La levure, organisme modele de longue date, gardera ce statut a l'ere post-genomique : quatre ans apres la fin de son sequencage par un consortium de laboratoires travaillant en reseau, elle devient en effet l'organisme eucaryote le mieux compris.

Appropriate combination of specific inhibitors of electron transport in the cytochrome bc1 segment of the respiratory chain of Saccharomyces cerevisiae allows the rapid resolution of three spectral forms of mitochondrial cytochrome b. (1) Addition of 3-(3,4-dichlorophenyl)-1,1-dimethylurea (DCMU) to aerobic yeast submitochondrial particles preincubated with cyanide and mucidin in the presence of NADH reveals cytochrome b-561.5. (2) Addition of funiculosin to aerobic yeast submitochondrial particles preincubated with cyanide, mucidin and n-heptylhydroxyquinolineN-oxide in the presence of NADH reveals cytochrome b-558 independently of cytochrome b-561.5 and cytochrome b-565. (3) Specific resolution of cytochrome b-565 can be obtained either by addition of mucidin to aerobic submitochondrial particles preincubated with cyanide, DCMU and NADH, or by addition of antimycin plus an oxygen pulse to NADH-reduced particles, preincubated with cyanide, in the presence of ascorbate plus TMPD, or by addition of antimycin A in the presence of oxidized TMPD to aerobically NADH-reduced particles.

The mutant uvsρ 72 of Saccharomyces cerevisiae UV-sensitive for rho- production displays slower growth on media containing non-fermentable carbon sources such as glycerol or lactate. The slower growth on glycerol is not due to any deficiency in glycerol catabolism or mitochondrial oxidative phosphorylation. No modifications of the sensitivity to ethidium bromide of the mitochondrial ATPase activity could be detected. A mathematical model is presented which accounts for slower growth of uvsρ 72 on the sole basis of the continuous and elevated rho- production in the mutant strain. This model, which estimates the rate of mutation from the rate of growth and vice versa, has been verified experimentally in the case of uvsρ 72. The model has been generalised, so that it can be used for any microbial population subject to constant and high rates of any type of mutation providing that the mutant is stable, and either unable to grow or able to grow at this own rate different from that of the parental strain.

A kinetic method based on the measurement of NADH oxidase activity in yeast submitochondrial particles incubated in the simultaneous presence of an inhibitor of high affinity such as antimycin and an inhibitor of low affinity such as 3(3,4‐dichlorophenyl)‐1,1‐dimethylurea (diuron) permits estimation of two distinct dissociation constants for diuron, Kd.1= 3.2 μM and Kd,2= 21.5 μM. To take into account the fact that these inhibitors do not act on the rate‐limiting step of the NADH oxidase activity, the Kröger and Klingenberg kinetic model [Eur. J. Biochem. (1973) 39, 313–323] has been applied using a new method for estimating the maximal activities of the NADH: Q reductase activity (Vred= 280 nmol O × min−1× mg −1) and of the QH2: O2 oxidase activity (Vox= 850 nmol O × min−1× mg−1) (Q = ubiquinone). These latter values permit the estimation of two distinct dissociation constants for another weakly bound inhibitor, n‐heptyl‐hydroxyquinoline‐N‐oxide: Kh,1= 0.23 μM and Kh,2= 5 μM. Only one dissociation constant, Kd= 23 μM for diuron is sufficient to characterize the ‘extra‐reduction’ of cytochrome b566. Even though diuron does not induce the ‘red‐shift’ of cytochrome b562, the combined addition of suboptimal concentrations of n‐heptyl‐hydroxyquinoline‐N‐oxide and diuron, permits estimation of a single dissociation constant which is lower than 6.6 μM diuron for a site which can bind either diuron or n‐heptyl‐hydroxyquinoline oxide but which will yield a ‘red‐shift’ of cytochrome b562 when occupied by n‐heptyl‐hydroxyquinoline‐N‐oxide only.

3-(3,4-dichlorophenyl)-1,1-dimethylurea (diuron), an inhibitor of cytochrome b oxidation, has been used for the selection of three resistant mutants (diur) of Saccharomyces cerevisiae. The mutant diur-64 exhibits in vivo cross-resistance to antimycin A while diur-34 and diur-1 are more sensitive to antimycin A than the parental strain. The three mutants exhibit mitochondrial inheritance according to the following criteria: mitotic segregation of diuron-resistant and diuron-sensitive diploids is obtained among the diploid progeny of a cross between diur and dius; non-Mendelian segregation of diuron resistance (4:0) is observed in spores of tetrads issued from diuron-resistant diploid; extensive ethidium bromide treatment leads to the formation of Q- mutants which no longer transmit diur and dius alleles. Evidence for two distinct diuron-resistant loci were obtained by allelism tests. Recombination analysis shows that diuron-resistance is not located in the polar region of the mitochondrial genome. The diur loci are not linked to the erythromycin locus since the upper limit in recombinants frequency (26%) for a non-polar region is obtained between diur and eryr. A low recombinants frequency (3%) is observed in crosses between diur-34 mutation and the two mutants cob1 and cob2 suggesting that diur-34 might be located between these two cytochrome-b-deficient loci. The resistance to diuron is also expressed in vitro since the oxidation rates of succinate by sonicated submitochondrial particles from the mutants are clearly less sensitive to diuron than that of the wild type.

Two mutants modified in their sensitivity to glucose repression have been obtained from a “petite-negative” yeast species: Schizosaccharomyces pombe 972h−. Compared to glycerol grown cells, the respiration of cells grown in the presence of 10% glucose is decreased by a factor of 3 for the wild strain and by a factor of 24 for strain COB5 which is therefore designated as a superrepressed mutant. Respiration of a derepressed mutant COB6 is totally insensitive to glucose repression. The effects of glucose on the absorption peaks of respiratory pigments parallel the effects of glucose on respiration. Using a derepressing medium that limits cellular division, full restoration of respiration and respiratory pigments of the superrepressed strain was achieved.

6 stable mutants with modified mitochondrial ATPase activity were obtained from a petite-negative yeast Schizosaccharomyces pombe. All the mutants have lost oligomycin-sensitive ATPase. Compared to the wild strain, cells of the strains RD14, RD15, M126 and Res4 have lost about 75% of the particulate dio-9 sensitive ATPase activity. Two other strains: M53 and RD32 have lost more than 95% of the total dio-9 sensitive ATPase activity per cell. M126, Res4, M53 and RD15 were shown to be of chromosomal heredity. All mutants are pleiotropic and show various cytochrome deficiencies.