Molecular cloning and expression in E. coli of a yeast gene coding for beta-galactosidase

The yeast Kluyveromyces lactis synthesizes a beta-galactosidase (EC 3.2.1.32) which is inducible by lactose. We have isolated the gene that codes for this enzyme using recombinant DNA techniques. K. lactis DNA was partially digested with the restriction endonuclease Eco R1 and joined to Eco R1-digested pBR322 plasmid DNA using DNA ligase. ligase. A lac-mutant of Escherichia coli lacking the structural gene for beta-galactosidase was transformed with ligated DNA. Three lac+ transformants containing recombinant plasmids were selected. Two of the plasmids (pK15 and pK17) contain four Eco R1-K. lactis DNA fragments having molecular weights of 2.2, 1.4, 0.55 and 0.5 x 10(6) daltons. The other plasmid (pK16) lacks the smallest fragment. E. coli carrying any of these plasmids produce beta-galactosidase activity that has a sedimentation coefficient and immunological determinants that are nearly identical to K. lactis beta-galactosidase and distinctly different from E. coli beta-galactosidase. DNA-DNA hybridization studies show that the four Eco R1 fragments in pK15 hybridize to K. lactis but not to E. coli DNA.