-species in these E. coli mutants was not affected by that mutation. Secondly evidence is presented for a correlation between mutations in structural genes of aminoacid biosynthetic enzymes and alterations in concentrations of cognate isoaccepting tRNAs in two different E. coli strains, auxotrophic for threonine, isoleucine/valine and leucine, and arginine respectively.

Relationship between messenger ribonucleic acid and enzyme levels specified by the leucine operon of Escherichia coli K-12

The levels of leucine-forming enzymes in Escherichia coli K-12 varied over a several thousand-fold range, depending upon conditions of growth. The highest levels were achieved by growing auxotrophs in a chemostat under conditions of leucine limitation. Under such conditions, enzyme levels were increased 45- to 90-fold relative to cells grown in minimal medium containing leucine (the latter values arbitrarily called 1). Leucine operon-specific messenger ribonucleic acid levels were elevated to about the same extent as enzyme levels in cells grown in a chemostat. Growth in media of greater complexity resulted in progressively lower levels of leucine-forming enzymes, reaching a value of less than 0.02 for growth in a medium containing tryptone broth and yeast extract. The levels of leucine operon-specified enzymes and messenger ribonucleic acid were also measured in strains containing about 25 copies of plasmid pCV1(ColE1-leu) per chromosome. For such strains grown in minimal medium, enzyme levels were proportional to the number of plasmids per cell. Furthermore, they followed the same trends as those described above upon derepression in a chemostat or upon repression following growth in rich media. Leucine messenger ribonucleic acid, measured both by pulse-labeling and hybridization-competition experiments, was roughly proportional to enzyme levels over this entire range. For a plasmid-containing strain grown in a chemostat under conditions of leucine limitation (about 100 plasmids per chromosome), about 27% of pulse-labeled ribonucleic acid was coded for by genes in or adjacent to the leucine operon, and 10% of the total protein was beta-isopropylmalate dehydrogenase.

Biochemical characterization of an Escherichia coli hisT strain

An Escherichia coli hisT strain was characterized biochemically and shown to contain altered transfer ribonucleic acid and to be altered in the regulation of amino acid biosynthesis.

Partial derepression of the isoleucine-valine enzymes during methionine starvation is Salmonella typhimurium

Methionine starvation of methionine auxotrophs in the presence of excess branched-chain amino acids results in a partial derepression of the isoleucine and valine enzymes. Reversed-phase chromatography indicated that isoleucine, valine and leucine tRNA were altered during methionine starvation. In addition, the total tRNA isolated from cells under these conditions were undermethylated. The observed derepression may be caused by the inability of methyl-deficient tRNA's to participate adequately in normal regulatory functions.

Detection of messenger RNA from the isoleucine--valine operons of Salmonella typhimurium by heterologous DNA-RNA hybridization: involvement of transfer RNA in transcriptional repression

A hybridization assay using Escherichia coli K-12 DNA isolated from the specialized transducing bacteriophage gammaCI857St68h80 dilv was used to examine the rate of synthesis of the messenger RNA's (mRNA) derived from the isoleucine-valine (ilv) gene cluster of Salmonella typhimurium. In all cases examined, changes in ilv enzyme levels could be correlated with changes in the rate of synthesis of ilv mRNA. Several well characterized regulatory mutants of S. typhimurium had rates of synthesis of ilv mRNA 3 to 8-fold higher than the repressed wild-type strain. The increased rates of ilv mRNA synthesis found in a hisT strain as well as in isoleucyl-and leucyl-tRNA SYNTHETASE MUTANTS, STRONGLY SUGGESTS A ROLE FOR BRANCHED-CHAIN AMINOACYL-TRNA's in transcriptional control.

Repression of the tyrosine, lysine, and methionine biosynthetic pathways in a hisT mutant of Salmonella typhimurium

A comparison was made of the repressibility of certain enzymes in the tyrosine, methionine, and lysine biosynthetic pathways in wild-type Salmonella typhimurium and a hisT mutant. The results show that (i) tyrosine represses the synthesis of the tyrosine-sensitive 3-deoxy-D-arabino-heptulsonic acid 7-phosphate synthetase and the tyrosine aminotransferase to the same extent in a hisT mutant as in wild type and (ii) there is no detectable alteration in the extent to which methionine represses O-succinylhomoserine synthetase or in the extent to which lysine represses the lysine-sensitive beta-aspartokinase as a result of the hisT mutation.

Threonine deaminase from Escherichia coli: feedback-hypersensitive enzyme from a genetic regulatory mutant

A mutation, ilvA538, in the gene coding for the biosynthetic L-threonine deaminase of Escherichia coli K-12 has previously been demonstrated to have pleiotropic regulatory effects leading to low and invariant expression of some of the isoleucine-valine biosynthetic enzyme, and altered expression of the branched-chain aminoacyl-tRNA synthetases. Strain PS187, which carries the ilvA538 allele, has a partial growth requirement for L-isoleucine and is characterized by a sensitivity to growth inhibition by L-leucine. The experiments reported here demonstrate that the L-threonine deaminase produced by strain PS187 is hypersensitive to inhibition by the pathway end product L-isoleucine. In addition, L-leucine, which acts at relatively high concentrations in vitro as an inhibitor of L-threonine deaminase from the wild type, is a more potent inhibitor of the activity of the mutant enzyme. Forty-six derivatives of strain PS187 were isolated as spontaneous mutants resistant to the growth-inhibitory effects of L-leucine. Two of these, strains MSR14 and MSR16, produce an L-threonine deaminase that is more resistant than the wild type to L-isoleucine inhibition, and intermediate between the wild type and strain PS187 with respect to L-leucine inhibition. Strains MSR14 and MSR16 produce L-threonine deaminase and dihydroxyacid dehydrase, the ilvD gene product, at the low levels characteristic of the parent strain. Other L-leucine-resistant derivatives of strain PS187 produce higher levels of the feedback-hypersensitive L-threonine deaminase. Thus, the sensitivity to growth inhibition by L-leucine observed with strain PS187 appears to be related both to the hypersensitivity of L-threonine deaminase to inhibition of catalytic activity and to the low level of ilv gene expression. The results reported here indicated that L-threonine deaminase is structurally altered in strain PS187, and thus provide further support for the proposal that L-threonine deaminase participates as a genetic regulatory element for the expression of the branched-chain amino acid biosynthetic enzymes.

The regulation of arginine biosynthesis in Saccharomyces cerevisiae. The specificity of argR- mutations and the general control of amino-acid biosynthesis

The regulation of arginine biosynthetic enzymes in yeast is subjected to a double control. One level of arginine enzyme synthesis is under the control of an apo-repressor, called ARGR. ARGR molecules control specifically the arginine pathway. A second level of control of arginine biosynthesis has been disclosed. It also controls tryptophan, histidine, lysine, isoleucine-valine and probably many more biosyntheses. The general mechanism is turned on in leaky mutants in any of the amino acid pathways mentioned above.

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.

tRNAs undermethylation in a met-regulatory mutant of Saccharomyces cerevisiae

A study of in vivo and in vitro methylation of tRNAs in regulatory mutants affected in methionine-mediated repression (eth2, eth3, eth10) has led to the following results: 1) The eth2-2 carrying strain presents a great undermethylation of its tRNAs of the same order of magnitude as observed during methionine starvation of methionine auxotrophs. 2) This undermethylation leads to a shift of the tRNAIII met peak on a BD cellulose column, while tRNAIII met peak is unchanged. 3) The study of a double mutant strain carrying eth2 and met2 mutations has shown that this undermethylation is a consequence of the high internal pool of methionine. 4) Undermethylation unequally affects the different bases and the different tRNA species.