Regulation of homoserine O-transacetylase, first step in methionine biosyntheis in Saccharomyces cerevisiae

Amino acid biosynthesis: new architectures in allosteric enzymes

This review focuses on the allosteric controls in the Aspartate-derived and the branched-chain amino acid biosynthetic pathways examined both from kinetic and structural points of view. The objective is to show the differences that exist among the plant and microbial worlds concerning the allosteric regulation of these pathways and to unveil the structural bases of this diversity. Indeed, crystallographic structures of enzymes from these pathways have been determined in bacteria, fungi and plants, providing a wonderful opportunity to obtain insight into the acquisition and modulation of allosteric controls in the course of evolution. This will be examined using two enzymes, threonine synthase and the ACT domain containing enzyme aspartate kinase. In a last part, as many enzymes in these pathways display regulatory domains containing the conserved ACT module, the organization of ACT domains in this kind of allosteric enzymes will be reviewed, providing explanations for the variety of allosteric effectors and type of controls observed.

Direct sulfhydrylation for methionine biosynthesis in Leptospira meyeri

A gene library of the Leptospira meyeri serovar semaranga strain Veldrat S.173 DNA has been constructed in a mobilizable cosmid with inserts of up to 40 kb. It was demonstrated that a Leptospira DNA fragment carrying metY complemented Escherichia coli strains carrying mutations in metB. The latter gene encodes cystathionine gamma-synthase, an enzyme which catalyzes the second step of the methionine biosynthetic pathway. The metY gene is 1,304 bp long and encodes a 443-amino-acid protein with a molecular mass of 45 kDa as determined by sodium dodecyl sulfate-polyacrylamide gel electrophoresis. The deduced amino acid sequence of the Leptospira metY product has a high degree of similarity to those of O-acetylhomoserine sulfhydrylases from Aspergillus nidulans and Saccharomyces cerevisiae. A lower degree of sequence similarity was also found with bacterial cystathionine gamma-synthase. The L. meyeri metY gene was overexpressed under the control of the T7 promoter. MetY exhibits an O-acetylhomoserine sulfhydrylase activity. Genetic, enzymatic, and physiological studies reveal that the transsulfuration pathway via cystathionine does not exist in L. meyeri, in contrast to the situation found for fungi and some bacteria. Our results indicate, therefore, that the L. meyeri MetY enzyme is able to perform direct sulfhydrylation for methionine biosynthesis by using O-acetylhomoserine as a substrate.

Metabolism of sulfur amino acids in Saccharomyces cerevisiae

Sulfur amino acid biosynthesis in Saccharomyces cerevisiae involves a large number of enzymes required for the de novo biosynthesis of methionine and cysteine and the recycling of organic sulfur metabolites. This review summarizes the details of these processes and analyzes the molecular data which have been acquired in this metabolic area. Sulfur biochemistry appears not to be unique through terrestrial life, and S. cerevisiae is one of the species of sulfate-assimilatory organisms possessing a larger set of enzymes for sulfur metabolism. The review also deals with several enzyme deficiencies that lead to a nutritional requirement for organic sulfur, although they do not correspond to defects within the biosynthetic pathway. In S. cerevisiae, the sulfur amino acid biosynthetic pathway is tightly controlled: in response to an increase in the amount of intracellular S-adenosylmethionine (AdoMet), transcription of the coregulated genes is turned off. The second part of the review is devoted to the molecular mechanisms underlying this regulation. The coordinated response to AdoMet requires two cis-acting promoter elements. One centers on the sequence TCACGTG, which also constitutes a component of all S. cerevisiae centromeres. Situated upstream of the sulfur genes, this element is the binding site of a transcription activation complex consisting of a basic helix-loop-helix factor, Cbf1p, and two basic leucine zipper factors, Met4p and Met28p. Molecular studies have unraveled the specific functions for each subunit of the Cbf1p-Met4p-Met28p complex as well as the modalities of its assembly on the DNA. The Cbf1p-Met4p-Met28p complex contains only one transcription activation module, the Met4p subunit. Detailed mutational analysis of Met4p has elucidated its functional organization. In addition to its activation and bZIP domains, Met4p contains two regulatory domains, called the inhibitory region and the auxiliary domain. When the level of intracellular AdoMet increases, the transcription activation function of Met4 is prevented by Met30p, which binds to the Met4 inhibitory region. In addition to the Cbf1p-Met4p-Met28p complex, transcriptional regulation involves two zinc finger-containing proteins, Met31p and Met32p. The AdoMet-mediated control of the sulfur amino acid pathway illustrates the molecular strategies used by eucaryotic cells to couple gene expression to metabolic changes.

