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

Identification of MET10-932 and characterization as an allele reducing hydrogen sulfide formation in wine strains of Saccharomyces cerevisiae

A vineyard isolate of the yeast Saccharomyces cerevisiae, UCD932, was identified as a strain producing little or no detectable hydrogen sulfide during wine fermentation. Genetic analysis revealed that this trait segregated as a single genetic determinant. The gene also conferred a white colony phenotype on BiGGY agar (bismuth-glucose-glycine-yeast agar), which is thought to indicate low basal levels of sulfite reductase activity. However, this isolate does not display a requirement for S-containing amino acids, indicating that the sulfate reduction pathway is fully operational. Genetic crosses against known mutations conferring white colony color on BiGGY agar identified the gene leading to reduced H(2)S formation as an allele of MET10 (MET10-932), which encodes a catalytic subunit of sulfite reductase. Sequence analysis of MET10-932 revealed several corresponding amino acid differences in relation to laboratory strain S288C. Allele differences for other genes of the sulfate reduction pathway were also detected in UCD932. The MET10 allele of UCD932 was found to be unique in comparison to the sequences of several other vineyard isolates with differing levels of production of H(2)S. Replacing the MET10 allele of high-H(2)S-producing strains with MET10-932 prevented H(2)S formation by those strains. A single mutative change, corresponding to T662K, in MET10-932 resulted in a loss of H(2)S production. The role of site 662 in sulfide reduction was further analyzed by changing the encoded amino acid at this position. A change back to threonine or to the conservative serine fully restored the H(2)S formation conferred by this allele. In addition to T662K, arginine, tryptophan, and glutamic acid substitutions similarly reduced sulfide formation.

Sodium-induced GCN4 expression controls the accumulation of the 5' to 3' RNA degradation inhibitor, 3'-phosphoadenosine 5'-phosphate

Most cytoplasmic mRNAs are decapped and digested by the 5'-3'-exonuclease Xrn1p in Saccharomyces cerevisiae. The activity of Xrn1p is naturally inhibited in the presence of 3'-phosphoadenosine 5'-phosphate (pAp), a metabolite produced during sulfate assimilation that is quickly metabolized to AMP by the enzymatic activity of Hal2p. However, pAp accumulates and 5'-3' degradation decreases in the presence of ions known to inhibit Hal2p activity, such as sodium or lithium. We have shown that yeast cells can better adapt to the presence of sodium than lithium because of their ability to reduce pAp accumulation by activating HAL2 expression in a Gcn4p-dependent response, a regulatory loop that is likely to be conserved in different yeast species. We have thus identified a new role for the transcriptional activity of Gcn4p in maintaining an active mRNA degradation pathway under conditions of sodium stress. Since deregulation of proteins involved in different metabolic pathways is observed in xrn1Delta mutants, the maintenance of mRNA degradation capacity is likely to be important for the accurate and rapid adaptation of gene expression to salt stress.

MET17 and hydrogen sulfide formation in Saccharomyces cerevisiae

Commercial isolates of Saccharomyces cerevisiae differ in the production of hydrogen sulfide (H(2)S) during fermentation, which has been attributed to variation in the ability to incorporate reduced sulfur into organic compounds. We transformed two commercial strains (UCD522 and UCD713) with a plasmid overexpressing the MET17 gene, which encodes the bifunctional O-acetylserine/O-acetylhomoserine sulfhydrylase (OAS/OAH SHLase), to test the hypothesis that the level of activity of this enzyme limits reduced sulfur incorporation, leading to H(2)S release. Overexpression of MET17 resulted in a 10- to 70-fold increase in OAS/OAH SHLase activity in UCD522 but had no impact on the level of H(2)S produced. In contrast, OAS/OAH SHLase activity was not as highly expressed in transformants of UCD713 (0.5- to 10-fold) but resulted in greatly reduced H(2)S formation. Overexpression of OAS/OAH SHLase activity was greater in UCD713 when grown under low-nitrogen conditions, but the impact on reduction of H(2)S was greater under high-nitrogen conditions. Thus, there was not a good correlation between the level of enzyme activity and H(2)S production. We measured cellular levels of cysteine to determine the impact of overexpression of OAS/OAH SHLase activity on sulfur incorporation. While Met17p activity was not correlated with increased cysteine production, conditions that led to elevated cytoplasmic levels of cysteine also reduced H(2)S formation. Our data do not support the simple hypothesis that variation in OAS/OAH SHLase activity is correlated with H(2)S production and release.

