Pleiotropic effects induced by modification deficiency next to the anticodon of tRNA from Salmonella typhimurium LT2.

A strain of Salmonella typhimurium LT2 was isolated, which harbors a mutation acting as an antisuppressor toward an amber suppressor derivative, supF30, of tRNATyr1. The mutant is deficient in cis-2-methylthioribosylzeatin[N6-(4-hydroxyisopentenyl)-2-me thylthioadenosine, ms2io6A], which is a modification normally present next to the anticodon (position 37) in tRNA reading codons starting with uridine. The gene miaA, defective in the mutant, is located close to and counterclockwise of the purA gene at 96 min on the chromosomal map of S. typhimurium with the gene order mutL miaA purA. Growth rate of the mutant was reduced 20 to 50%, and the effect was more pronounced in media supporting fast growth. Translational chain elongation rate at 37 degrees C was reduced from 16 amino acids per s in the wild-type cell to 11 amino acids per s in the miaA1 mutant in the four different growth media tested. The cellular yield in limiting glucose, glycerol, or succinate medium was reduced for the miaAI mutant compared with wild-type cells, with 49, 41, and 57% reductions, respectively. The miaAI mutation renders the cell more sensitive or resistant toward several amino acid analogs, suggesting that the deficiency in ms2io6A influences the regulation of several amino acid biosynthetic operons. We suggest that tRNAPhe, lacking ms2io6A, translates a UUU codon in the early histidine leader sequence with lowered efficiency, leading to repression of the his operon.     

Sulfur-Deficient Transfer Ribonucleic Acid in a Cysteine-Requiring, “Relaxed” Mutant of Escherichia coli

A cysteine-requiring mutant of the parent strain Escherichia coli Hfr Cavalli (RC(rel), Met(-), lambda) has been isolated. The mutant was selected by using replica plating after mutagenesis by N-methyl-N'-nitro-N-nitrosoguanidine. The mutation appears to be in the gene for sulfite reductase, since the mutant could utilize sulfide but not sulfite as a sulfur source. The mutant was found to be RC(rel) with respect to both methionine and cysteine. During cysteine starvation, transfer ribonucleic acid (tRNA) deficient in 4-thiouracil was produced, and in vivo studies indicate that this tRNA can accept sulfur groups to a greater extent than normal tRNA. Further, there were differences both in the rate and extent of amino acid acceptance between normal and sulfur-deficient tRNA. This suggests that thionucleotides are involved in at least one of the biological functions of the tRNA molecule.     

Mutant aminoacyl-tRNA synthetase that compensates for a mutation in the major identity determinant of its tRNA

A single G3.U70 base pair in the acceptor helix is the major determinant for the identity of alanine transfer RNAs (Hou & Schimmel, 1988). Introduction of this base pair into foreign tRNA sequences confers alanine acceptance on them. Moreover, small RNA helices with as few as seven base pairs can be aminoacylated with alanine, provided that they encode the critical base pair (Francklyn & Schimmel, 1989). Alteration of G3.U70 to G3.C70 abolishes aminoacylation with alanine in vivo and in vitro. We describe here the mutagenesis and selection of a single point mutation in Escherichia coli Ala-tRNA synthetase that compensates for a G3.C70 mutation in tRNAAla. The mutation maps to a region previously implicated as proximal to the acceptor end of the bound tRNA. In contrast to the wild-type enzyme, the mutant charges small RNA helices that encode a G3.C70 base pair. However, the mutant enzyme retains specificity for alanine tRNA and can serve as the sole source of Ala-tRNA synthetase in vivo. The results demonstrate the capacity of an aminoacyl-tRNA synthetase to compensate through a single amino acid substitution for mutations in the major determinant of its cognate tRNA.     

Relaxed control of RNA synthesis during nutritional shiftdowns of hisU mutant of Salmonella typhimurium.

Among the classes of histidine regulatory mutants isolated in Salmonella typhimurium, three of these mutants (hisT, hisW and hisU) exhibit pleiotrophic effects on the regulation of expression of other amino acid biosynthetic operons (1, 2). While the regulatory patterns of the hisT mutants are explained by the defective tRNA pseudouridylate synthetase (3), the exact function of the hisW and hisU loci are not as clearly defined, although both mutants exhibit reductions in the relative amino acid acceptance activity of several tRNA's (4). In studies of tRNA synthesis and processing in one such hisU mutant (hisU1820), we unexpectedly observed continued RNA synthesis during nutritional (carbon and energy source) transitions. It was also shown that this relaxed control of stable RNA formation is independent of the relA gene product.     

