Physiological Studies of β-Galactosidase Induction in Kluyveromyces lactis

Abstract

We examined the kinetics of β-galactosidase (EC 3.2.1.23) induction in the yeast Kluyveromyces lactis . Enzyme activity began to increase 10 to 15 min, about 1/10 of a cell generation, after the addition of inducer and continued to increase linearly for from 7 to 9 cell generations before reaching a maximum, some 125- to 150-fold above the basal level of uninduced cells. Thereafter, as long as logarithmic growth was maintained, enzyme levels remained high, but enzyme levels dropped to a value only 5- to 10-fold above the basal level if cells entered stationary phase. Enzyme induction required the constant presence of inducer, since removal of inducer caused a reduction in enzyme level. Three nongratuitous inducers of β-galactosidase activity, lactose, galactose, and lactobionic acid, were identified. Several inducers of the lac operon of Escherichia coli , including methyl-, isopropyl- and phenyl-1-thio-β- d -galactoside, and thioallolactose did not induce β-galactosidase in K. lactis even though they entered the cell. The maximum rate of enzyme induction was only achieved with lactose concentrations of greater than 1 to 2 mM. The initial differential rate of β-galactosidase appearance after induction was reduced in medium containing glucose, indicating transient carbon catabolite repression. However, glucose did not exclude lactose from K. lactis , it did not cause permanent carbon catabolite repression of β-galactosidase synthesis, and it did not prevent lactose utilization. These three results are in direct contrast to those observed for lactose utilization in E. coli . Furthermore, these results, along with our observation that K. lactis grew slightly faster on lactose than on glucose, indicate that this organism has evolved an efficient system for utilizing lactose.