Methionine adenosyltransferase and ethionine resistance in Saccharomyces cerevisiae

Effect of Methionine on Gene Expression in Komagataella phaffii Cells

Komagataella phaffii yeast plays a prominent role in modern biotechnology as a recombinant protein producer. For efficient use of this yeast, it is essential to study the effects of different media components on its growth and gene expression. We investigated the effect of methionine on gene expression in K. phaffii cells using RNA-seq analysis. Several gene groups exhibited altered expression when K. phaffii cells were cultured in a medium with methanol and methionine, compared to a medium without this amino acid. Methionine primarily affects the expression of genes involved in its biosynthesis, fatty acid metabolism, and methanol utilization. The AOX1 gene promoter, which is widely used for heterologous expression in K. phaffii, is downregulated in methionine-containing media. Despite great progress in the development of K. phaffii strain engineering techniques, a sensitive adjustment of cultivation conditions is required to achieve a high yield of the target product. The revealed effect of methionine on K. phaffii gene expression is important for optimizing media recipes and cultivation strategies aimed at maximizing the efficiency of recombinant product synthesis.

A single-nucleotide mutation in a gene encoding S-adenosylmethionine synthetase is associated with methionine over-accumulation phenotype in Arabidopsis thaliana

Met-overaccumulating mutants provide a powerful genetic tool for examining both the regulation of the Met biosynthetic pathway and in vivo developmental responses of gene expression to altered Met levels. We have previously reported the identification of two Arabidopsis thaliana Met over-accumulation (mto) mutants, mto1-1 and mto2-1, that carry mutations in the genes encoding cystathionine gamma-synthase (CGS) and threonine synthase (TS), respectively. A third mutant, mto3-1, has recently been reported to carry a mutation in the gene encoding S-adenosylmethionine synthetase 3 (SAMS3). Here, we report the isolation of a new ethionine-resistant A. thaliana mutant that over-accumulates soluble Met approximately 20-fold in young rosettes. The causal mutation was determined to be a single, recessive mutation that was mapped to chromosome 3. Sequence analysis identified a single nucleotide change in the gene encoding SAMS3 that was distinct from the mto3-1 mutation and altered the amino acid sequence of the enzyme active site. This mutation was therefore referred to as mto3-2. Although Met over-accumulation in the mto3-2 mutant was similar to that in the mto2-1 mutant, CGS mRNA levels did not respond to the mto3-2 mutation and were similar to that in equivalent wild-type plants.

A dominant negative effect of eth-1r, a mutant allele of the Neurospora crassa S-adenosylmethionine synthetase-encoding gene conferring resistance to the methionine toxic analogue ethionine

eth-1r, a thermosensitive allele of the Neurospora crassa S-adenosylmethionine (AdoMet) synthetase gene that confers ethionine resistance, has been cloned and sequenced. Replacement of an aspartic amino acid residue (D48-->N48), perfectly conserved in prokaryotic, fungal and higher eukaryotic AdoMet synthetases, was found responsible for both thermosensitivity and ethionine resistance conferred by eth-1r. Gene fusion constructs, designed to overexpress eth-1r in vivo, render transformant cells resistant to ethionine. Dominance of ethionine resistance was further demonstrated in eth-1+/eth-1r partial diploids carrying identical gene doses of both alleles. Heterozygous eth-1+/eth-1r cells have, at the same time, both the thermotolerance conferred by eth-1+ and the ethionine-resistant phenotype conferred by eth-1r. AdoMet levels and AdoMet synthetase activities were dramatically decreased in heterozygous eth-1+/ eth-1r cells. We propose that this negative effect exerted by eth-1r results from the in vivo formation of heteromeric eth-1+/eth-1r AdoMet synthetase molecules.

Characterization of soybean tissue culture cell lines resistant to methionine analogs

Several hundred soybean [Glycine max (L.) Merr.] cell lines resistant to ethionine were isolated either with or without chemical mutagenesis. of these, 26 were found to contain 2 to 22 times higher than normal levels of uncombined methionine. These 26 cell lines also contained higher than normal levels of S-adenosylmethionine and S-methylmethionine, but the levels of free lysine, threonine, cysteine, valine, tyrosine and phenylalanine were not elevated. Isoleucine levels were only slightly elevated. These results suggest that the regulation of methionine synthesis in vivo is more likely to be later in the pathway (after homoserine phosphate) than early in the pathway.

