A mutant (nmr3) of Saccharomyces cerevisiae has been selected which is conditionally resistant to ethionine and thienylalanine. This nmr3 mutation differs from metp 1 and gap which are impaired in specific methionine permease and in general animo-acid permease, respectively, and from ethionine-resistant mutations sam 1 or sam 2 (Cherest et al., 1978). The nmr3 mutation has several pleiotropic effects: 1. Synthesis of catabolic NAD-specific glutamate dehydrogenase is impaired, specific enzyme activities under inducing conditions being about 10 to 20% of those observed in the wild-type strain. 2. Non-specific induction of arginase by e.g. leucine is absent. 3. Specific arginase activities in cultures with inducing nitrogenous compounds such as arginine or diamino acids are significantly lower than in the wild-type strain. 4. Nitrogen metabolite repression of arginase, ornithine transaminase and allantoinase by amino acids is greatly relieved, while that by ammonia is reinforced in some cases. The latter effect may be due to an increased inhibition of inducer uptake by ammonia. 5. The nmr3 mutant is resistant to ethionine and thienylalanine and to a lesser extent to some other toxic amino-acid analogues in glucose-ammonia medium; in glucose-proline medium the mutant is not resistant but in some cases more sensitive than the wild-type strain. NMR3 appears to represent a control element of nitrogen metabolism exerting effects on enzyme induction, on nitrogen metabolite repression and on amino-acid accumulation.

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Journal Article

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Middelhoven WJ, Hoogkamer-te Niet MC

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