Cloning and characterization of a mammalian lithium-sensitive bisphosphate 3'-nucleotidase inhibited by inositol 1,4-bisphosphate

Discovery of a structurally conserved metal-dependent lithium-inhibited phosphomonoesterase protein family has identified several potential cellular targets of lithium as used to treat manic depression. Here we describe identification of a novel family member using a "computer cloning" strategy. Human and murine cDNA clones encoded proteins sharing 92% identity and were highly expressed in kidney. Native and recombinant protein harbored intrinsic magnesium-dependent bisphosphate nucleotidase activity (BPntase), which removed the 3'-phosphate from 3'-5' bisphosphate nucleosides and 3'-phosphoadenosine 5'-phosphosulfate with Km and Vmax values of 0.5 microM and 40 micromol/min/mg. Lithium uncompetitively inhibited activity with a Ki of 157 microM. Interestingly, BPntase was competitively inhibited by inositol 1,4-bisphosphate with a Ki of 15 microM. Expression of mammalian BPntase complemented defects in hal2/met22 mutant yeast. These data suggest that BPntase's physiologic role in nucleotide metabolism may be regulated by inositol signaling pathways. The presence of high levels of BPntase in the kidney are provocative in light of the roles of bisphosphorylated nucleotides in regulating salt tolerance, sulfur assimilation, detoxification, and lithium toxicity. We propose that inhibition of human BPntase may account for lithium-induced nephrotoxicity, which may be overcome by supplementation of current therapeutic regimes with inhibitors of nucleotide biosynthesis, such as methionine.

Lysine production byBrevibacterium linens and its mutants: Activities and regulation of enzymes of the lysine biosynthetic pathway

Activity and regulation of key enzymes of the lysine biosynthetic pathway were investigated inBrevibacterium linens, a natural excretor of lysine, its lysine-overproducing homoserine auxotroph (Hom(-1)) and its auxotrophic and multianalogue-resistant high-yielding mutant (AEC NV 20(r)50). The activity of aspartate kinase (AK) and aspartaldehydate dehydrogenase (AD) was maximum during the mid-exponential phase of growth and decreased therafter. The mutants showed 10 and 20% more activity of AK and AD than the wild-type lysine excretor.B. linens (natural excretor) has a single AK and AD repressed and inhibited bivalently by lysine and threonine. Lysine slightly repressed and inhibited dihydrodipicolinate synthase (DS) and diaminopimelate decarboxylase (DD) of the wild type and of the mutant Hom(-1). The mutant AEC NV 20(r)50 showed DS and DD to be insensitive to lysine inhibition and repression. Persistence of a major part of the maximal activity of these enzymes during the late stationary phase of growth allowed prolonged synthesis and excretion of lysine. Stepwise addition of resistance to the different analogues of lysine in the mutant AEC NV20(r)50 resulted in an increase of enzyme activity and reduced repressibilities of enzymes that contributed to the high yield of lysine.

Lithium toxicity in yeast is due to the inhibition of RNA processing enzymes

Hal2p is an enzyme that converts pAp (adenosine 3',5' bisphosphate), a product of sulfate assimilation, into 5' AMP and Pi. Overexpression of Hal2p confers lithium resistance in yeast, and its activity is inhibited by submillimolar amounts of Li+ in vitro. Here we report that pAp accumulation in HAL2 mutants inhibits the 5'-->3' exoribonucleases Xrn1p and Rat1p. Li+ treatment of a wild-type yeast strain also inhibits the exonucleases, as a result of pAp accumulation due to inhibition of Hal2p; 5' processing of the 5.8S rRNA and snoRNAs, degradation of pre-rRNA spacer fragments and mRNA turnover are inhibited. Lithium also inhibits the activity of RNase MRP by a mechanism which is not mediated by pAp. A mutation in the RNase MRP RNA confers Li+ hypersensitivity and is synthetically lethal with mutations in either HAL2 or XRN1. We propose that Li+ toxicity in yeast is due to synthetic lethality evoked between Xrn1p and RNase MRP. Similar mechanisms may contribute to the effects of Li+ on development and in human neurobiology.

Determination of sulphite reductase activity and its response to assimilable nitrogen status in a commercial Saccharomyces cerevisiae wine yeast