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.

Role of hydrosulfide ions (HS-) in methylmercury resistance in Saccharomyces cerevisiae

Methylmercury-resistant mutants were obtained from Saccharomyces cerevisiae. They were divided into two complementation groups, met2 (homoserine O-acetyltransferase deficiency) and met15 (enzyme deficiency unknown), as reported previously. It was found that met15 was allelic to met17 (O-acetylserine and O-acetylhomoserine sulfhydrylase deficiency). Methylmercury toxicity was counteracted by exogenously added HS-, and both met2 and met17 (met15) mutants overproduced H2S. On the basis of these results, we conclude that met2 and met17 (met15) cause accumulation of hydrosulfide ions in the cell and that the increased level of hydrosulfide is responsible for detoxification of methylmercury.

Roles of O-acetyl-L-homoserine sulfhydrylases in micro-organisms

O-Acetyl-L-homoserine sulfhydrylase (EC 4.2.99.10) is essential for certain micro-organisms, functioning as a homocysteine synthase in the pathway of methionine synthesis. It participates in an alternative pathway of L-homocysteine synthesis for those microbes in which homocysteine is synthesized mainly via cystathionine. The protein can also catalyze the de novo synthesis of L-cysteine and O-alkyl-L-homoserine in some microorganisms. The enzyme possibly recycles the methylthio group of methionine.

Partial purification and some properties of homoserine O-acetyltransferase of a methionine auxotroph of Saccharomyces cerevisiae

A wild-type strain and six methionine auxotrophs of Saccharomyces cerevisiae were cultured in a synthetic medium supplemented with 0.1 mM L-cysteine or L-methionine and analyzed for the synthesis of homoserine O-acetyltransferase (EC 2.3.1.31). Among them, four mutant strains exhibited enzyme activity in cell extracts. Methionine added to the synthetic medium at concentrations higher than 0.1 mM repressed enzyme synthesis in two of these strains. The enzyme was partially purified (3,500-fold) from an extract of a mutant strain through ammonium sulfate fractionation and chromatography on columns of DEAE-cellulose, Phenyl-Sepharose C1-4B, and Sephadex G-150. The enzyme exhibited optimal pH at 7.5 for activity and at 7.8 for stability. The reaction product was ascertained to be O-acetyl-L-homoserine by confirming that it produced L-homocysteine in an O-acetyl-L-homoserine sulfhydrylase reaction. The Km for L-homoserine was 1.0 mM, and for acetyl coenzyme A it was 0.027 mM. The molecular weight of the enzyme was estimated to be approximately 104,000 by Sephadex G-150 column chromatography and 101,000 by sucrose density gradient centrifugation. The isoelectric point was at pH 4.0. Of the hydroxy amino acids examined, the enzyme showed reactivity only to L-homoserine. Succinyl coenzyme A was not an acyl donor. In the absence of L-homoserine, acetyl coenzyme A was deacylated by the enzyme, with a Km of 0.012 mM. S-Adenosylmethionine and S-adenosylhomocysteine slightly inhibited the enzyme, but methionine had no effect.

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.

Molecular genetics of met 17 and met 25 mutants of Saccharomyces cerevisiae: intragenic complementation between mutations of a single structural gene

We cloned the MET 17 gene of Saccharomyces cerevisiae by functional complementation after transformation of a yeast met 17 mutant. Restriction mapping and nucleotide sequencing of the MET 17 clones revealed that these were from the same genomic region as clones isolated previously and shown to contain the MET 25 gene encoding the enzyme O-acetylhomoserine, O-acetylserine sulphydrylase (OAH-OAS sulphydrylase). Transformation studies with MET 25 clones showed that the MET 17 and MET 25 functions were both endoced in a single transcription unit. We conclude that met 17 and met 25 are both mutations in the structural gene for the OAH-OAS sulphydrylase subunit and that each affects a different functional domain of the enzyme allowing subunit complementation in the met 17 X met 25 diploid. Enzyme assays indicated that the diploid, although not requiring methionine, had a low OAH-OAS sulphydrylase activity (10% of wild type). This is consistent with MET 17 and MET 25 being the same gene. We found that both met 17 and met 25 mutants were devoid of 3' phospho-adenosine 5' phospho-sulphite (PAPS) reductase activity and that this activity was fully restored in the met 17 X met 25 diploid. The possible interactions between OAH-OAS sulphydrylase and PAPS reductase are discussed.

