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

Synthetic gene circuit-mediated monitoring of endogenous metabolites: identification of GAL11 as a novel multicopy enhancer of s-adenosylmethionine level in yeast

Monitoring levels of key metabolites in living cells comprises a critical step in various investigations. The simplest approach to this goal is a fluorescent reporter gene using an endogenous promoter responsive to the metabolite. However, such a promoter is often not identified or even present in the species of interest. An alternative can be a synthetic gene circuit based on a heterologous pair consisting of a promoter and a transcription factor known to respond to the metabolite. We exploited the met operator and MetJ repressor of Escherichia coli, the interaction between which depends on S-adenosylmethionine (SAM), to construct synthetic gene circuits that report SAM levels in Saccharomyces cerevisiae. Using a dual-input circuit that outputs selection marker genes in a doxycycline-tunable manner, we screened a genomic library to identify GAL11 as a novel multicopy enhancer of SAM levels. These results demonstrate the potential and utility of synthetic gene circuit-mediated metabolite monitoring.

Combined proteome and metabolite-profiling analyses reveal surprising insights into yeast sulfur metabolism

Metabolomics is considered as an emerging new tool for functional proteomics in the identification of new protein function or in projects aiming at modeling whole cell metabolism. When combined with proteome studies, metabolite-profiling analyses revealed unanticipated insights into the yeast sulfur pathway. In response to cadmium, the observed overproduction of glutathione, essential for the detoxification of the metal, can be entirely accounted for by a marked drop in sulfur-containing protein synthesis and a redirection of sulfur metabolite fluxes to the glutathione pathway. A kinetic analysis showed sequential and dramatic changes in intermediate sulfur metabolite pools within the first hours of the treatment. Strikingly, whereas proteome and metabolic data were positively correlated under cadmium conditions, proteome and metabolic data were negatively correlated during other growth conditions, i.e. methionine supplementation or sulfate starvation. These differences can be explained by alternative mechanisms in the regulation of Met4, the activator of the sulfur pathway. Whereas Met4 activity is controlled by the cellular cysteine content in response to sulfur source and availability, the present study suggests that Met4 activation under cadmium conditions is cysteine-independent. The results clearly indicate that the metabolic state of a cell cannot be safely predicted based solely on proteomic and/or gene expression data. Combined metabolome and proteome studies are necessary to draw a comprehensive and integrated view of cell metabolism.

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.

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.

The synthesis of the two S-adenosyl-methionine synthetases is differently regulated in Saccharomyces cerevisiae

S-adenosyl-L-methionine (AdoMet) is synthesized by transfer of the adenosyl moiety of ATP to the sulfur atom of methionine. This reaction is catalysed by AdoMet synthetase. In all eukaryotic organisms studied so far, multiple forms of AdoMet synthetases have been reported and from their recent study, it appears that AdoMet synthetase is an exceptionally well conserved enzyme through evolution. In Saccharomyces cerevisiae, we have demonstrated the existence of two AdoMet synthetases encoded by genes SAM1 and SAM2. Yeast, which is able to concentrate exogenously added AdoMet, is thus a particularly useful biological system to understand the role and the physiological significance of the preservation of two almost identical AdoMet synthetases. The analysis of the expression of the two SAM genes in different genetic backgrounds during growth under different conditions shows that the expression of SAM1 and SAM2 is regulated differently. The regulation of SAM1 expression is identical to that of other genes implicated in AdoMet metabolism, whereas SAM2 shows a specific pattern of regulation. A careful analysis of the expression of the two genes and of the variations in the methionine and AdoMet intracellular pools during the growth of different strains lead us to postulate the existence of two different AdoMet pools, each one supplied by a different AdoMet synthetase but in equilibrium with each other. This could be a means of storing AdoMet whenever this metabolite is overproduced, thus avoiding the degradation of a metabolite the synthesis of which is energetically expensive.

Methylated proteins and amino acids in the ribosomes of Saccharomyces cerevisiae

The occurrence of methylated proteins in the ribosomes of Saccharomyces cerevisiae was investigated by tracing the transfer of radioactive methyl groups from S-adenosyl methionine, taken up by growing cells, into the protein moiety of ribosomes. It was estimated that the large subunit contained about 10 protein-bound methyl groups distributed mainly among proteins YL23, YL32 and YL1. The small subunit contained at most 2-4 methyl groups in proteins. Methyl groups could be transferred in vitro to proteins YL23 and YL32 in extracts from cultures of an S-adenosyl methionine auxotroph deprived of the methyl-group donor. In the most heavily methylated proteins the methylated amino acids formed in vitro were the same as those found in vivo (monomethyllysine and dimethyllysine in YL32; dimethyl and trimethyllsine in YL23). It is concluded that the enzymatic reaction in vitro faithfully saturates with methyl groups the target amino acids which are normally fully methylated in vivo.

