Cadmium is very toxic at low concentrations, but the basis for its toxicity is not clearly understood. We analyzed the proteomic response of yeast cells to acute cadmium stress and identified 54 induced and 43 repressed proteins. A striking result is the strong induction of 9 enzymes of the sulfur amino acid biosynthetic pathway. Accordingly, we observed that glutathione synthesis is strongly increased in response to cadmium treatment. Several proteins with antioxidant properties were also induced. The induction of nine proteins is dependent upon the transactivator Yap1p, consistent with the cadmium hypersensitive phenotype of the YAP1-disrupted strain. Most of these proteins are also overexpressed in a strain overexpressing Yap1p, a result that correlates with the cadmium hyper-resistant phenotype of this strain. Two of these Yap1p-dependent proteins, thioredoxin and thioredoxin reductase, play an important role in cadmium tolerance because strains lacking the corresponding genes are hypersensitive to this metal. Altogether, our data indicate that the two cellular thiol redox systems, glutathione and thioredoxin, are essential for cellular defense against cadmium.Heavy metals represent major environmental hazards to human health. In particular, cadmium is very toxic and probably carcinogenic at low concentrations. However, the biological effects of this metal and the mechanism of its toxicity are not yet clearly understood. It has been proposed that Cd 2ϩ ions might displace Zn 2ϩ and Fe 2ϩ in proteins (1), resulting in their inactivation and in the release of free iron, which might generate highly reactive hydroxyl radicals (OH ⅐ ) (2). In support of this hypothesis, a major effect of cadmium is oxidative stress (3), particularly lipid peroxidation (1). However, it is not known whether these effects are responsible for the extreme toxicity of the metal.Living organisms use several mechanisms to counter cadmium toxicity. In bacteria, efflux pumps are able to export toxic ions outside the cell (4). In higher eukaryotes, Cd 2ϩ is sequestered by metallothioneins through their high cysteine content (5). Cadmium can also be detoxified by chelation to GSH or to phytochelatin, a glutathione polymer of general structure (␥-Glu-Cys) n -Gly synthesized from GSH in plants and in the yeast Schizosaccharomyces pombe. Cd 2ϩ -phytochelatin and Cd 2ϩ ⅐ (GSH) 2 complexes are transported into the vacuole by ATPbinding cassette transporters (6 -8).Yap1p and Skn7p are yeast transcription factors that regulate the adaptive response to oxidative stress (9 -11). Strains lacking either transcription factor are sensitive to H 2 O 2 and are defective in the induction by H 2 O 2 of several enzymes with antioxidant properties (9). Yap1p is also important in cadmium tolerance because yap1-deleted strains are very sensitive to cadmium, and strains overexpressing YAP1 are hyper-resistant to this toxic metal (12). The contribution of Skn7p to the cadmium response is more complex, because skn7-deleted strains are hyper-resistant to cadmium (9)...

Oxygen is a major determinant of both survival and mortality of aerobic organisms. For the facultative anaerobe Lactococcus lactis, oxygen has negative effects on both growth and survival. We show here that oxygen can be beneficial to L. lactis if heme is present during aerated growth. The growth period is extended and long-term survival is markedly improved compared to results obtained under the usual fermentation conditions. We considered that improved growth and survival could be due to the capacity of L. lactis to undergo respiration. To test this idea, we confirmed that the metabolic behavior of lactococci in the presence of oxygen and hemin is consistent with respiration and is most pronounced late in growth. We then used a genetic approach to show the following. (i) The cydA gene, encoding cytochrome d oxidase, is required for respiration and plays a direct role in oxygen utilization. cydA expression is induced late in growth under respiration conditions. (ii) The hemZ gene, encoding ferrochelatase, which converts protoporphyrin IX to heme, is needed for respiration if the precursor, rather than the final heme product, is present in the medium. Surprisingly, survival improved by respiration is observed in a superoxide dismutase-deficient strain, a result which emphasizes the physiological differences between fermenting and respiring lactococci. These studies confirm respiratory metabolism in L. lactis and suggest that this organism may be better adapted to respiration than to traditional fermentative metabolism.The toxic cellular effects of oxygen are a major factor in aging and mortality (3, 28). Oxygen toxicity is attributed to the activity of reactive oxygen species that attack proteins, lipids, and nucleic acids (15). Effects of oxygen have been extensively studied by use of bacterial models, principally with the facultatively respiring bacterium Escherichia coli (see references 7 and 13 for reviews). In this model, respiration itself is implicated as a source of oxidative damage in E. coli (8, 9, 18, 20, 27, and 36). It has been suggested that the shutdown of respiration in nutrient-limited conditions may reduce reactive oxygen species levels and thereby improve E. coli survival. Recent evidence further suggests that survival is favored by shifting cells to anaerobic conditions during entry into stationary phase (9).Current information on the effects of oxygen is mainly based on respiring organisms. As such, the question of what anaerobes do in the presence of oxidative stress has been explored little. It is presumed that these organisms cope with stress in much the same way as aerobes, except that their defense systems, which may include superoxide dismutases (SODs) and catalases, may be more limited. However, there has been no demonstration to date that responses of anaerobes to an oxidative environment are predictable from the behavior of respiring bacteria.The effects of oxygen have been examined with Lactococcus lactis, a gram-positive facultative anaerobe with a fermentative metabolism that ca...