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The effect of individual environmental conditions (pH, pO(2), temperature, salinity, concentration of ethanol, propanol, tryptone and yeast extract) on the specific growth rate as well as ethanol and glycerol production rate of Saccharomyces cerevisiae S288C was mapped during the fermentative growth in aerobic auxo-accelerostat cultures. The obtained steady-state values of the glycerol to ethanol formation ratio (0.1 mol mol(-1)) corresponding to those predicted from the stoichiometric model of fermentative yeast growth showed that the complete repression of respiration was obtained in auxostat culture and that the model is suitable for calculation of Y(ATP) and Q(ATP) values for the aerobic fermentative growth. Smooth decrease in the culture pH and dissolved oxygen concentration (pO2) down to the critical values of 2.3 and 0.8%, respectively, resulted in decrease in growth yield (Y(ATP)) and specific growth rate, however the specific ATP production rate (Q(ATP)) stayed almost constant. Increase in the concentration of biomass (>0.8 g dwt l(-1)), propanol (>2 g l(-1)) or NaCl (>15 g l(-1)) lead at first to the decrease in the specific growth rate and Q(ATP), while Y(ATP) was affected only at higher concentrations. The observed decrease in Q(ATP) was caused by indirect rather than direct inhibition of glycolysis. The increase in tryptone concentration resulted in an increase in the specific growth rate from 0.44 to 0.62 h(-1) and Y(ATP) from 12.5 to 18.5 mol ATP g dwt(-1). This study demonstrates that the auxo-accelerostat method, besides being an efficient tool for obtaining the culture characteristics, provides also decent conditions for the experiments elucidating the control mechanisms of cell growth.

Molecular mechanisms leading to glutathione (GSH) over-accumulation in a Saccharomyces cerevisiae strain produced by UV irradiation-induced random mutagenesis were studied. The mutant accumulated GSH but also cysteine and γ-glutamylcysteine in concentrations that were several fold higher than in its wild-type parent strain under all studied cultivation conditions (chemostat, fed-batch, and turbidostat). Transcript analyses along with shotgun proteome quantification indicated a difference in the expression of a number of genes and proteins, the most pronounced of which were several fold higher expression of CYS3, but also that of GSH1 and its transcriptional activator YAP1. This together with the higher intracellular cysteine concentration is most likely the primary factor underlying GSH over-accumulation in the mutant. Comparative sequencing of GSH1 and the fed-batch experiments with continuous cysteine addition demonstrated that the feedback inhibition of Gsh1p by GSH was still operational in the mutant.