Effects of three mutant genes, and , on catabolite repression of mitochondrial cytochromes and the first two enzymes of haem biosynthesis were compared. The mutation gave no resistance to glucose, whereas endowed both cytochromes and 5-aminolaevulinate dehydratase with resistance, but did not alter the effect of glucose on 5-aminolaevulinate synthase. The mutation caused repression resistance of cytochromes and of the two haem biosynthetic enzymes. strains also accumulated intracellular 5-aminolaevulinate. Co-inheritance of the latter traits, sensitivity to maltose inhibition and ability to grow on raffinose in the presence of 2-deoxyglucose, demonstrated that the pleiotropic phenotype is a function of the single gene . Revertants which grew on maltose regained sensitivity to deoxyglucose and exhibited normal sensitivity of cytochromes and haem biosynthesis enzymes to repression. Addition of the mutation, which renders cytochromes resistant to repression, to a strain did not produce the same effect on 5-aminolaevulinate synthase as . It is concluded that repression patterns of haem and cytochrome biosynthesis are substantially affected by and but not by .


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