S. cerevisiae and sulfur: a unique way to deal with the environment.

Saccharomyces cerevisiae is by far the best-studied unicellular eukaryote. Although yeast cells are very similar to higher eukaryotes in many respects, there is striking evidence that S. cerevisiae is not a perfect model for a eukaryotic cell (cf. 1). Here we report that yeast proteins contain a significantly lower amount of cysteine residues compared to other eukaryotes. Explanations for this phenomenon could not be found in the sulfur metabolism of yeast, which showed no major differences from other organisms (2-4). However, previous examinations could link a defect in sulfate uptake of S. cerevisiae to an increased resistance against toxic substances like selenate and chromate in the environment, which share the same permeases (5-7). This environmental problem might have caused S. cerevisiae to down-regulate its sulfate uptake and therefore lead to a lower amount of available sulfur in the cell, making it necessary to replace all dispensable sulfur amino acids in proteins. We show in two examples that S. cerevisiae proteins contain only such cysteine residues that are structurally or functionally needed. Therefore, we conclude that S. cerevisiae has solved a widespread environmental problem in a specific way which might be unique among eukaryotes.