The liberation of H2S is a common problem afflicting wine fermentation. Sulphite reductase activity of a commercial wine yeast was investigated to define its involvement in this process. The activity studied here differed from those characterized previously from cider and bakers' yeasts by displaying a greater sensitivity to cold, low ionic strength and possibly, proteolytic action. These differences necessitated the development of a new method of quantification. Through this method, the onset of H2S liberation was shown not to be a result of variations in the levels of sulphite reductase activity. Thus, high levels of activity which occurred during the exponential phase of growth were not necessarily accompanied by the liberation of H2S. Similarly, nitrogen-starved cultures which liberated H2S showed no corresponding increase in sulphite reductase activity from prestarvation levels. In fact, rates of H2S liberation from cultures and in enzyme assays agreed closely. A short-term independence of sulphite reductase activity from culture nitrogen status was therefore evident. The only influence of nitrogen was achieved in its absence when enzyme activity decayed with a half-life (4.25 h) which was comparable to that induced by the presence of cycloheximide (5.75 h). A proposed transcriptional control mechanism mediated by methionine derivatives was only partly effective in this strain although an in vitro inhibitory effect of methionine was implicated. These data therefore support the notion that H2S liberation in response to nitrogen starvation stems from a failure of metabolism to sequester H2S which continues to be formed, at least initially, at prestarvation rates.

Antifungal azoxybacilin exhibits activity by inhibiting gene expression of sulfite reductase

Azoxybacilin, produced by Bacillus cereus, has a broad spectrum of antifungal activity in methionine-free medium and has been suggested to inhibit sulfite fixation. We have further investigated the mode of action by which azoxybacilin kills fungi. The compound inhibited the incorporation of [35S] sulfate into acid-insoluble fractions of Saccharomyces cerevisiae under conditions in which virtually no inhibition was observed for DNA, RNA, or protein synthesis. It did not interfere with the activity of the enzymes for sulfate assimilation but clearly inhibited the induction of those enzymes when S. cerevisiae cells were transferred from rich medium to a synthetic methionine-free medium. Particularly strong inhibition was observed in the induction of sulfite reductase. Northern (RNA) analysis revealed that azoxybacilin decreased the level of mRNA of genes for sulfate assimilation, including MET10 for sulfite reductase and MET4, the transactivator of MET10 and other sulfate assimilation genes. When activities of azoxybacilin were compared for mRNA and enzyme syntheses from MET10, the concentration required for inhibition of transcription of the gene was about 10 times higher (50% inhibitory concentration = 30 micrograms/ml) than that required for inhibition of induction of enzyme synthesis (50% inhibitory concentration = 3 micrograms/ml). The data suggest that azoxybacilin acts on at least two steps in the expression of sulfite reductase; the transcriptional activation of MET4 and a posttranscriptional regulation in MET10 expression. We conclude that azoxybacilin exhibits antifungal activity by interfering with the regulation of expression of sulfite reductase activity.

Regulation of hydrogen sulfide liberation in wine-producing Saccharomyces cerevisiae strains by assimilable nitrogen

Saccharomyces cerevisiae wine-producing yeast cultures grown under model winemaking conditions could be induced to liberate hydrogen sulfide (H2S) by starvation for assimilable nitrogen. The amount of H2S produced was dependent on the yeast strain, the sulfur precursor compound, the culture growth rate, and the activity of the sulfite reductase enzyme (EC 1.8.1.2) immediately before nitrogen depletion. Increased H2S formation relative to its utilization by metabolism was not a consequence of a de novo synthesis of sulfite reductase. The greatest amount of H2S was produced when nitrogen became depleted during the exponential phase of growth or during growth on amino acids capable of supporting short doubling times. Both sulfate and sulfite were able to act as substrates for the generation of H2S in the absence of assimilable nitrogen; however, sulfate reduction was tightly regulated, leading to limited H2S liberation, whereas sulfite reduction appeared to be uncontrolled. In addition to ammonium, most amino acids were able to suppress the liberation of excess H2S when added as sole sources of nitrogen, particularly for one of the strains studied. Cysteine was the most notable exception, inducing the liberation of H2S at levels exceeding that of the nitrogen-depleted control. Threonine and proline also proved to be poor substitutes for ammonium. These data suggest that any compound that can efficiently generate sulfide-binding nitrogenous precursors of organic sulfur compounds will prevent the liberation of excess H2S.

Methionine-mediated lethality in yeast cells at elevated temperature

Saccharomyces cerevisiae cells grown at 30 degrees C in minimal medium containing methionine lose viability upon transfer to 45 degrees C, whereas cells grown in the absence of methionine survive. Cellular levels of two intermediates in the sulfate assimilation pathway, adenosine 5'-phosphosulfate (APS) and adenosine 5'-phosphosulfate 3'-phosphate, are increased by a posttranslational mechanism after sudden elevation of temperature in yeast cultures grown in the absence of methionine. Yeast cells unable to synthesize APS because of repression by methionine or mutation of the MET3 gene do not survive the temperature shift. Thus, methionine-mediated lethality at elevated temperature is linked to the inability to synthesize APS. The results demonstrate that APS plays an important role in thermotolerance.

