Induction of β-Galactosidase in Lactobacillus plantarum

Abstract

β-galactosidase (β-galactoside galactohydrolase, EC 3.2.1.23) is inducible in Lactobacillus plantarum by d -galactose or thiomethyl galactoside, and to a much lesser extent by lactose, isopropyl thiomethyl galactoside, and d -fucose. Isopropyl thiomethyl galactoside is a competitive inhibitor of the enzyme with a K i of 4.2 mM. The K m of the crude enzyme for o -nitrophenyl β- d -galactoside is 0.87 mM. Induction also requires a source of energy and amino acids. Chloramphenicol and actinomycin D inhibited induction. d -Glucose, d -fructose and to a lesser extent maltose and d -mannitol inhibited enzyme synthesis. Methyl-alpha- d -glucopyranoside was not inhibitory. Glucose exerts its effect through its ability to exclude galactose or lactose entry into the cell. The uptake of lactose and the metabolism of galactose by preinduced cells is severely inhibited by glucose. But neither galactose nor lactose severely affected the uptake of glucose by preinduced cells. Thus, glucose acts through catabolite inhibition, i.e., transport of inducer rather than repression through transcription or related mechanisms. This is supported by the inability of cyclic nucleotides to relieve the inhibition produced by glucose or to stimulate induction. Furthermore, intracellularly produced glucose did not inhibit enzyme synthesis. Acetate and mevalonate, the precursors of membrane lipids, stimulate induction independently of their effect on growth. Homobiotin partially abolished the acetate effect but did not inhibit induction or growth.