A mutation in Escherichia coli tRNA nucleotidyltransferase that affects only AMP incorporation is in a sequence often associated with nucleotide-binding proteins

Escherichia coli strain 5C15 contains a mutation in the cca gene that decreases AMP incorporation by tRNA nucleotidyltransferase while leaving CMP incorporation unaffected. Earlier studies of the purified mutant enzyme suggested that the mutation was localized to the AMP-incorporating site. In order to analyze this mutation in more detail, the cca gene from strain 5C15 was cloned into plasmid pUC8. Analysis of tRNA nucleotidyltransferase activity in extracts of a strain transformed with this plasmid demonstrated an elevated level of CMP incorporation, but low AMP incorporation, as expected from the properties of the original mutant. Sequence analysis of the mutant cca gene revealed only a single G to A point mutation leading to a glycine to aspartic acid substitution at position 70 of the peptide chain. The amino acid change was localized to one of two Gly-X-Gly-X-X-Gly sequences present in the protein. This sequence has been identified previously near the nucleotide-binding domain of various proteins, but it has not been noted in enzymes that incorporate nucleotide residues. However, other sequences often associated with ATP-binding domains are not found in tRNA nucleotidyltransferase. The implications of these findings for our understanding of nucleotide-binding domains are discussed.     

The Escherichia coli argU10(Ts) phenotype is caused by a reduction in the cellular level of the argU tRNA for the rare codons AGA and AGG

The Escherichia coli argU10(Ts) mutation in the argU gene, encoding the minor tRNA(Arg) species for the rare codons AGA and AGG, causes pleiotropic defects, including growth inhibition at high temperatures, as well as the Pin phenotype at 30 degrees C. In the present study, we first showed that the codon selectivity and the arginine-accepting activity of the argU tRNA are both essential for complementing the temperature-sensitive growth, indicating that this defect is caused at the level of translation. An in vitro analysis of the effects of the argU10(Ts) mutation on tRNA functions revealed that the affinity with elongation factor Tu-GTP of the argU10(Ts) mutant tRNA is impaired at 30 and 43 degrees C, and this defect is more serious at the higher temperature. The arginine acceptance is also impaired significantly but to similar extents at the two temperatures. An in vivo analysis of aminoacylation levels showed that 30% of the argU10(Ts) tRNA molecules in the mutant cells are actually deacylated at 30 degrees C, while most of the argU tRNA molecules in the wild-type cells are aminoacylated. Furthermore, the cellular level of this mutant tRNA is one-tenth that of the wild-type argU tRNA. At 43 degrees C, the cellular level of the argU10(Ts) tRNA is further reduced to a trace amount, while neither the cellular abundance nor the aminoacylation level of the wild-type argU tRNA changes. We concluded that the phenotypic properties of the argU10(Ts) mutant result from these reduced intracellular levels of the tRNA, which are probably caused by the defective interactions with elongation factor Tu and arginyl-tRNA synthetase.     

Functional compensation of a recognition-defective transfer RNA by a distal base pair substitution.

A single G3:U70 base pair in the acceptor helix is the major determinant of alanine acceptance in alanine transfer RNAs. Transfer of this base pair into other transfer RNAs confers alanine acceptance. A G3:C70 substitution eliminates alanine acceptance in vivo and in vitro. In this work, a population of mutagenized G3:C70 alanine tRNA amber suppressors was subjected to a selection for mutations that compensate for the inactivating G3:C70 substitution. No compensatory mutations located in the acceptor helix were obtained. Instead, a U27:U43 substitution that replaced the wild-type C27:G43 in the anticodon stem created a U27:U43/G3:C70 mutant alanine tRNA that inserts alanine at amber codons in vivo. The U27:U43 substitution is at a location where previous footprinting work established an RNA-protein contact. Thus, this mutation may act by functionally coupling a distal part of the tRNA structure to the active site.     

Activation of the Bacillus subtilis hut operon at the onset of stationary growth phase in nutrient sporulation medium results primarily from the relief of amino acid repression of histidine transport.