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.

Cystathionine accumulation in Saccharomyces cerevisiae

A cysteine-dependent strain of Saccharomyces cerevisiae and its prototrophic revertants accumulated cystathionine in cells. The cystathionine accumulation was caused by a single mutation having a high incidence of gene conversion. The mutation was designated cys3 and was shown to cause loss of gamma-cystathionase activity. Cysteine dependence of the initial strain was determined by two linked and interacting mutations, cys3 and cys1 . Since cys1 mutations cause a loss of serine acetyltransferase activity, our observation led to the conclusion that S. cerevisiae synthesizes cysteine by sulfhydrylation of serine with hydrogen sulfide and by cleavage of cystathionine which is synthesized from serine and homocysteine.

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.

Methionine metabolism in BHK cells: selection and characterization of ethionine resistant clones

The selection of clones resistant to methionine antagonists was undertaken on baby hamster Kidney cells grown in a methionine free medium, supplemented with homocystine, folic acid and hydroxo-B12. Clones resistant to 30 mug/ml ethionine were isolated after mutagenesis at an induced mutation frequency of 2.3 X 10(-5). An ethionine resistant clone, ETH 304, was extensively studied. The resistant cells excreted methionine in the culture medium and the intracellular pools of methionine and SAM were two to five times greater in the resistant clone than in the wild type cells. A semidominant ethionine resistant phenotype was observed in hybrids between the wild type and this resistant clone. Measurement of the specific activity of menadione reductase, B12 methyltransferase and ATP: L-methionine S-adenosyl-transferase in crude extracts of the wild type showed a repressive action of methionine on the level of the three enzymes. However, the ethionine resistant clone ETH 304 was not modified in this function. Menadione reductase is feedback-inhibited by SAM in wild type cells. The enzyme of the ethionine resistant clone was significantly less sensitive to SAM. When a comparison of thermal stability was made between the wild type and ethionine resistant clone enzymes, it was found that the thermal stability of the latter was modified. Three other ethionine resistant clones, independantly isolated, were similarly affected in the properties of menadione reductase. These results suggest that the pathway of re-use of S-adenosyl homocysteine, produced during methylation reactions, is highly regulated by methionine and SAM.

Enzymatic lesions in methionine mutants of Aspergillus nidulans: role and regulation of an alternative pathway for cysteine and methionine synthesis

In Aspergillus nidulans the pathway involving cystathionine formation is the main one for homocysteine synthesis. Mutants lacking cystathionine gamma-synthase or beta-cystathionase are auxotrophs suppressible by: (i) mutations in the main pathway of cysteine synthesis (cysA1, cysB1, and cysC1), (ii) mutations causing stimulation of cysteine catabolism (su101), and (iii) mutations in a presumed regulatory gene (suAmeth). A relative shortage of cysteine in the first group of suppressors causes a derepression of homocysteine synthase, the enzyme involved in the alternative pathway of homocysteine synthesis. A similar derepression is observed in the suAmeth strain. Homocysteine synthesized by this pathway serves as precursor for cysteine and methionine synthesis. A mutant with altered homocysteine synthase is a prototroph, indicating that this enzyme is not essential for the fungus.

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.

Insensitivity of homocitrate synthase in extracts of Penicillium chyrosogenum to feedback inhibition by lysine

We previously reported that lysine inhibits in vivo homocitrate synthesis in the lysine bradytroph, Penicillium chrysogenum L(1), and that such feedback inhibition could explain the known lysine inhibition of penicillin formation. In the present study, it was found that dialyzed cell-free extracts of mutant L(1) converted [1-(14)C]acetate to homocitrate. This homocitrate synthase activity was extremely labile but could be stabilized by high salt concentrations. The pH optimum of the reaction was 6.9, and the K(m) was 5.5 mM with respect to alpha-ketoglutarate. The reaction was also dependent upon the presence of Mg(2+), adenosine 5'-triphosphate, and coenzyme A. Surprisingly, the activity in these crude extracts was not inhibited by lysine. Benzylpenicillin at a high concentration (20 mM) partially inhibited the enzyme, an effect that was enhanced by lysine. Casein hydrolysate also partially inhibited the enzyme.

