Tetrahydrofolate biosynthesis in plants: molecular and functional characterization of dihydrofolate synthetase and three isoforms of folylpolyglutamate synthetase in Arabidopsis thaliana

Tetrahydrofolate coenzymes involved in one-carbon (C1) metabolism are polyglutamylated. In organisms that synthesize tetrahydrofolate de novo, dihydrofolate synthetase (DHFS) and folylpolyglutamate synthetase (FPGS) catalyze the attachment of glutamate residues to the folate molecule. In this study we isolated cDNAs coding a DHFS and three isoforms of FPGS from Arabidopsis thaliana. The function of each enzyme was demonstrated by complementation of yeast mutants deficient in DHFS or FPGS activity, and by measuring in vitro glutamate incorporation into dihydrofolate or tetrahydrofolate. DHFS is present exclusively in the mitochondria, making this compartment the sole site of synthesis of dihydrofolate in the plant cell. In contrast, FPGS is present as distinct isoforms in the mitochondria, the cytosol, and the chloroplast. Each isoform is encoded by a separate gene, a situation that is unique among eukaryotes. The compartmentation of FPGS isoforms is in agreement with the predominance of gamma-glutamyl-conjugated tetrahydrofolate derivatives and the presence of serine hydroxymethyltransferase and C1-tetrahydrofolate interconverting enzymes in the cytosol, the mitochondria, and the plastids. Thus, the combination of FPGS with these folate-mediated reactions can supply each compartment with the polyglutamylated folate coenzymes required for the reactions of C1 metabolism. Also, the multicompartmentation of FPGS in the plant cell suggests that the transported forms of folate are unconjugated.

Polyglutamylation of folate coenzymes is necessary for methionine biosynthesis and maintenance of intact mitochondrial genome in Saccharomyces cerevisiae

One-carbon metabolism is essential to provide activated one-carbon units in the biosynthesis of methionine, purines, and thymidylate. The major forms of folates in vivo are polyglutamylated derivatives. In organisms that synthesize folate coenzymes de novo, the addition of the glutamyl side chains is achieved by the action of two enzymes, dihydrofolate synthetase and folylpolyglutamate synthetase. We report here the characterization and molecular analysis of the two glutamate-adding enzymes of Saccharomyces cerevisiae. We show that dihydrofolate synthetase catalyzing the binding of the first glutamyl side chain to dihydropteroate yielding dihydrofolate is encoded by the YMR113w gene that we propose to rename FOL3. Mutant cells bearing a fol3 mutation require folinic acid for growth and have no dihydrofolate synthetase activity. We show also that folylpolyglutamate synthetase, which catalyzes the extension of the glutamate chains of the folate coenzymes, is encoded by the MET7 gene. Folylpolyglutamate synthetase activity is required for methionine synthesis and for maintenance of mitochondrial DNA. We have tested whether two folylpolyglutamate synthetases could be encoded by the MET7 gene, by the use of alternative initiation codons. Our results show that the loss of mitochondrial functions in met7 mutant cells is not because of the absence of a mitochondrial folylpolyglutamate synthetase.

Molecular characterization of two high affinity sulfate transporters in Saccharomyces cerevisiae

Strains resistant to the toxic analogues of sulfate, selenate and chromate have been isolated. Their genetic analysis allowed us to identify four genes. One, called MET28, encodes a transcriptional factor. The three other genes, called SUL1, SUL2 and SUL3, encode proteins involved in sulfate transport. The sequence of Sul1p and Sul2p indicate that they are integral membrane proteins exhibiting, respectively, 11 and 10 transmembrane domains. Moreover, Sul1p and Sul2p share a high degree of similarity. Sulfate transport kinetic studies made with parental and mutant strains show that, as expected from genetic results, Saccharomyces cerevisiae has two high affinity sulfate transport systems. Sul3p has been shown to be involved in the transcriptional regulation of the SUL2 gene.

