ORIGINAL PAPER Angel F. Maris á Andre L. K. AssumpcËaÄo á Diego Bonatto Martin Brendel á JoaÄo AntoÃnio P. Henriques Diauxic shift-induced stress resistance against hydroperoxides in Saccharomyces cerevisiae is not an adaptive stress response and does not depend on functional mitochondria Received: 8 September 2000 / Accepted: 5 January 2001 / Published online: 22 March 2001 Ó Springer-Verlag 2001 Abstract Respiring Saccharomyces cerevisiae cells grown on a non-fermentable carbon source are intrinsically more resistant to several stresses, including oxidative stress. The mechanisms leading to increased stress re- sistance are not yet well understood. Active mitochon- dria are the major source of intracellular reactive oxygen species ROS), which could cause the up-regulation of the antioxidant defense systems. We investigated the role of mitochondria in the intrinsic stress resistance against the hydroperoxides H 2 O 2 and tert-butylhydroperoxide 4 h after a shift in carbon source. We found that, inde- pendently of functional mitochondria, the yeast acquired the intrinsic resistance of respiring cells against hydroperoxides solely as a response to a change of carbon source in the growth medium. Furthermore, utilizing reporter gene fusion constructs, we monitored the expression of the c-glutamylcysteinyl synthetase encoded by GSH1) and the two superoxide dismutases encoded by SOD1 and SOD2) during the metabolic transition from fermentation to respiration; and we detected an up-regulation of all three genes during the diauxic shift. Overall available data allowed us to propose that the antioxidant system of S. cerevisiae could be considered as a class of genes under glucose/ carbon catabolite regulation. This control system is dierent from the well-known adaptive response to oxidative stress. Keywords Mitochondria á Oxidative stress á Glucose repression á Adaptive response Introduction In air, Saccharomyces cerevisiae has the ability to grow with either anaerobic or aerobic metabolism, depending on the carbon source. When abundant glucose is avail- able, this yeast shuts down all alternative carbon source- utilizing processes and catabolizes glucose mainly by fermentation reviewed in Entian and Barnett 1992). In rich yeast extract/peptone/dextrose; YPD) media, glu- cose consumption allows a generation time of about 90 min during the exponential phase of growth ExpG), generating ethanol as the major by-product. When glu- cose concentration falls below 0.2%, cells stop dividing for a few hours and, by altered gene expression, several metabolic processes change and allow the de novo bio- synthesis of functional mitochondria Pon and Schatz 1991; De Winde et al. 1996, 1997). After this lag-phase in the growth curve called the diauxic shift), growth is resumed at a slower rate 3±4 h generation time in YPD media) and cells now consume, by respiration, the ethanol, glycerol, and other by-products from the for- mer glucose catabolism Westerbeek-Mares et al. 1988; Choder 1993). After the cells double a further 2±3 times, the culture's cell density is so high that essential nutri- ents are exhausted. Yeast cells cease dividing and adapt to starvation conditions, entering a stress-resistant stage known as the stationary phase stat-phase; Fuge and Werner-Washburne 1997). Glucose per se is a powerful signaling molecule in yeast. Its presence or absence in culture media alone is sucient to trigger the major metabolic changes of the diauxic transition. Therefore, cells can be forced into a diauxic shift by replacing a glucose-rich medium with a glucose-free medium containing a non-fermentable car- bon source, i.e., ethanol or glycerol. However, as the natural diauxic shift-induced changes begin well before complete glucose exhaustion, this allows a dierential response of groups of genes and hence metabolic processes) to dierent thresholds of glucose limitation De Winde et al. 1997). The forced diauxic shift, with its Curr Genet 2001) 39: 137±149 DOI 10.1007/s002940100194 Communicated by: K. Wolf A. F. Maris á A. L. K. AssumpcË aÄ o á D. Bonatto J. A. P. Henriques &) Departamento de BiofõÂsica, Centro de Biotecnologia, UFRGS, Av. Bento GoncËalves, 9500 Porto Alegre, Rio Grande do Sul, Brazil E-mail: pegas@dna.cbiot.ufrgs.br M. Brendel Institut fuÈr Mikrobiologie, J.W. Goethe-UniversitaÈt, Theodor-Stern-Kai 7, Haus 75, 60590 Frankfurt, Germany