At least four regulatory genes control sulphur metabolite repression in Aspergillus nidulans

Mutations in four genes: sconA (formerly suA25meth, mapA25), sconB (formerly mapB1), sconC and sconD, the last two identified in this work, relieve a group of sulphur amino acid biosynthetic enzymes from methionine-mediated sulphur metabolite repression. Exogenous methionine has no effect on sulphate assimilation in the mutant strains, whereas in the wild type it causes almost complete elimination of sulphate incorporation. In both mutant and wild-type strains methionine is efficiently taken up and metabolized to S-adenosylmethionine, homocysteine and other compounds, scon mutants also show elevated levels of folate-metabolizing enzymes which results from the large pool of homocysteine found in these strains. The folate enzymes appear to be inducible by homocysteine and repressible by methionine (or S-adenosylmethionine).

Biosynthesis of sulphur amino acids in Saccharomyces cerevisiae: regulatory roles of methionine and S-adenosylmethionine reassessed

cys4-1, a mutation in the reverse trans-sulphuration pathway, relieves the sulphate assimilation pathway and homocysteine synthase from methionine-mediated repression. Since the mutation blocks the synthesis of cysteine from methionine downstream from homocysteine, this indicates that neither methionine nor S-adenosylmethionine serve as low-molecular-mass effectors in this regulatory system, contradicting earlier hypotheses.

Four major transcriptional responses in the methionine/threonine biosynthetic pathway of Saccharomyces cerevisiae

Genes encoding enzymes in the threonine/methionine biosynthetic pathway were cloned and used to investigate their transcriptional response to signals known to affect gene expression on the basis of enzyme specific-activities. Four major responses were evident: strong repression by methionine of MET3, MET5 and MET14, as previously described for MET3, MET2 and MET25; weak repression by methionine of MET6; weak stimulation by methionine but no response to threonine was seen for THR1, HOM2 and HOM3; no response to any of the signals tested, for HOM6 and MES1. In a BOR3 mutant, THR1, HOM2 and HOM3 mRNA levels were increased slightly. The stimulation of transcription by methionine for HOM2, HOM3 and THR1 is mediated by the GCN4 gene product and hence these genes are under the general amino acid control. In addition to the strong repression by methionine, MET5 is also regulated by the general control.

Posttranscriptional regulation of the expression of MET2 gene of Saccharomyces cerevisiae

The first step of the specific pathway for methionine biosynthesis in the yeast Saccharomyces cerevisiae is catalyzed by the enzyme L-homoserine-O-acetyltransferase (HSTase) (EC 2.3.1.31), encoded by the MET2 gene. In order to ascertain whether there is a posttranscriptional control on the MET2 gene expression, as suggested by previous results on the expression of the cloned gene, systems for high inducible expression of MET2 gene were constructed. In these constructs the MET2 gene was cloned in yeast expression vectors under the control of an inducible yeast GAL promoter element so that the MET2 was transcribed at very high levels under induced conditions. Measurements of the specific mRNA levels showed a strong stimulation of MET2 gene transcription in yeast transformants grown on galactose as carbon source, corresponding to 50-100-fold the repressed conditions, while only a 2-fold increase of the enzymatic activity was observed. In addition, no evidence of a strong induced polypeptide of appropriate size on two dimensional gel electrophoresis was obtained. To understand the functional role of the non-coding 5' region of MET2 mRNA, we performed either a partial and a complete deletion of the 5' leader sequence, but even with these constructs an elevated mRNA level was not associated to a marked increase of the HSTase activity. These data support the idea of a posttranscriptional regulation of MET2 gene expression and show that the untranslated region of the specific mRNA is not involved in this regulatory mechanism.

Rapid purification and characterization of homoserine dehydrogenase from Saccharomyces cerevisiae

Homoserine dehydrogenase of Saccharomyces cerevisiae has been rapidly purified to homogeneity by heat and acid treatments, ammonium sulfate fractionation, and chromatography on Matrex Gel Red A and Q-Sepharose columns. The final preparation migrated as a single entity upon sodium dodecyl sulfate-polyacrylamide gel electrophoresis with a Mr of 40,000. The Mr of the native enzyme was 81,000 as determined by gel filtration, suggesting that the enzyme is composed of two identical subunits. This feature was also confirmed by cross-linking analysis using the bifunctional reagent dimethyl suberimidate. Feedback inhibition by L-methionine and L-threonine was observed using the purified enzyme. The enzyme was markedly stabilized against heat treatment at high salt concentrations. Additions of feedback inhibitors or high concentrations of salts failed to cause any dissociation or aggregation of the enzyme subunits unlike enzymes from other sources such as Rhodospirillum rubrum. The enzyme denatured in 3 M guanidine-HCl was refolded by simple dilution with a concomitant restoration of the activity. Cross-linking analysis of the renaturation process suggested that the formation of the dimer is required for activity expression. Amino acid sequence analysis of peptides obtained by digestion of the enzyme protein with Achromobacter lyticus protease I revealed that several amino acid residues are strictly conserved among homoserine dehydrogenases from S. cerevisiae, Escherichia coli, and Bacillus subtilis.

