SUMMARY: The hydrolysis of lactose in was shown to be catalysed by β-D-galactosidase. To isolate lactose catabolic genes, a gene library of DNA was constructed in bacteriophage λ47.1 and recombinants expressing β-D-galactosidase activity were detected using 5-bromo-4-chloro-indoyl-β-D-galactoside (X-Gal). The gene was cloned on a 7·8 kb DNA restriction fragment into pBR322 to generate the recombinant plasmid pHG1, which also encoded lactose permease, thiogalactoside transacetylase and lactose repressor protein. The position and orientation of the four genes were determined by subcloning and transposon mutagenesis. The β-D-galactosidase, lactose permease and thiogalactoside transacetylase genes constitute an operon controlled by the repressor protein. β-D-Galactosides induced expression of the genes in while glucose had no effect. The nucleotide sequence of the presumptive regulatory region of the operon was determined and compared with the corresponding sequence. The operators and the 5’ end of the genes showed strong identity. The catabolite activator protein binding sequence, present in the promoter, was absent from the corresponding region.


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