Some Effects of Douglas Fir Terpenes on Certain Microorganisms

The Douglas fir terpene α-pinene was shown to inhibit the growth of a variety of bacteria and a yeast. Other terpenes of the Douglas fir, including limonene, camphene, and isobornyl acetate, were also inhibitory to Bacillus thuringiensis . All terpenes were inhibitory at concentrations normally present in the fir needle diet of Douglas fir tussock moth larvae. The presence of such terpenes in the diet of these insects was found to strongly influence the infectivity of B. thuringiensis spores for the Douglas fir tussock moth larvae. The terpene α-pinene destroyed the cellular integrity and modified mitochondrial activity in certain microorganisms.

Stimulation of yeast ascospore germination and outgrowth by S-adenosylmethionine

The supplementation of S-adenosylmethionine (SAM) to germination medium stimulated the accumulation of [14C]uracil from the medium into germinating cells, as well as its incorporation into ribonucleic acid during germination and outgrowth of ascospores of Saccharomyces cerevisiae. In addition to uracil, the accumulation of leucine, cytosine, serine, and methionine was also stimulated by the extracellular addition of this sulfonium compound. The SAM-stimulatory effect was dose dependent; half-maximal stimulation was observed at about 50 muM. The effect exerted by SAM supplementation appeared to be specific for SAM and for germination and outgrowth. In the absence of SAM biosynthesis (in the presence of cycloleucine), spores were inhibited in their ability to accumulate label, whereas the supplementation of SAM completely reversed the cycloleucine-induced inhibition of accumulation. In addition to accumulation and incorporation, the kinetics of bud formation during outgrowth were also stimulated by exogenous SAM. The stimulation of budding by SAM was amplified in an ethionine-resistant strain. These observations suggest that SAM may be essential for the initiation of cell division during the breaking of spore dormancy.

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.

Genetics and biochemistry of cryptopleurine resistance in the yeast Saccharomyces cerevisiae

Protein synthesis by ribosomes from several cryptopleurine-resistant yeast mutants is also resistant to emetine and tubulosine. These mutants can be classified into two different types: Class I mutants which display high levels of resistance to emetine and tubulosine and Class II mutants that are only weakly resistant to tubulosine and are slightly more sensitive to emetine than those of Class I. Apparently all mutants have similar levels of resistance to cryptopleurine. The distinct phenotypes of Class I and Class II strains are expressed through their 40S ribosomal subunit. Genetic analysis has shown that the mutations to cryptopleuring resistance are allelic and that in a particular case (strain CRY6) the pleiotropic phenotype is a result of the expression of the cry1 locus. It is suggested that Class I and Class II mutants arise from two independent mutational events within The cry1 allel. In heterozygous (+/cry1) diploids both the sensitive and the resistant genes are expressed as shown by studies of the action of cryptopleurine on polyphenylalanine-synthesizing systems derived from each parental sensitive and resistant haploid strain and heterozygous diploid strains. The apparent dominance of sensitivity over resistance which may be observed in vivo in heterozygous (+/cry1) diploids has been explained in terms of the mode of action of the inhibitors.

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.

Assay and regulation of S-adenosylmethionine synthetase in Saccharomyces cerevisiae and Candida utilis

A simple and sensitive assay for S-adenosylmethionine (SAM) synthetase is described which depends on the quantitative separation of the product, [14CH3]S-adenosylmethionine, from the substrate, L-[14CH3]methionine, on a Bio-Rex 70 column. L-Methionine protects the enzyme during preparation of cell extracts by sonic treatment but causes repression of enzyme activity during growth of Candida utilis. The presence of 5 mM methionine in the growth medium repressed SAM synthetase specific activity threefold compared to the specific acitivity of the enzyme isolated from cells grown in unsupplemented medium. Conversely, the presence of methionine in the growth medium resulted in an 80-fold increase in the intracellular concentration of SAM as compared to the Sam accumulated intracellularly in unsupplemented cultures.