Structure of the HOM2 gene of Saccharomyces cerevisiae and regulation of its expression

In Saccharomyces cerevisiae the HOM2 gene encodes aspartic semi-aldehyde dehydrogenase (ASA DH). The synthesis of this enzyme had been shown to be derepressed by growth in the presence of high concentrations of methionine. In the present work we have cloned and sequenced the HOM2 gene and found that the promoter region of this gene bears one copy of the consensus sequence for general control of amino acid synthesis. This prompted us to study the regulation of the expression of the HOM2 gene. We have found that ASA DH is the first reported enzyme of the related threonine and methionine pathway to be regulated by the general control of amino acid synthesis.

The Saccharomyces cerevisiae MET3 gene: nucleotide sequence and relationship of the 5' non-coding region to that of MET25

In Saccharomyces cerevisiae, the expression of several genes implicated in methionine biosynthesis is co-regulated by a specific negative control. To elucidate the molecular basis of this regulation, we have cloned two of these genes, MET3 and MET25. The sequence of MET25 has already been determined (Kerjan et al. 1986). Here, we report the nucleotide sequence of the MET3 gene along with its 5' and 3' flanking regions. Plasmids bearing different deletions upstream of the transcribed region of MET3 were constructed. They were introduced into yeast cells and tested for their ability to complement met3 mutations and to respond to regulation by exogenous methionine. The regulatory region was located within a 100 bp region. The sequence of this regulatory region was compared with that of MET25. A short common sequence which occurs 250-280 bp upstream of the translation initiation codon of the gene was found. This sequence is a good candidate for the cis-acting regulatory element.

Genetic and biochemical study of threonine-overproducing mutants of Saccharomyces cerevisiae

Three threonine-overproducing mutants were obtained as prototrophic revertants of a hom3 mutant strain of Saccharomyces cerevisiae. The gene HOM3 codes for aspartokinase (aspartate kinase; EC 2.7.2.4), the first enzyme of the threonine-methionine biosynthetic route, which is subjected to feedback inhibition by threonine. Enzymatic studies indicated that aspartokinase from the revertants has lost the feedback inhibition, resulting in overproduction of threonine. These revertants also bore one or two additional mutations, named tex1-1 and tex2-1, which alone or jointly made possible the excretion of the threonine accumulated. The effect of these two genes on excretion is potentiated by excess inositol in the medium.

The general control of amino acid biosynthetic genes in the yeast Saccharomyces cerevisiae

Enzymes in diverse amino acid biosynthetic pathways in Saccharomyces cerevisiae are subject to a general amino acid control in which starvation for any amino acid leads to increased levels of the mRNAs encoding these enzymes. The short nucleotide sequence TGACTC, found nontandemly repeated upstream from the coregulated structural genes, serves as a cis-acting site for positive regulation of transcription. Multiple trans-acting repressors and activators have been identified. Most of these factors act indirectly by regulating the level of an activator encoded by the GCN4 gene. This regulation occurs at the level of GCN4 translation and is mediated by sequences in the long 5' leader of GCN4 mRNA. The GCN4 protein is the most likely candidate for the transcriptional activator that interacts with the TGACTC sequences at the structural genes.

Molecular cloning and regulation of the expression of the MET2 gene of Saccharomyces cerevisiae

The MET2 gene of Saccharomyces cerevisiae, which codes for homoserine-O-acetyltransferase, a key enzyme in methionine biosynthesis, was isolated by complementation of a met2 mutant strain of S. cerevisiae with a yeast gene bank. A 3.9-kb genomic fragment contains the entire gene, as demonstrated by genetic and molecular analysis of the integrative transformants. A polyadenylated mRNA of 1700 nt is detected by Northern blot hybridization with a MET2 probe. The level of this mRNA decreases by addition of exogenous methionine or of S-adenosylmethionine, suggesting a transcriptional regulation. The level of specific mRNA and the enzyme activity found in transformants that bear the MET2 gene on a multicopy plasmid suggest that also a post-transcriptional regulatory mechanism may be operative in budding yeast.