Characteristics and relationships of mercury-resistant mutants and methionine auxotrophs of yeast

Approximately one-half of the mutants of Saccharomyces cerevisiae that are selected as resistant to methyl mercury are also found to require methionine. Eighty-four percent of these met mutations occur at the met15 locus, and the remaining 16% occur at the met2 locus. Surprisingly, the methionine-requiring mutants are recovered at a much higher frequency on methionineless media than on media supplemented with methionine. Growth patterns of the met mutants on media having a continuous concentration gradient of methionine and mercury compounds indicate that, at a critical concentration of the mercury compounds, the methionine requirement of certain met mutants is partially or completely alleviated. This was found for met2, met15, and to a lesser extent for met6, but not for any other methionine mutants. This loss of methionine requirement is produced with methyl mercury, phenyl mercury, and mercuric chloride although met2 and met15 strains can be shown to be resistant only to methyl mercury. Other methionine auxotrophs are not resistant to any of the three mercury compounds. The met2 and met15 mutants, but not the other methionine auxotrophs, develop a sheen of an unidentified product when grown on media with mercuric chloride but not with methyl mercury or phenyl mercury. It is suggested that met2 and met15 mutants produce a simple diffusible substance, which detoxifies methyl mercury, which reacts with mercuric chloride to produce a sheen, and which is the cause of the methionine requirement.

Induction and repression in the S-adenosylmethionine and methionine biosynthetic systems of Saccharomyces cerevisiae

Two methionine biosynthetic enzymes and the methionine adenosyltransferase are repressed in Saccharomyces cerevisiae when grown under conditions where the intracellular levels of S-adenosylmethionine are high. The nature of the co-repressor molecule of this repression was investigated by following the intracellular levels of methionine, S-adenosylmethionine, and S-adenosylhomocysteine, as well as enzyme activities, after growth under various conditions. Under all of the conditions found to repress these enzymes, there is an accompanying induction of the S-adenosylmethionine-homocysteine methyltransferase which suggests that this enzyme may play a key role in the regulation of S-adenosylmethionine and methionine balance and synthesis. S-methylmethionine also induces the methyltransferase, but unlike S-adenosylmethionine, it does not repress the methionine adenosyltransferase or other methionine biosynthetic enzymes tested.

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.

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

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

Regulation of S-adenosylmethionine synthetase in Escherichia coli

Addition of methionine to the growth medium of Escherichia coli K-12 leads to a reduction in the specific activity of S-adenosylmethionine (SAM) synthetase. Thus the enzyme appears to be repressible rather than inducible. Mutant strains (probably metJ(-)) are constitutive for SAM synthetase as well as for the methionine biosynthetic enzymes, suggesting that the regulatory systems for these enzymes have at least some elements in common. Cells grown to stationary phase in complete medium, which have low specific activities of the enzymes, were routinely used for derepression experiments. The lag in growth and derepression when these cells are incubated in minimal medium is shortened by threonine. Ethionine, norleucine, and alpha-methylmethionine are poor substrates or nonsubstrates for SAM synthetase and are ineffective repressors. Selenomethionine, a better substrate for SAM synthetase than methionine, is also slightly more effective at repression than methionine. Although SAM is considered to be a likely candidate for the corepressor in the control of the methionine biosynthetic enzymes, addition of SAM to the growth medium does not cause repression. Measurement of SAM uptake shows that too little is taken into the cells to have a significant effect, even if it were active in the control system.

Role of homocysteine synthetase in an alternate route for methionine biosynthesis in Saccharomyces cerevisiae

In vivo studies have shown that, in the absence of homoserine-O-transacetylase activity (locus met(2)), the C(4)-carbon moiety of ethionine is utilized (provided the ethionine resistance gene eth-2r is present) by methionine auxotrophs, except for met(8) mutants (homocysteine synthetase-deficient). Concomitant utilization of sulfur and methyl group from methylmercaptan or S-methylcysteine has been demonstrated. In the absence of added methylated intermediates, the methyl group of methionine formed from ethionine is derived from serine. In vitro studies with crude extracts of Saccharomyces cerevisiae have demonstrated that this synthesis of methionine occurs by the following reactions: CH(3)-SH + ethionine right harpoon over left harpoon methionine + C(2)H(5)SH and S-methylcysteine + ethionine right harpoon over left harpoon methionine + S-ethylcysteine. In the forward direction, the second product of the second reaction was shown to be S-ethylcysteine; this reaction has also been found reversible, leading to ethionine formation. Genetic and kinetic data have shown that homocysteine synthetase catalyzes these two reactions, at 0.3% of the rate it catalyzes direct homocysteine synthesis: O-Ac-homoserine + Na(2)S --> homocysteine + acetate. The three reactions are lost together in a met(8) mutant and are recovered to the same extent in spontaneous prototrophic revertants from this strain. Methionine-mediated regulation of enzyme synthesis affects the three activities and is modified to the same extent by the presence of the recessive allele (eth-2r) of the regulatory gene eth-2. Affinities of the enzyme for substrates of both types of reactions are of the same order of magnitude. Moreover, ethionine, the substrate of the second reaction, inhibits the third reaction, whereas O-acetyl-homoserine, the substrate of the third reaction, inhibits the second reaction. An enzymatic cleavage of S-methylcysteine, leading to methylmercaptan production, has been shown to occur in crude yeast extracts. It is concluded that the enzyme homocysteine synthetase participates in the two alternate pathways leading to methionine biosynthesis in S. cerevisiae, one involving O-acetyl-homoserine and H(2)S, the other involving the 4-carbon chain of ethionine and a mercaptyl donor. Participation of the two types of reactions catalyzed by homocysteine synthetase, in in vivo methionine synthesis, has been shown to occur in a met(2) partial revertant.