During growth of Bacillus subtilis in nutrient sporulation medium containing histidine (DSM-His medium), the expression of histidase, the first enzyme in the histidine-degradative pathway (hut), is derepressed 40- to 200-fold at the onset of stationary phase. To identify the gene products responsible for this regulation, histidase expression was examined in various hut regulatory mutants as well as in mutants defective in stationary-phase gene regulation. Histidase expression during growth in DSM-His medium was significantly altered only in a strain containing the hutC1 mutation. The hutC1 mutation allows the hut operon to be expressed in the absence of its inducer, histidine. During logarithmic growth in DSM-His medium, histidase levels were 25-fold higher in the HutC mutant than in wild-type cells. Moreover, histidase expression in the HutC mutant increased only four- to eightfold after the end of exponential growth in DSM-His medium. This suggests that histidine transport is reduced in wild-type cells during exponential growth in DSM-His medium and that this reduction is largely responsible for the repression of hut expression in cells growing logarithmically in this medium. Indeed, the rate of histidine uptake in DSM-His medium was fourfold lower in exponentially growing cells than in stationary-phase cells. The observation that the degradation of histidine is inhibited when B. subtilis is growing rapidly in medium containing a mixture of amino acids suggests that a hierarchy of amino acid utilization may be present in this bacterium.     

The methylthio group (ms2) of N6-(4-hydroxyisopentenyl)-2-methylthioadenosine (ms2io6A) present next to the anticodon contributes to the decoding efficiency of the tRNA.

A Salmonella typhimurium LT2 mutant which harbors a mutation (miaB2508::Tn10dCm) that results in a reduction in the activities of the amber suppressors supF30 (tRNA(CUATyr)), supD10 (tRNA(CUASer)), and supJ60 (tRNA(CUALeu)) was isolated. The mutant was deficient in the methylthio group (ms2) of N6-(4-hydroxyisopentenyl)-2-methylthioadenosine (ms2io6A), a modified nucleoside that is normally present next to the anticodon (position 37) in tRNAs that read codons that start with uridine. Consequently, the mutant had i6A37 instead of ms2io6A37 in its tRNA. Only small amounts of io6A37 was found. We suggest that the synthesis of ms2io6A occurs in the following order: A-37-->i6A37-->ms2i6A37-->ms2io6A37. The mutation miaB2508::Tn10dCm was 60% linked to the nag gene (min 15) and 40% linked to the fur gene and is located counterclockwise from both of these genes. The growth rates of the mutant in four growth media did not significantly deviate from those of a wild-type strain. The polypeptide chain elongation rate was also unaffected in the mutant. However, the miaB2508::Tn10dCm mutation rendered the cell more resistant or sensitive, compared with a wild-type cell, to several amino acid analogs, suggesting that this mutation influences the regulation of several amino acid biosynthetic operons. The efficiencies of the aforementioned amber suppressors were decreased to as low as 16%, depending on the suppressor and the codon context monitored, demonstrating that the ms2 group of ms2io6A contributes to the decoding efficiency of tRNA. However, the major impact of the ms2io6 modification in the decoding process comes from the io6 group alone or from the combination of the ms2 and io6 groups, not from the ms2 group alone.     

Suppression of amber codons in vivo as evidence that mutants derived from Escherichia coli initiator tRNA can act at the step of elongation in protein synthesis

The absence of a Watson-Crick base pair at the end of the amino acid acceptor stem is one of the features which distinguishes prokaryotic initiator tRNAs as a class from all other tRNAs. We show that this structural feature prevents Escherichia coli initiator tRNA from acting as an elongator in protein synthesis in vivo. We generated a mutant of E. coli initiator tRNA in which the anticodon sequence is changed from CAU to CUA (the T35A36 mutant). This mutant tRNA has the potential to read the amber termination codon UAG. We then coupled this mutation to others which change the C1.A72 mismatch at the end of the acceptor stem to either a U1:A72 base pair (T1 mutant) or a C1:G72 base pair (G72 mutant). Transformation of E. coli CA274 (HfrC Su- lacZ125am trpEam) with multicopy plasmids carrying the mutant initiator tRNA genes show that mutant tRNAs carrying changes in both the anticodon sequence and the acceptor stem suppress amber codons in vivo, whereas mutant tRNA with changes in the anticodon sequence alone does not. Mutant tRNAs with the above anticodon sequence change are aminoacylated with glutamine in vitro. Measurement of kinetic parameters for aminoacylation by E. coli glutaminyl-tRNA synthetase show that both the nature of the base pair at the end of the acceptor stem and the presence or absence of a base pair at this position can affect aminoacylation kinetics. We discuss the implications of this result on recognition of tRNAs by E. coli glutaminyl-tRNA synthetase.     