The MET2 gene of Saccharomyces cerevisiae: molecular cloning and nucleotide sequence

A 5.1-kb DNA fragment from Saccharomyces cerevisiae, which complements a yeast met2 mutant strain, has been cloned. This fragment contains the wild-type MET2 gene which codes for the homoserine O-transacetylase, one of the methionine biosynthetic enzymes. The presence of the MET2 gene has been shown by integrative transformation experiments and genetic analyses of the resulting transformants. The complete nucleotide sequence of a 2826-bp DNA fragment carrying the MET2 gene has been determined. The sequence contains one major open reading frame of 438 codons, giving a calculated Mr of 48,370 for the encoded protein. We have identified the transcriptional product of the MET2 gene and estimated its size at 1650 nucleotides.

Partial deprivation of GTP initiates meiosis and sporulation in Saccharomyces cerevisiae

We have investigated the physiological conditions under which meiosis and the ensuing sporulation of Saccharomyces cerevisiae are initiated. Initiation of sporulation occurs in response to carbon, nitrogen, phosphorus, or sulfur deprivation, and also, when met auxotrophs are partially starved for methionine, but not after starvation of other amino acid auxotrophs. It also occurs after partial starvation of pur or gua auxotrophs for guanine but not after starvation of ura auxotrophs for uracil. Under all these sporulation conditions the concentrations of both guanine nucleotides (GTP) and S-adenosylmethionine (SAM) decrease whereas those of other nucleotides show no trend. We show that the decrease of guanine nucleotides is essential for the initiation of meiosis and sporulation: when a gua auxotroph, also lacking one of the two SAM synthetases, is starved for guanine but supplemented with 0.1 mM methionine, GTP decreases while SAM slightly increases and yet the cells sporulate.

Three regulatory systems control production of glutamine synthetase in Saccharomyces cerevisiae

Production of glutamine synthetase in Saccharomyces cerevisiae is controlled by three regulatory systems. One system responds to glutamine levels and depends on the positively acting GLN3 product. This system mediates derepression of glutamine synthetase in response to pyrimidine limitation as well, but genetic evidence argues that this is an indirect effect of depletion of the glutamine pool. The second system is general amino acid control, which couples derepression of a variety of biosynthetic enzymes to starvation for many single amino acids. This system operates through the positive regulatory element GCN4. Expression of histidinol dehydrogenase, which is under general control, is not stimulated by glutamine limitation. A third system responds to purine limitation. No specific regulatory element has been identified, but depression of glutamine synthetase is observed during purine starvation in gln3 gcn4 double mutants. This demonstrates that a separate purine regulatory element must exist. Pulse-labeling and immunoprecipitation experiments indicate that all three systems control glutamine synthetase at the level of subunit synthesis.

Three regulatory systems control production of glutamine synthetase in Saccharomyces cerevisiae

Production of glutamine synthetase in Saccharomyces cerevisiae is controlled by three regulatory systems. One system responds to glutamine levels and depends on the positively acting GLN3 product. This system mediates derepression of glutamine synthetase in response to pyrimidine limitation as well, but genetic evidence argues that this is an indirect effect of depletion of the glutamine pool. The second system is general amino acid control, which couples derepression of a variety of biosynthetic enzymes to starvation for many single amino acids. This system operates through the positive regulatory element GCN4. Expression of histidinol dehydrogenase, which is under general control, is not stimulated by glutamine limitation. A third system responds to purine limitation. No specific regulatory element has been identified, but depression of glutamine synthetase is observed during purine starvation in gln3 gcn4 double mutants. This demonstrates that a separate purine regulatory element must exist. Pulse-labeling and immunoprecipitation experiments indicate that all three systems control glutamine synthetase at the level of subunit synthesis.

Genetic and biochemical study of threonine-overproducing mutants of Saccharomyces cerevisiae

Three threonine-overproducing mutants were obtained as prototrophic revertants of a hom3 mutant strain of Saccharomyces cerevisiae. The gene HOM3 codes for aspartokinase (aspartate kinase; EC 2.7.2.4), the first enzyme of the threonine-methionine biosynthetic route, which is subjected to feedback inhibition by threonine. Enzymatic studies indicated that aspartokinase from the revertants has lost the feedback inhibition, resulting in overproduction of threonine. These revertants also bore one or two additional mutations, named tex1-1 and tex2-1, which alone or jointly made possible the excretion of the threonine accumulated. The effect of these two genes on excretion is potentiated by excess inositol in the medium.

