Methionine biosynthesis from the 4-carbon skeleton of ethionine in Saccharomyces cerevisiae

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.

Methionine adenosyltransferase and ethionine resistance in Saccharomyces cerevisiae

The methionine adenosyltransferase is repressed in Saccharomyces cerevisiae during growth in the presence of excess methionine. The relationship of this repression to the level of intracellular S-adenosylmethionine is discussed. In conjunction with these studies, an ethionine-resistant mutant has been investigated which has a low level of methionine adenosyltransferase under all conditions tested. The mechanism of ethionine resistance in the latter strain apparently depends on its inability to form large quantities of intracellular S-adenosylethionine. With respect to the methionine adenosyltransferase, there is no apparent interaction between ethionine-resistant and ethionine-sensitive alleles when both are present in the heterozygous diploid.

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.