OCCURRENCE OF β-GALACTOSIDASE IN THE GENUS SHIGELLA

BETA-GALACTOSIDASE OF STREPTOCOCCUS LACTIS

Citti, J. E. (Oregon State University, Corvallis), W. E. Sandine, and P. R. Elliker. beta-Galactosidase of Streptococcus lactis. J. Bacteriol. 89:937-942. 1965.-Synthesis of beta-galactosidase by several strains of Streptococcus lactis was induced by lactose. The rate of hydrolysis of o-nitrophenyl-beta-d-galactopyranoside was used to measure enzyme activity. The enzyme of all but one strain was unstable when whole cells were sonic-treated or treated with toluene; the enzyme of one strain of S. lactis was stable to these treatments, which resulted in at least a fivefold increase in activity over that found in whole cells. The optimal assay conditions for toluene-treated cells of this strain involved incubation at 37 C in pH 7.0 sodium phosphate buffer. Lactose was the most effective inducer of enzyme synthesis. Methyl-beta-d-thiogalactopyranoside, isopropyl-beta-d-thiogalactopyranoside, and galactose were also inducers of the enzyme, but were not as effective as lactose. Melibiose, maltose, and calcium lactobionate were poor inducers of enzyme synthesis. Exogenously supplied glucose repressed enzyme synthesis. The means of control of induced beta-galactosidase synthesis in S. lactis was similar to that in Escherichia coli.

Induction and characterization of -galactosidase in an extreme thermophile

A thermostable beta-galactosidase (EC 3.2.1.23; beta-dgalactoside galactohydrolase) was found to be inducible in an extreme thermophile resembling Thermus aquaticus. Enzyme induction was achieved by the addition of lactose, galactose, or the alpha-galactoside, melibiose, to growing cultures. The addition of glucose to induced cultures had a repressive effect on further enzyme synthesis. The enzyme was purified 78-fold, and the optimum temperature and pH for activity were determined to be 80 C and pH 5.0, respectively. The enzyme was activated by both manganese and ferrous iron. Sulfhydryl activation and thermal stabilization indicate that the thermophilic beta-galactosidase is a sulfhydryl enzyme. Kinetic determinations at 80 C established a K(m) of 2.0 x 10(-3)m for the chromogenic substrate o-nitrophenyl beta-d-galactopyranoside (ONPG) and a K(1) of 7.5 x 10(-3)m for lactose. The Arrhenius energy of activation (for the hydrolysis of ONPG) was calculated to be 13.7 kcal/mole. A molecular weight of 5.7 x 10(5) daltons was estimated by elution of the enzyme from Sephadex 4B.

Delayed Lactose Fermentation by Enterobacteriaceae

Goodman , R. E. (University of California, Los Angeles), and M. J. Pickett . Delayed lactose fermentation by Enterobacteriaceae . J. Bacteriol. 92: 318–327. 1966.—When 171 Citrobacter freundii strains and 14 Paracolobactrum arizonae strains examined, 51 of the C. freundii strains and 13 of the P. arizonae strains were found to be delayed or negative lactose fermenters. Of the slow fermenters, 65% yielded rapidly fermenting mutants in cultures undergoing delayed fermentation. Lactose fermentation could generally be hastened by increasing lactose concentrations. Many organisms which fermented lactose slowly grew readily on a medium containing lactose as the sole carbon source. Regardless of their ability to ferment lactose, all strains of C. freundii and P. arizonae investigated could be shown to possess β-galactosidase. Delayed fermenters failed to take up lactose from the culture medium, whereas prompt fermenters did so readily. The β-galactosidases of 12 strains of enteric bacteria were studied in crude cell extracts with respect to specific activity, stability, and activity at varying substrate ( o -nitrophenyl-β- d -galactopyranoside) concentrations, at varying p H, and in the presence of sodium, potassium, and magnesium. The widely varying specific activities and the approximate similarity of the Michaelis constants (about 2 × 10 −4 m ) suggested that the strains investigated produced differing amounts of β-galactosidase. Moreover, qualitative differences in the enzymes provided evidence that these strains synthesized different molecular forms of β-galactosidase. The results suggested that organisms which ferment lactose only after a prolonged delay do so because they possess multiple defects in their lactose-metabolizing machinery.

Properties and regulation of the beta-D-galactosidase in Shigella dysenteriae and in Escherichia coli-Shigella dysenteriae hybrids

Sarkar, S. (Massachusetts Institute of Technology, Cambridge). Properties and regulation of the beta-d-galactosidase in Shigella dysenteriae and in Escherichia coli-Shigella dysenteriae hybrids. J. Bacteriol. 91:1477-1488. 1966.-Shigella dysenteriae strain 60 has a beta-d-galactosidase related to that of Escherichia coli but more heat-sensitive and with a turnover number about 10 times lower. Hybridization by transduction produces strains with enzymes of intermediate properties by recombination within the z gene. Both E. coli and S. dysenteriae have a regulatory i(+) gene. Recombination between i(-) mutants of the two organisms leads to restoration of the i(+) genotype. In S. dysenteriae 60, most of the i(-) mutants are subject to genetic suppression by suppressor mutations at unlinked loci. The effect of these suppressors on the products of the suppressed i(-) genes is discussed.