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.