FIG. 3.

Model to explain the effects of lacR mutations on succinate-mediated repression of α-galactoside utilization. The model depicts the α-galactoside (α-gal) and β-galactoside (β-gal) transport systems when wild-type (top) and lacR mutant (bottom) cells are growing on succinate plus an α-galactoside. The model postulates that succinate-mediated catabolite repression of the melA-agp operon is due, in part, to succinate causing inducer exclusion of α-galactosides. In the lacR mutant, the overproduction of β-galactoside transporters would allow some α-galactosides into the cell, thus short-circuiting succinate-mediated repression. The details of the model are described in the text.