Inducible System for the Utilization of β-Glucosides in Escherichia coli II. Description of Mutant Types and Genetic Analysis

Two types of mutants obtained by treating β- gl + cells with nitrosoguanidine are described. One type, β- gl + c , is constitutive for the biosynthesis of the aryl β-glucoside splitting enzyme(s) and for the β-glucoside permease; the other (β- gl + sal − ) has lost the capacity to ferment salicin, but has retained the capacity to ferment arbutin and other aryl β-glucosides. By two successive mutational steps, β- gl + sal − c double mutants can be obtained. Determinations of the enzymatic splitting of salicin and p -nitrophenyl β-glucoside by β- gl + sal − cells and extracts showed that these mutants have lost the capacity to split salicin but do split p -nitrophenyl β-glucoside; they possess the β-glucoside permease, and in them salicin is a gratuitous inducer for enzyme and permease biosynthesis. Studies on a β- gl + strain, which splits salicin as well as p -nitrophenyl β-glucoside, have shown that the splitting of salicin is more temperature-sensitive than that of p -nitrophenyl β-glucoside and other β-glucosides. Other properties of the two activities are similar. Interrupted mating experiments and cotransduction with P1kc phage showed that the genetic determinants of the β-glucoside system map between the pyrE and ile loci. Three distinct mutational sites were found and are presumed to have the following functions: β- glA , a structural gene for an aryl β-glucoside splitting enzyme; β- glB , either the structural gene for the β-glucoside-permease or a regulatory gene; and β- glC , a regulatory gene (or site). Escherichia coli wild-type strains are of the genotype A + B − C + . The β- gl + mutation determining the ability to ferment β-glucosides is considered to be a permease or regulatory mutation, and the resulting genotype is A + B + C + . The β- gl + sal − phenotype results from a mutation in the β- glA gene (genotype A′ B + C + ), and the constitutive phenotype results from a mutation in the β- glC gene, the genotypes A + B + C a and A′ B + C a corresponding to the phenotypes β- gl + c and β- gl + sal − c .