Genetic and biochemical evidence of sucrose fermentation by maltase in yeast

Summary Strain 1403-7A, which carries the MAL4 gene responsible for constitutive maltase synthesis, can ferment sucrose in the absence of sucrose genes. Sucrose fermentation cannot be separated from maltose fermentation either by genetic recombination or by mutation. Crude extracts of strain 1403-7A also lack the classical invertase, and fractionation of such extracts by gel filtration results in a peak of maltase activity which corresponds exactly to the activity with respect to sucrose hydrolysis. Moreover, in vitro, both of these disaccharides are hydrolyzed maximally at pH 6.4 to 6.8. It is suggested that, as long as sucrose can penetrate the cell, maltase, if present at high level in any strain, should be able to hydrolyze sucrose and therefore permit its fermentation. We have, however, identified in one of our yeast stocks a single recessive gene (ssf gene) which specifically interferes with sucrose fermentation in strain 1403-7A, probably by limiting the penetration of sucrose.