Mutations conferring quantitative and qualitative increases in β-galactosidase activity inEscherichia coli

Summary

Sodium lactobionate is not utilized as a carbon source byEscherichia coli because it is only poorly bound and hydrolyzed by β-galactosidase and it does not induce the formation of the enzyme. However, treatment with N-methyl-N′-nitro-N-nitrosoguanidine produced 32 independent mutants able to grow on lactobionate. Most of the mutants formed β-galactosidase constitutively, 29 of them having mutations in the regulatory gene and one possibly in the operator. In addition, the mutants possessed quantitatively—or qualitatively—altered β-galactosidase. In 28 mutants the β-galactosidase activity was 1.5 to 4.5 times that of the wild-type. The enzymes of these mutants were unaltered in thermostability and substrate binding. One enzyme that was titrated immunologically possessed a molecular activity indentical with the wild-type enzyme. These mutants appear to contain extra copies of the gene for β-galactosidase. The spontaneous mutation rate to constitutivity was 6.3x10^-3 and to the formation of apparently extra genes, 9.2x10^-3.

The β-galactosidases of three mutants were qualitatively changed as judged from their increased thermosensitivity, altered substrate-binding constants and greatly increased ability to hydrolyze lactose and lactobionate. Affinity for 0-nitrophenyl-β-galactoside and galactose was increased by the mutations while that for lactose was decreased; maximum velocities for the hydrolysis of 0-nitrophenyl-β-galactoside were also decreased. Relative to their rates of hydrolysis of 0-nitrophenyl-β-galactoside, these altered enzymes hydrolyzed lactose at 6 to 8 times, and lactobionate up to 23 times, the rate given by the normal enzyme. The mutations appear to increase the hydrophobic nature of the enzyme near the aglycon binding site and facilitate the hydrolysis of more hydrophilic galactosides. The lactobionic acid positive character could be transferred to other bacteria by sexual conjugation when the enzyme changes were qualitative, but not when they were quantitative.