A number of strains varying in steady-state level of assembled bc1 complex were used to test the conclusions from inhibitor titration experiments with isolated mitochondria that, in cells of Saccharomyces cerevisiae grown on non-fermentable carbon sources, the control coefficient of the bc1 complex on the mitochondrial respiratory capacity equals 1 and the respiratory chain consists of supermolecular respiratory units [Boumans, Grivell and Berden (1998) J. Biol. Chem. 273, 4872–4877]. In addition, the control coefficient of mitochondrial respiration on the growth rate was determined. It was found that a reduced level of bc1 complex is accompanied by an almost parallel decrease in steady-state level of cytochrome c oxidase. Since the linear relationship between level of active bc1 complex and respiratory capacity still holds, it is concluded that cytochrome c oxidase has disappeared from respiratory units that are already deficient in the bc1 complex and that the cytochrome c oxidase in a respiratory unit is destabilized when the bc1 complex is deficient. The control coefficient of the bc1 complex, and thus of mitochondrial electron-transfer capacity, on respiration of intact cells (without uncoupler added) is 0.20. Addition of uncoupler results in an increase in the coefficient to 0.36. Thus changing the respiratory state changes the distribution of control, increasing the control coefficient of electron-transfer activity as the respiratory state goes towards State 3u. Rates of growth of the strains on different carbon sources were determined and subsequently fitted to calculate control coefficients of the bc1 complex (and therefore of the respiratory capacity) on growth. Little variation was found between lactate-, ethanol- and glycerol-containing media, control coefficients being around 0.18 at pH 5. At pH 7 the control coefficient increased to 0.57, indicative of a higher dependence of the cell on ATP derived from oxidative phosphorylation. During growth on glucose-containing medium, the bc1 complex has no control on the growth rate, as indicated by the fact that all strains, including a respiratory-deficient strain, grow as fast as the wild-type. However, the presence of respiratory capacity in the wild-type does result in a higher growth yield compared with the respiratory-deficient strain, indicating that, in contrast with what is generally assumed, in S. cerevisiae the ‘Pasteur effect ’ is not restricted to special experimental conditions.

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