Identification of 3,4-dihydroxy-5-hexaprenylbenzoic acid as an intermediate in the biosynthesis of ubiquinone-6 by Saccharomyces cerevisiae

The mutant strain of Saccharomyces cerevisiae E3-24 is unable to synthesize ubiquinone-6. When this mutant is grown in the presence of p-hydroxy[U-14C]benzoate or p-hydroxy[carboxy-14C]benzoate, a radioactive compound accumulates. This new metabolite has been isolated and identified as 3,4-dihydroxy-5-hexaprenylbenzoate (3,4-DHHB). Aerobically grown prototrophic strains of S. cerevisiae were found to contain only low levels of this compound. When strain X963-18C, blocked at homoserine O-transacetylase (in methionine biosynthesis), was deprived of methionine, ubiquinone biosynthesis ceased, and 3,4-DHHB was observed to accumulate. This suggested that S-adenosylmethionine (SAM) could be the methyl donor for 3,4-DHHB. Restoration of methionine to the cultures released this block and resulted in the conversion of 3,4-DHHB to ubiquinone-6, demonstrating a precursor--product relationship. The identification of 3,4-DHHB as an intermediate in ubiquinone biosynthesis in yeast establishes an alternate pathway for ubiquinone biosynthesis in eukaryotes.

Sulfate uptake in Saccharomyces cerevisiae: biochemical and genetic study

Sulfate uptake is the first step of the sulfate assimilation pathway, which has been shown in our laboratory to be part of the methionine biosynthetic pathway. Kinetic study of sulfate uptake has shown a biphasic curve in a Lineweaver-Burk plot. The analysis of this plot indicates that two enzymes participate in sulfate uptake. One (permease I) has a high affinity for the substrate (K(m) = 0.005 mM); the other (permease II) shows a much lower affinity for sulfate (K(m) = 0.35 mM). Regulation of the synthesis of both permeases is under the control of exogenous methionine or S-adenosylmethionine. It was shown, moreover, that synthesis of sulfate permeases is coordinated with the synthesis of the other methionine biosynthetic enzymes thus far studied in our laboratory. An additional specific regulation of sulfate permeases by inhibition of their activity by endogenous sulfate and adenosyl phosphosulfate (an intermediate metabolite in sulfate assimilation) has been shown. A mutant unable to concentrate sulfate has been selected. This strain carried mutations in two independent genes. These two mutations, separated in two different strains, lead to modified kinetics of sulfate uptake. The study of these strains leads us to postulate that there is an interaction in situ between the products of these two genes.

Methionine biosynthesis in Saccharomyces cerevisiae. II. Gene-enzyme relationships in the sulfate assimilation pathway

In Saccharomyces cerevisiae, the products of eleven different genes are needed for a functional sulfate assimilation pathway. Only five enzymatic steps are known in this pathway. The study of the gene-enzyme relationships has shown that the enzymes catalysing two of these steps are probably heteropolymeric. Moreover, mutations in three unlinked genes lead to multiple enzymatic losses. Different hypotheses are made to account for these results.

Methionine biosynthesis in Saccharomyces cerevisiae. I. Genetical analysis of auxotrophic mutants

In order to analyse how many structural genes are implicated in the specific steps of the biosynthesis of methionine in Sacch. cerevisiae, a hundred mutants were studied by complementation. 21 groups were defined named MET1 to MET25. Neither recombination between independent mutants of the same complementation group nor linkage between different groups was found. Preliminary to biochemical studies, mutants of each complementation group were tested for their capacity to utilize various precursors of methionine.

Methionine-and S-adenosyl methionine-mediated repression in a methionyl-transfer ribonucleic-acid synthetase mutant of Saccharomyces cerevisiae

A Saccharomyces cerevisiae mutant strain unable to grow at 38 C and bearing a modified methionyl-transfer ribonucleic acid (tRNA) synthetase has been studied. It has been shown that, in this mutant, the percentage of tRNAmet charged in vivo paralleled the degree of repressibility of methionine biosynthetic enzymes by exogenous methionine. On the contrary, the repression mediated by exogenous S-adenosylmethionine does not correlate with complete acylation of tRNAmet. Althought McLaughlin and Hartwell reported previously that the thermosensitivity and the defect in the methionyl-tRNA synthetase were due to the same genetic lesion (1969), no diffenence could be found in the methionyl-tRNA synthetase activity or in the pattern of repressibility of methionine biosynthetic pathway after growth at the premissive and at a semipermissive temperature. It appears that the mutant also exhibits some other modified characters that render unlikely the existence of only one genetic lesion in this strain. A genetic study of this mutant was undertaken which led to the conclusion that the thermosensitivity and the other defects are not related to the methionyl-tRNA synthetase modification. It was shown that the modified repressibility of methionine biosynthetic enzymes by methionine and the lack of acylation of tRNAmet in vivo follow the methionyl-tRNA synthetase modification. These results are in favor of the idea that methionyl-tRNAmet, more likely than methionine, is implicated in the regulation of the biosynthesis of methionine.