A heteromeric complex containing the centromere binding factor 1 and two basic leucine zipper factors, Met4 and Met28, mediates the transcription activation of yeast sulfur metabolism

Transcription activation of sulfur metabolism in yeast is dependent on two DNA binding factors, the centromere binding factor 1 (Cbf1) and Met4. While the role of Met4 was clearly established by showing that it acts as a transcription activator, the precise function in transcription of the multi-functional factor Cbf1 remains more elusive. We report here the identification of a new transcription factor Met28 which participates in the regulation of sulfur metabolism. Cloning and sequencing of MET28 revealed that it encodes a new member of the basic leucine zipper DNA binding factor family. We also demonstrate that Met28 possesses no intrinsic transcription activation capabilities. Studies of the DNA binding characteristics of Met28 led us to identify in gel mobility assays a heteromeric complex containing Cbf1, Met4 and Met28. We further demonstrated that the presence of Cbf1 and Met4 stimulates the binding of Met28 to DNA. 'Two-hybrid' studies allowed us to carry out preliminary investigations on the binary protein-protein interactions involved in the formation of the Cbf1-Met4-Met28 complex. Our results give evidence that the leucine zippers of Met4 and Met28, along with the basic helix-loop-helix domain of Cbf1, provide the protein surfaces mediating these interactions. All these results suggest that the multi-functional factor Cbf1 functions in transcription activation by tethering specific activating factors to the DNA.

Met30p, a yeast transcriptional inhibitor that responds to S-adenosylmethionine, is an essential protein with WD40 repeats

A specific repression mechanism regulates the biosynthesis of sulfur amino acids in Saccharomyces cerevisiae. When the intracellular S-adenosylmethionine (AdoMet) concentration increases, transcription of the sulfur genes is repressed. Using a specific reporter system, we have isolated mutations impairing the AdoMet-mediated transcriptional regulation of the sulfur network. These mutations identified a new gene, MET30, and were shown to also affect the regulation of the methyl cycle. The MET30 gene was isolated and sequenced. Sequence analysis reveals that Met30p contains five copies of the WD40 motif within its carboxy-terminal part, like the yeast transcriptional repressors Hir1p and Tup1p. We identified one target of Met30p as Met4p, a transcriptional activator regulating the sulfate assimilation pathway. By the two-hybrid method, we showed that Met30p interacts with Met4p and identified a region of Met4p involved in this interaction. Further analysis reveals that expression of Met30p is essential for cell viability.

Two divergent MET10 genes, one from Saccharomyces cerevisiae and one from Saccharomyces carlsbergensis, encode the alpha subunit of sulfite reductase and specify potential binding sites for FAD and NADPH

The yeast assimilatory sulfate reductase is a complex enzyme that is responsible for conversion of sulfite into sulfide. To obtain information on the nature of this enzyme, we isolated and sequenced the MET10 gene of Saccharomyces cerevisiae and a divergent MET10 allele from Saccharomyces carlsbergensis. The polypeptides deduced from the identically sized open reading frames (1,035 amino acids) of both MET10 genes have molecular masses of around 115 kDa and are 88% identical to each other. The transcript of S. cerevisiae MET10 has a size comparable to that of the open reading frame and is transcriptionally repressed by methionine in a way similar to that seen for other MET genes of S. cerevisiae. Distinct homology was found between the putative MET10-encoded polypeptide and flavin-interacting parts of the sulfite reductase flavoprotein subunit (encoded by cysJ) from Escherichia coli and several other flavoproteins. A significant N-terminal homology to pyruvate flavodoxin oxidoreductase (encoded by nifJ) from Klebsiella pneumoniae, together with a lack of obvious flavin mononucleotide-binding motifs in the MET10 deduced amino acid sequence, suggests that the yeast assimilatory sulfite reductase is a distinct type of sulfite reductase.