Role of Histidine Transfer Ribonucleic Acid in Regulation of Synthesis of Histidyl-Transfer Ribonucleic Acid Synthetase of Salmonella typhimurium

The role of histidine transfer ribonucleic acid (tRNA) in repression of synthesis of histidyl-tRNA synthetase was examined in two strains of Salmonella typhimurium, one of which was a histidine tRNA (hisR) mutant possessing 52% of the wild-type (hisR(+)) histidine tRNA and a derepressed level of the histidine biosynthetic enzymes during histidine-unrestricted growth. Histidine-restricted growth caused a derepression of the rate of formation of histidyl-tRNA synthetase in both strains. In the case of the wild-type strain, addition of histidine to the derepressed culture caused a repression of synthesis of histidyl-tRNA synthetase for at least one generation of growth. In contrast, when histidine was restored to the derepressed hisR mutant culture, synthesis of histidyl-tRNA synthetase was continued at the initial derepressed rate. These results suggest that histidine must be attached to histidine tRNA for repression of synthesis of histidyl-tRNA synthetase.     

Purification and characterization of a mutant ATP phosphoribosyltransferase hypersensitive to histidine feedback inhibition.

A mutant form of ATP phosphoribosyltranferase (EC 2.4.2.17), hisG1708c, which results in abnormally slow growth of Salmonella typhimurium at 20 °C was purified to homogeneity and kinetic and chemical behavior were characterized. Initial velocity steady-state substrate kinetics of wild-type and mutant enzymes at 37 °C were consistent with sequential kinetics and demonstrated that standard assay concentrations of substrates were sufficient to substantially saturate both enzymes. Nearly time-independent inhibition by histidine at 37 °C could be obtained only after incubation in the presence of product and histidine. Studies at 37 °C showed that the mutant enzyme is 24 times more sensitive to histidine than the wild type in a negatively cooperative manner instead of the positively cooperative manner seen for wild type. Pure mutant enzyme exhibits two major electrophoretic species of native enzyme. Although one less cysteine is titratable in native mutant enzyme, the amino acid compositions of mutant and wild-type enzymes are similar. Histidine produces an ultraviolet difference spectrum in mutant enzyme closely resembling that produced in wild type. Binding of histidyl-tRNA to mutant enzyme is substantially inhibited by histidine. It is concluded that the hisG1708c mutation alters some conformational processes coupled to the histidine binding site while not affecting others.     

Functional tRNAs with altered 3' ends.

The CCA trinucleotide is a universally conserved feature of the 3' end of tRNAs, where it serves as the site of amino acid attachment. Despite this extreme conservation, we have isolated functional mutants of tRNA(His) and tRNA(Val1) with altered CCA ends. A mutant that leads to de-repression of the histidine biosynthetic operon in Salmonella typhimurium has been characterized and found to have the CCA end of the sole tRNA(His) species mutated to UCA. However, constructed mutants of tRNA(His) with ACA or GCA ends appeared to be nonfunctional in vivo. Mutants of Escherichia coli tRNA(Val1) with GCA or ACA ends were isolated on the basis of their ability to promote frameshifting at a specific sequence. These same tRNA(Val1) mutants also caused read-through of stop codons that were one, or in some instances two, codons downstream of the valine codon decoded by the mutant tRNA. A startling implication of these data is that disruption of interactions between the CCA end of the tRNA and the large ribosomal subunit promotes these aberrant codon-anticodon interactions on the small ribosomal subunit.     

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.     

Genetic and Molecular Analysis of a Trna(leu) Missense Suppressor of Nudc3, a Mutation That Blocks Nuclear Migration in Aspergillus Nidulans

NudC encodes a protein of unknown biochemical function that is required for nuclear migration. In an attempt to define its function by identifying interacting proteins, a screen for extragenic suppressors of the temperature-sensitive nudC3 mutation was undertaken that identified nine snc genes. Here we demonstrate that nudC3 has a missense mutation at amino acid 146 that causes leucine to be replaced by proline and that sncB69 encodes a mutant tRNA(Leu) that corrects the mutation. The sncB69 mutation deletes a single nucleotide in the anticodon of a tRNA(Leu) that changes its normal (5')CAG(3') leucine anticodon to the proline anticodon (5')CGG(3'), which presumably allows incorporation of leucine at the mutant nudC3 proline codon 146 and thereby causes suppression of the nudC3 mutant phenotype.     

