lacY mutant of Escherichia coli with altered physiology of lactose induction.

A mutant of Escherichia coli is described that grew on lactose only in the presence of isopropylthiogalactoside. This cell contained a defect in the lacY gene that resulted in the formation of a transport system with a poor affinity for lactose. The inability to grow on lactose alone was due to the failure of induction by this disaccharide. This failure of inducation was presumably due to a defect in lactose accumulation which resulted in significant reduction in the formation of allo-lactose, the true inducer of lac operon. These results are consistent with the view that the capacity to accumulate lactose plays an important physiological role in the induction of the enzymes necessary for its utilization.