Cystathionine Metabolism in Methionine Auxotrophic and Wild-Type Strains of Saccharomyces cerevisiae

Abstract

The role of cystathionine in methionine biosynthesis in wild-type and auxotrophic strains of Saccharomyces cerevisiae was studied. Homocysteine and cysteinerequiring mutants were selected for detailed study. Exogenously supplied cystathionine, although actively transported by all strains tested, could not satisfy the organic sulfur requirements of the mutants. Cell-free extracts of the wild-type, homocysteine, and cysteine auxotrophs were shown to cleave cystathionine. Pyruvic acid and homocysteine were identified as the products of this cleavage. A mutant containing an enzyme which could cleave cystathionine to homocysteine in cellfree experiments was unable to use cystathionine as a methionine precursor in the intact organisms. The significance of this finding is discussed.

Adaptive Biosynthesis of Thiazole and Bypassing of the Thiazole Requirement in Exobasidium by Acetyl Phosphate, Tween and Ethanol

“…In earlier studies of the thiazole requirement in Exobasidium (Sundstrom 1968) it was stated that neither homoserine + cysteine nor cystathionine added to culture media of thiazole-or methionine-requiring Exobasidium could replace methionine. The inability to utilize cystathionine as precursor in methionine synthesis may depend on lack of uptake or on the incapacity to utilize exogenously supplied cystathionine, as has been shown for yeast (Sorsoli et al 1968). Further, it is probable that Exobasidium in thiamine-deficient glucosehomoserine media produces insufficient amounts of acetyl CoA.…”

Influence of Methionine Pool Composition on the Formation of Methyl-Deficient Transfer Ribonucleic Acid in Saccharomyces cerevisiae

“…1. Although mostly taken from work on S. cerevisiae (3,6,7,29,32), this scheme is partly inferred from work on other fungi (13,25). In particular, the role of cystathionine is at present unclear; although it may be taken up by yeast cells (29), it will not support growth of any methionine auxotroph thus far described (21,22).…”

“…The sulfhydration reaction has been shown to occur in S. cerevisiae, and cystathionine may not be an obligatory intermediate in methionine biosynthesis in these species (3). S. cerevisiae is, however, known to contain ,-cystathionase for the cleavage of cystathionine to homocysteine and pyruvate (5,29). Homocysteine is methylated to yield methionine, utilizing a folate derivative as methyl donor (2, 8; E. Burton and W. Sakami, Fed.…”