SUMMARY: A wild-type strain of the yeast was grown in continuous culture on inulin-, fructose- and glucose-limited media. In the presence of organic nitrogen, all three carbon sources supported an extremely rapid changeover to a homogeneous mutant cell population that exhibited hyperproduction of β-fructofuranosidase (exo-inulase). The non-specific role of the carbon substrate in the takeover suggested co-regulation of inulase synthesis and early glycolytic pathway enzymes such as hexokinase, resulting in gratuitous hyperproduction of inulase. The rate of mutant appearance increased linearly with a decrease in dilution rate as well as carbon/nitrogen ratio, implying that the responsible forces were related to the residual concentration of the limiting carbon substrate. Under optimum conditions, mutant cells were detected within only 40 h (2.9 generations) from the start of continuous culture, at least 6.5 times faster than predicted. The maximum rate of displacement was as high as 54% per generation, or more than 12 times greater than predicted from the differential growth rate between the wild-type and mutant strain. These extraordinarily rapid takeovers point to a very fast and ongoing genetic change triggered by growth under severe stress.


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