SUMMARY: The Carbondale yeast stock contains cultures which are characterized by the delayed fermentation of galactose. Long-term adaptation to galactose results from gene-mutation followed by the selection of the mutant in the presence of galactose. The original non-fermenter and the derived fermenter mutant contain no galactozymase and possess no selective advantage over each other when grown in glucose. Three different methods (variance analysis, fluctuation-redistribution tests, and chemostat experiments) which have been used with success in the detection of spontaneous mutations showed that the mutation to ability to ferment galactose did not occur spontaneously in these stocks. Exposure of non-dividing non-fermenter cells of the long-term fermenter to galactose induced the mutation to the fermenter type in a small but relatively constant number of cells, while exposure to glucose, ribose or lactose was ineffective; dividing cells were not susceptible to themutagenic action of galactose. Prolonging the exposure of the cells on galactose agar plates increased the number of mutations. It is inferred that the induction of the mutation results from the impingement of galactose on the gene-surface thus modelling the gene-surf'ace into a mirror-image of the substrate.


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