Cysteine biosynthesis in Saccharomyces cerevisiae occurs through the transsulfuration pathway which has been built up by enzyme recruitment

The transsulfuration pathways allow the interconversion of homocysteine and cysteine with the intermediary formation of cystathionine. The various organisms studied up to now incorporate reduced sulfur into a three- or a four-carbon chain and use differently the transsulfuration pathways to synthesize sulfur amino acids. In enteric bacteria, the synthesis of cysteine is the first step of organic sulfur metabolism and homocysteine is derived from cysteine. Fungi are capable of incorporating reduced sulfur into a four-carbon chain, and they possess two operating transsulfuration pathways. By contrast, synthesis of cysteine from homocysteine is the only existing transsulfuration pathway in mammals. In Saccharomyces cerevisiae, genetic, phenotypic, and enzymatic study of mutants has allowed us to demonstrate that homocysteine is the first sulfur amino acid to be synthesized and cysteine is derived only from homocysteine (H. Cherest and Y. Surdin-Kerjan, Genetics 130:51-58, 1992). We report here the cloning of genes STR4 and STR1, encoding cystathionine beta-synthase and cystathionine gamma-lyase, respectively. The only phenotypic consequence of the inactivation of STR1 or STR4 is cysteine auxotrophy. The sequencing of gene STR4 has allowed us to compare all of the known sequences of transsulfuration enzymes and enzymes catalyzing the incorporation of reduced sulfur in carbon chains. These comparisons reveal a partition into two families based on sequence motifs. This partition mainly correlates with similarities in the catalytic mechanisms of these enzymes.

Genetic analysis of a new mutation conferring cysteine auxotrophy in Saccharomyces cerevisiae: updating of the sulfur metabolism pathway

We have identified a mutation in a gene of Saccharomyces cerevisiae, STR1, that leads to a strict nutritional requirement for cysteine. The str1-1 mutation decreases to an undetectable level the cystathionine gamma-lyase activity. This enzyme catalyzes one of the two reactions involved in the transsulfuration pathway that yields cysteine from homocysteine with the intermediary formation of cystathionine. The phenotype induced by this mutation implies that, in S. cerevisiae, the sulfur atom of sulfide resulting from the reductive assimilation of sulfate is incorporated into a four carbon backbone yielding homocysteine, which, in turn, is the precursor of the biosynthesis of both cysteine and methionine. This also reveals that the direct synthesis of cysteine by incorporation of the sulfur atom into a three carbon backbone as found in Escherichia coli does not occur in S. cerevisiae. The study of the meiotic progeny of diploid strains heterozygous at the STR1 locus has shown that the str1-1 mutation undergoes a particularly high frequency of meiotic gene conversion.

Identification of the structural gene for glucose-6-phosphate dehydrogenase in yeast. Inactivation leads to a nutritional requirement for organic sulfur

Cloning of the MET19 gene revealed that it encodes the glucose-6-phosphate dehydrogenase from yeast. Sequence analysis showed a high degree of similarity between the yeast and the human enzymes. The cloned gene has allowed the construction of a glucose-6-phosphate dehydrogenase null mutant. The only phenotype of such a strain is an absolute requirement for an organic sulfur source, i.e. methionine, S-adenosylmethionine (AdoMet), cysteine, glutathione or homocysteine. The phenotype of this null mutant raises some new questions about the exact functions of the pentose phosphate pathway which was usually considered as the main cellular source of NADPH. Moreover, results reported here show that an increase of the AdoMet pool represses the transcription of the glucose-6-phosphate dehydrogenase gene. This regulation acts on the glucose-6-phosphate dehydrogenase biosynthesis but does not affect the synthesis of 6-phosphogluconate dehydrogenase. That AdoMet controls a part of the pentose phosphate pathway sheds new light on mechanisms regulating the relative fluxes of carbon utilization through the pentose phosphate pathway and glycolysis.

