Isolation and characterization of mutants of Kluyveromyces lactis defective in lactose transport.

Mutants of Kluyveromyces lactis defective in lactose transport were identified among lactose-resistant revertants of lactose-sensitive strains. The mutations are closely linked to the beta-galactosidase gene, LAC4, and they are located in a previously identified gene, LAC12, which has been shown to code for a lactose permease. Our data establish that LAC12 is the only lactose permease gene in K. lactis. The lactose permease also transports galactose. LAC12 is transcribed in a direction opposite to that of LAC4, there being about 2.5 kb between their transcription start sites. Transcription of LAC12 is inducible as is that of all other structural genes in the lactose-galactose regulon of K. lactis.