The cysteine desulfurase IscS is required for synthesis of all five thiolated nucleosides present in tRNA from Salmonella enterica serovar typhimurium

Deficiency of a modified nucleoside in tRNA often mediates suppression of +1 frameshift mutations. In Salmonella enterica serovar Typhimurium strain TR970 (hisC3737), which requires histidine for growth, a potential +1 frameshifting site, CCC-CAA-UAA, exists within the frameshifting window created by insertion of a C in the hisC gene. This site may be suppressed by peptidyl-tRNAProcmo5UGG (cmo(5)U is uridine-5-oxyacetic acid), making a frameshift when decoding the near-cognate codon CCC, provided that a pause occurs by, e.g., a slow entry of the tRNAGlnmnm5s2UUG (mnm(5)s(2)U is 5-methylaminomethyl-2-thiouridine) to the CAA codon located in the A site. We selected mutants of strain TR970 that were able to grow without histidine, and one such mutant (iscS51) was shown to have an amino acid substitution in the L-cysteine desulfurase IscS. Moreover, the levels of all five thiolated nucleosides 2-thiocytidine, mnm(5)s(2)U, 5-carboxymethylaminomethyl-2-thiouridine, 4-thiouridine, and N-6-(4-hydroxyisopentenyl)-2-methylthioadenosine present in the tRNA of S. enterica were reduced in the iscS51 mutant. In logarithmically growing cells of Escherichia coli, a deletion of the iscS gene resulted in nondetectable levels of all thiolated nucleosides in tRNA except N-6-(4-hydroxyisopentenyl)-2-methylthioadenosine, which was present at only 1.6% of the wild-type level. After prolonged incubation of cells in stationary phase, a 20% level of 2-thiocytidine and a 2% level of N-6-(4-hydroxyisopentenyl)-2-methylthioadenosine was observed, whereas no 4-thiouridine, 5-carboxymethylaminomethyl-2-thiouridine, or mnm(5)s(2)U was found. We attribute the frameshifting ability mediated by the iscS51 mutation to a slow decoding of CAA by the tRNAGlnmnm5s2UUG due to mnm(5)s(2)U deficiency. Since the growth rate of the iscS deletion mutant in rich medium was similar to that of a mutant (mnmA) lacking only mnm(5)s(2)U, we suggest that the major cause for the reduced growth rate of the iscS deletion mutant is the lack of mnm(5)s(2)U and 5-carboxymethylaminomethyl-2-thiouridine and not the lack of any of the other three thiolated nucleosides that are also absent in the iscS deletion mutant.     

Regulation of histidine biosynthetic enzymes in a mutant of Escherichia coli with an altered histidyl-tRNA synthetase

Summary A mutant of Escherichia coli was isolated that grew at a normal rate in minimal medium at 26°C, grew at a normal rate in minimal medium at 37°C only if exogenous histidine was supplied, and grew more slowly than normal at 42°C even in the presence of histidine. In very rich media the growth rate of the mutant was normal at 26°C and 30°C, but not at 37°C or 42°C. It may be described as a temperature-conditional histidine bradytroph with a decreased ceiling to its growth rate.The histidyl-tRNA synthetase of the mutant was found to be abnormal; in crude extracts the enzyme activity was less stable and had approximately a tenfold higher apparent K _Mfor histidine than normal.Under many growth conditions the histidine biosynthetic enzymes in the mutant were derepressed several hundred fold compared to the wild strain, even in the presence of exogenous genous histidine. In general, the degree of derepression in the mutant was proportional to the difference in growth rate between the mutant and normal strains; this relationship, however, did not hold below 30°C or above 37°C.The properties of the mutant could be related to the properties of its histidyl-tRNA synthetase by assuming that the enzyme participates both in protein synthesis and in histidine biosynthetic enzyme regulation and that at low temperature it functions relatively more effectively in protein synthesis than in repression, while at high temperature it functions relatively more effectively in repression.Abbreviations used tRNA transfer RNA - AICAR aminoimidazole carboxamide ribose-5-phosphate