Elements involved in S-adenosylmethionine-mediated regulation of the Saccharomyces cerevisiae MET25 gene

In Saccharomyces cerevisiae, the MET25 gene encodes O-acetylhomoserine sulfhydrylase. Synthesis of this enzyme is repressed by the presence of S-adenosylmethionine (AdoMet) in the growth medium. We identified cis elements required for MET25 expression by analyzing small deletions in the MET25 promoter region. The results revealed a regulatory region, acting as an upstream activation site, that activated transcription of MET25 in the absence of methionine or AdoMet. We found that, for the most part, repression of MET25 expression was due to a lack of activation at this site, reinforced by an independent repression mechanism. The activation region contained a repeated dyad sequence that is also found in the promoter regions of other unlinked but coordinately regulated genes (MET3, MET2, and SAM2). We show that the presence of the two dyads is necessary for maximal gene expression. Moreover, we demonstrate that in addition to this transcriptional regulation, a posttranscriptional regulation, probably targeted at the 5' region of mRNA, is involved in MET25 expression.

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.

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.

Nucleotide sequence of the Saccharomyces cerevisiae MET25 gene

To elucidate further the molecular basis of the specific regulatory mechanism modulating the expression of the genes implicated in methionine metabolism, we have cloned and characterized two genes, MET3 and MET25, and shown that the regulation of their expression is transcriptional. The sequence of the cloned yeast MET25 gene which encodes the O-acetyl homoserine - O-acetyl serine (OAH-OAS) sulfhydrylase is reported here along with its 5' and 3' flanking regions. The amino acid composition predicted from the DNA sequence is in good agreement with that determined by hydrolysis of the purified enzyme. In the 5' flanking region the signal for general amino acid control was not found, corroborating our previous finding that the synthesis of OAH-OAS sulfhydrylase is not submitted to general control. The transcription start points have been determined. The 5' and 3' flanking regions of the MET25 gene suggest initiation and termination signals similar to those associated with other yeast genes.

The expression of the MET25 gene of Saccharomyces cerevisiae is regulated transcriptionally

The MET25 gene of Saccharomyces cerevisiae was cloned by functional complementation after transformation of a yeast met25 mutant. Subcloning of the DNA fragment bearing MET25 located the gene on a 2.3 kb region. The gene was formally identified by integration at the chromosomal MET25 locus. The cloned MET25 gene was used as a probe to measure the MET25 messenger RNA in a wild-type strain grown under conditions which promoted or failed to promote repression of MET25 expression. It was found that, under repression conditions, MET25 messenger RNA was reduced tenfold when compared with non-repression conditions. This suggests that the expression of MET25 is regulated transcriptionally. The direction of transcription, the size of the transcript and the position of the transcribed part of the gene were determined. Deletion mapping of the regulatory region was carried out. Deleted plasmids were introduced back into yeast cells and tested for their ability to complement met25 mutations and to promote regulation of expression of the MET25 gene by exogenous methionine. By this method the regulatory region was found to be confined to a 130 bp region.

Transcriptional regulation of the MET3 gene of Saccharomyces cerevisiae

The MET3 gene, coding for ATP sulfurylase (ATPS), an enzyme implicated in methionine biosynthesis in Saccharomyces cerevisiae, was cloned by functional complementation, after transformation, of a yeast met3 mutant strain. The cloned MET3 gene was used as a probe to measure the specific MET3 messenger RNA in a wild-type strain grown under conditions which promote or fail to promote repression of ATPS synthesis. It was found that the level of MET3 messenger RNA is reduced ten-fold when the strain is grown under conditions where ATPS synthesis is repressed, suggesting that the MET3 expression is regulated transcriptionally. The direction of transcription and the size of the transcript have been determined.