Genetic mapping and physiological consequences of metE mutations of Bacillus subtilis

Three metE mutations of Bacillus subtilis, which cause cells to have a 25- to 200-fold decrease in L-methionine S-adenosyltransferase (EC 2.5.1.6) activity, were mapped between bioB and thr. The corresponding three metE mutants contained three- to fourfold less intracellular S-adenosylmethionine (SAM) but at least sevenfold more methionine than the metE+ strain when grown in synthetic medium. This indicates a strong feedback control of SAM on its synthesis. However, only the metE2 strain, with the lowest SAM concentration, grew at a slightly lower rate than the parent, which showed that an intracellular concentration of about 25 microM SAM was critical for growth at the normal rate. Neither DNA methylation (measured by bacteriophage luminal diameter 105 restriction) nor sporulation was affected at this low SAM concentration. Addition of methionine to the growth medium caused an increase in the pool of SAM in some but not all metE mutants. Coaddition of adenine did not change this result. However, the extent of sporulation (induced by mycophenolic acid) was decreased 50-fold in all mutants by the addition of methionine and adenine. Therefore, the combination of methionine and adenine suppresses sporulation regardless of whether it causes an increase in the level of SAM.

Inhibition of leucine transport in Saccharomyces by S-adenosylmethionine

S-Adenoxyl-L-methionine (SAM) inhibited leucine transport in Saccharomyces cerevisiae. By using a mutant defective in the active transport of SAM, we demonstrated that the inhibitory effect was exerted at an extracellular site. Cells preincubated wtih SAM for 120 min became refractory to its inhibitory effect, which was not a result of either the active transport or the metabolism of SAM. The quantitative recovery of labeled SAM from the incubation medium indicated that SAM, and not a metabolite, was the true inhibitory molecule. S-Adenosyl-L-homocysteine and S-adenosyl-L-ethionine also functioned as inhibitors of leucine transport, whereas S-adenosyl-D-methionine, S-adenosyl-D-homocystein, 5'-methylthioadenosine, 5'-dimethylthioadenosine, and adenosine lacked this property. Kinetic studies demonstrated that SAM was a competitive inhibitor of leucine transport.

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.

Specificity and genetics of S-adenosylmethionine transport in Saccharomyces cerevisiae

The specificity of a transport system for S-adenosylmethionine was determined through the use of structurally related derivatives. Of the compounds tested, the analogues S-adenosylethionine and S-inosylmethionine and the naturally occurring compounds S-adenosyl-(5')-3-methylthiopropylamine and S-adenosylhomocysteine competitively inhibited uptake of the sulfonium compound. Ki values for these compounds indicate that the order of affinity for the transport protein is S-adenosylmethionine congruent to S-adenosyl-(5')-3-methyl-thiopropylamine greater than S-adenosylethionine greater than S-inosylmethionine greater than S-adenosylhomocysteins. S-adenosyl-(2-hydroxy-4-methylthio)butyric acid exerted inhibition of a mixed type. S-insoyl-(2-hydroxy-4-methylthio)butyric acid, S-inosylhomocysteine, and S-ribosylhomocysteine were without effect. On the basis of the inhibition data, the methionine-amino, adenine-amino, and methyl groups were identified as group important in the binding of S-adenosylmethionine to the transport protein. Comparison is made with the specificities of various transmethylating enzymes utilizing S-adenosylmethionine. In addition, a number of conventional and temperature-sensitive S-adenosylmethionine transport mutants were isolated and analyzed in an attempt to identify the structural character of the specific transport protein(s). The data obtained suggest that only a single gene (a single polypeptide) is involved in specific S-adenosylmethionine transport. Apparent interallelic complementation supports the assumption that the functional form of the protein is composed of two or more copies of a monomer.

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.

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.