Cytochrome glucose repression

Another derepressive carbon source is melibiose, which was used in studies of mutants capable of achieving various levels of cytochrome c [37,39]. It was found that mutants low in cytochrome c are particularly sensitive with respect to glucose repression of cytochrome a. Diminished cytochrome a synthesis can, therefore, be a secondary effect of other cytochrome deficiences [37,39]. 

As noted above, all S. cerevisiae pleiotropic mutants described so far produce secondary p mutations at very high rates. This problem is easily solved by the use of S. pombe, where several distinct classes of chromosomal pleiotropic respiratory-deficient mutants have been described to date (Table V). Curiously, all pleiotropic strains of S. pombe reported so far are deficient in cytochrome aa and/or cytochrome oxidase. Early reports of additional deficiencies of the respiratory chain were hardly quantitative because of the poor basic knowledge of the respiratory chain in S. pombe and imperfect control of the interference produced by glucose repression. More recent papers report deficiencies in a cytochrome 561.5.Two types of ATPase deficiencies were also reported (a) the total absence or presence in reduced amount, of Dio-9-sensitive ATPase, and (b) the oligomycin-insensitive ATPase. 

The low-temperature spectrum of dithionite-reduced mutant cells and mitochondrial particles demonstrates the virtual absence of the 606 nm absorption peak of cytochrome aa in the mutant (Figs. 3D, 4D, and 8). Cytochrome c is clearly detected at 547 nm in intact mutant cells (Fig. 3D) and in intact mitochondria (Fig. 8). Cytochrome Ci is best observed at 551 nm in mitochondrial fractions of derepressed cells from which cytochrome c has been extracted (Fig. 4D). On the other hand, cytochrome and cytochrome 6555.5 in the mutant are best observed in submitochondrial particles from glucose-repressed cells. Small amounts of cytochromes 6557.5 and 6560 have also been detected in mitochondria from stationary cells grown in 1% glucose (Fig. 8). Cytochrome c oxidase activity of the mutant is about 2-5% of the activity of the wild-type in exponential as well as in stationary phase, and thus confirms the lack of a detectable cytochrome aa absorption peak. On the other hand, the marked decrease of succinate cytochrome c oxidoreductase, which was reported previously in repressed mutant homogenates, is somewhat less pronounced in fresh, intact mitochondria of derepressed mutant cells which exhibit 10-15% of the activity of the wild type. 

This is the reverse Pasteur or Crabtree effect and is also known as glucose inhibition or cataboHte repression. In the presence of higher sugar concentrations, synthesis of respiratory enzymes such as cytochromes is inhibited. 

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