Construction of hybrid plasmids containing the lysA gene of Escherichia coli: studies of expression in Escherichia coli and Saccharomyces cerevisiae

The lysA gene of Escherichia coli has been cloned from a lambda transducing phage on various plasmids, present in different copy numbers in bacterial cells. Synthesis of the product of this gene, diaminopimelate (DAP)-decarboxylase, and its regulation have been studied. Expression does not follow a simple gene dosage effect, maximal expression already being obtained with a six-copy plasmid. This result suggests that either a positive or an autogenous regulatory mechanism is involved. We also used one of the hybrid plasmids to look for expression of the bacterial lysA gene in Saccharomyces cerevisiae. The results indicate that the product of the E. coli gene is not actively translated in yeast.

The two methionine adenosyl transferases in Saccharomyces cerevisiae: evidence for the existence of dimeric enzymes

In Saccharomyces cerevisiae either of the two genes SAM1 and SAM2 is able to produce a functional methionine adenosyl transferase (MATI and MATII). In a wild-type strain, MATI and MATII are present in dimeric forms: MATI-MATI, MATII-MATII and perhaps MATI-MATII. A hypothesis is presented to explain the possible role of these different forms of methionine adenosyl transferase in S. cerevisiae.

S-adenosyl methionine requiring mutants in Saccharomyces cerevisiae: evidences for the existence of two methionine adenosyl transferases

Mutants requiring S-adenosyl methionine (SAM) for growth have been selected in Saccharomyces cerevisiae. Two classes of mutants have been found. One class corresponds to the simultaneous occurrence of mutations at two unlinked loci SAM1 and SAM2 and presents a strict SAM requirement for growth on any medium. The second class corresponds to special single mutations in the gene SAM2 which lead to a residual growth on minimal medium but to normal growth on SAM supplemented medium or on a complex medium like YPGA not containing any SAM. These genetic data can be taken as an indication that Saccharomyces cerevisiae possesses two isoenzymatic methionine adenosyl transferases (MAT). In addition, SAM1 and SAM2 loci have been identified respectively with the ETH-10 and ETH2 loci previously described. Biochemical evidences corroborate the genetic results. Two MAT activities can be dissociated in a wild type extract (MATI and MATII) by DEAE cellulose chromatography. Mutations at the SAM1 locus lead to the absence or to the modification of MATII whereas mutations at the SAM2 locus lead to the absence or to the modification of MATI. Moreover, some of our results seem to show that MATI and MATII are associated in vivo.

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.

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.

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.

S-adenosyl methionine-mediated repression of methionine biosynthetic enzymes in Saccharomyces cerevisiae

S-adenosylmethionine (SAM) has been shown to provoke repression of some methionine-specific enzymes in wild-type cells, namely, adenosine triphosphate sulfurylase, sulfite reductase, and homocysteine synthetase. Repressive effects observed in SAM-supplemented cultures should be due to SAM per se, since the intracellular pool of SAM increases while the intracellular pool of methionine remains low and constant. Derepression brought about by methionine limitation is accompanied by a severe decrease in SAM as well as methionine pool sizes, although methionine adenosyl transferase is slightly derepressed. Different hypotheses have been considered to account for the previously reported implication of methionyl transfer ribonucleic acid and the presently reported SAM effects in this regulatory process.

Relationship between methionyl transfer ribonucleic acid cellular content and synthesis of methionine enzymes in Saccharomyces cerevisiae

Derepression of some methionine biosynthetic enzymes (methionine group I enzymes) obtained in methionine limitation has been found to be accompanied by a significant lack of in vivo charging of bulk methionine transfer ribonucleic acid (tRNA(Met)) and in addition by a decreased rate of synthesis of all tRNAs. Under the same conditions, methionyl-tRNA synthetase (MTS) was derepressed rather than repressed. These results are in agreement with those previously published based on studies of a mutant with an impaired MTS (5) and reinforce the idea that the rate of synthesis of methionine group I enzymes can be related to the total content of methionyl (Met)-tRNA (Met) per cell. They also render unlikely that MTS could be a constituent of the regulatory signal.

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.

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.

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.