GENE DOSAGE AND GALACTOSE UTILIZATION BY SACCHAROMYCES TETRAPLOIDS.

In yeast, the enzymes required for steps (1) to ( 3 ) are “induced” enzymes which are synthesized only if galactose is present in the medium (DE ROBICHONSZULMAJSTER 1958). The nonallelic genes GA, and GA, are required in our strains of Saccharomyces for the rapid fermentation of galactose. GA, appears to control the transfer of galactose into the cell (DOUGLAS and CONDIE 1954) ; in the absence of GA,, utilization of this sugar is markedly dependent upon its concentration in the medium. The absence of GA, results, however, in a complete block of fermentation and respiration of galactose, since the cells cannot synthesize galactokinase (DE ROBICHON-SZULMAJSTER 1958). NELSON (1953) has shown that when glucose-grown cells are adapted aerobically to respire galactose, the required time of exposure to the inducer before rapid oxidation occurs decreases as the dosage of GA, increases. Further, in cells which are adapted anaerobically to galactose, the rate of fermentation also in-