Influence of methionine pool composition on the formation of methyl-deficient transfer ribonucleic acid in Saccharomyces cerevisiae

Methionine auxotrophs of Saccharomyces cerevisiae continue to synthesize ribonucleic acid (RNA) after methionine withdrawal. The newly synthesized transfer RNA (tRNA) is methyl-deficient in some strains, but not in all. Whether such tRNA will accumulate depends on the position of the block in the methionine pathway that is carried by the mutant strain. Free methionine rapidly decreases in the intracellular pool of all strains after its removal from the medium. Certain metabolites derived from methionine are removed from the pool relatively slowly after methionine withdrawal. Notable among these is S-adenosylhomocysteine, which is depleted less rapidly from those strains that accumulate methyl-deficient tRNA than from others. S-adenosylhomocysteine is a potent inhibitor of tRNA-methylating enzymes in vitro.

Serine transhydroxymethylase in methionine biosynthesis in Saccharomyces cerevisiae

Serine transhydroxymethylase appears to be the first enzyme in the synthesis of the methyl group of methionine. Properties of serine transhydroxymethylase activity as assayed by the production of formaldehyde were correlated with properties of cell-free extracts for the methylation of homocysteine deriving the methyl group from the beta-carbon of serine. The reaction required pyridoxal phosphate and tetrahydrofolic acid, and was characterized in cell-free extracts with respect to Michaelis constant, pH optimum, incubation time, and optimal enzyme concentration. The activity was sensitive to inhibition by methionine, and to a much greater extent by S-adenosylmethionine. Serine transhydroxymethylase and the methylation of homocysteine reactions were not repressed by methionine and were stimulated by glycine. The activities of cell-free extracts for these reactions were significantly higher in cells in exponential than in stationary growth. When cells were grown in 10 mm glycine, the activities remained high throughout the culture cycle. The data indicated that glycine rather than methionine is involved in the control of the formation of the enzyme.

Genetic and regulatory aspects of methionine biosynthesis in Saccharomyces cerevisiae

Methionine biosynthesis and regulation of four enzymatic steps involved in this pathway were studied in Saccharomyces cerevisiae, in relation to genes concerned with resistance to ethionine (eth(1) and eth(2)). Data presented in this paper and others favor a scheme which excludes cystathionine as an obligatory intermediate. Kinetic data are presented for homocysteine synthetase [K(m)(O-acetyl-l-homoserine) = 7 x 10(-3)m; K(i) (l-methionine) = 1.9 x 10(-3)m]. Enzymes catalyzing steps 3, 4, 5, and 9 were repressible by methionine. Enzyme 4 (homoserine-O-transacetylase) and enzyme 9 (homocysteine synthetase) were simultaneously derepressed in strains carrying the mutant allele eth(2) (r). Studies on diploid strains confirmed the dominance of the eth(2) (s) allele over eth(2) (r). Regulation of enzyme 3 (homoserine dehydrogenase) and enzyme 5 (adenosine triphosphate sulfurylase) is not modified by the allele eth(2) (r). The other gene eth(1) did not appear to participate in regulation of these four steps. Gene enzyme relationship was determined for three of the four steps studied (steps 3, 4, and 9). The structural genes concerned with the steps which are under the control of eth(2) (met(8): enzyme 9 and met(a): enzyme 4) segregate independently, and are unlinked to eth(2). These results are compatible with the idea that the gene eth(2) is responsible for the synthesis of a pleiotropic methionine repressor and suggest the existence of at least two different methionine repressors in S. cerevisiae. Implications of these findings in general regulatory mechanisms have been discussed.