Existence of two levels of repression in the biosynthesis of methionine in Saccharomyces cerevisiae: effect of lomofungin on enzyme synthesis

Derepression of a methionine biosynthetic enzyme (homocysteine synthase) has been studied after repression either by exogenous methionine or by exogenous S-adenosylmethionine (SAM). Lomofungin, which inhibits the synthesis of ribosomal precursor and messenger ribonucleic acid but not of protein in Saccharomyces cerevisiae, has been used in this system. It has been shown that the addition of this antibiotic prevents the derepression of homocysteine synthase after repression by exogenous methionine but not after repression by exogenous SAM. These experiments with lomofungin and the kinetics of repression after addition of methionine or SAM to the growth medium provide evidence that the repression induced by exogenous methionine acts at the transcriptional level whereas the repression induced by exogenous SAM acts at the translational level.

Biochemical and regulatory effects of methionine analogues in Saccharomyces cerevisiae

The effect of three methionine analogues, ethionine, selenomethionine, and trifluoromethionine, on the biosynthesis of methionine in Saccharomyces cerevisiae has been investigated. We have found the following to be true. (i) A sharp decrease in the endogenous methionine concentration occurs after the addition of any one of these analogues to growing cells. (ii) All of them can be transferred to methionine transfer ribonucleic acid in vitro as well as in vivo with, as a consequence, their incorporation into proteins. In the absence of radioactive trifluoromethionine, this conclusion results from experiments of an indirect nature and must be taken as an indication rather than a direct demonstration. (iii) Ethionine and selenomethionine can be activated as homologues of S-adenosylmethionine, whereas trifluoromethionine cannot. (iv) All of them can act as repressors of the methionine biosynthetic pathway. This has been shown by measuring the de novo rate of synthesis of methionine in a culture grown in the presence of any one of the three analogues.

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.

Regulation of tryptophan biosynthesis in Saccharomyces cerevisiae: mode of action of 5-methyl-tryptophan and 5-methyl-tryptophan-sensitive mutants

In a wild-type strain of Saccharomyces cerevisiae the tryptophan analogue dl-5-methyl-tryptophan (5MT) causes only a slight reduction of the growth rate. Uptake experiments indicate that the limited inhibition is partly due to low levels of 5MT inside the cell. On the other hand, this low concentration of 5MT leads to an increase in the activity of the tryptophan-biosynthetic enzymes. Evidence is presented that suggests that 5MT acts primarily through feedback inhibition of anthranilate synthase, the first enzyme of the pathway. A number of 5MT-sensitive mutants have been isolated, characterized, and assigned to one of the following three classes: class I, strains with altered activity and/or feedback sensitivity of anthranilate synthase; class II, strains with elevated uptake of 5MT; class III, mutants with altered regulation of the tryptophan-biosynthetic enzymes, which do not exhibit increases in activity in the presence of 5MT. This failure to exhibit increased enzyme activities in mutants of class III can also be observed after tryptophan starvation. Two mutants of class III show high sensitivity towards 3-amino-1,2,4-triazole. They can not exhibit derepression of some histidine- and arginine-biosynthetic enzymes under conditions that lead to an increase in these same enzymes in the wild-type strain.

Effects of regulatory mutations upon methionine biosynthesis in Saccharomyces cerevisiae: loci eth2-eth3-eth10

The effects of mutations occurring at three independent loci, eth2, eth3, and eth10, were studied on the basis of several criteria: level of resistance towards two methionine analogues (ethionine and selenomethionine), pool sizes of free methionine and S-adenosyl methionine (SAM) under different growth conditions, and susceptibility towards methionine-mediated repression and SAM-mediated repression of some enzymes involved in methionine biosynthesis (met group I enzymes). It was shown that: (i) the level of resistance towards both methionine analogues roughly correlates with the amount of methionine accumulated in the pool; (ii) the repressibility of met group I enzymes by exogenous methionine is either abolished or greatly lowered, depending upon the mutation studied; (iii) the repressibility of the same enzymes by exogenous SAM remains, in at least three mutants studied, close to that observed in a wild-type strain; (iv) the accumulation of SAM does not occur in the most extreme mutants either from endogenously overproduced or from exogenously supplied methionine: (v) the two methionine-activating enzymes, methionyl-transfer ribonucleic acid (tRNA) synthetase and methionine adenosyl transferase, do not seem modified in any of the mutants presented here; and (vi) the amount of tRNA(met) and its level of charging are alike in all strains. Thus, the three recessive mutations presented here affect methionine-mediated repression, both at the level of overall methionine biosynthesis which results in its accumulation in the pool, and at the level of the synthesis of met group I enzymes. The implications of these findings are discussed.