Evolution of a Second Gene for β-Galactosidase in Escherichia coli

Mutants ofE. coli K12 with deletions of the β-galactosidase gene (lacZ ) can reacquire the ability to hydrolyze β-galactosides during prolonged intense selection for growth on lactose. Full lactose competence is restored through a sequence of at least five mutations. Cell extracts of these derived strains hydrolyzeo -nitrophenyl-β-D-galactoside, the standard substrate for assay of β-galactosidase. The enzyme responsible for this activity differs in its immunological, kinetic, and sedimentation characteristics from thelacZ β-galactosidase of wild-typeE. coli . Its genetic determinant, designatedebg -5, maps at 59 min on theE. coli chromosome, whereas thelac operon maps at 10 min. We suggest that a gene not involved in lactose utilization has been progressively changed into a form capable of specifying a β-galactosidase and that this process is similar to that whereby genes with new functions are evolved by natural selection.