Primary structure of the lactose permease gene from the yeast Kluyveromyces lactis. Presence of an unusual transcript structure.

The LAC12 gene of Kluyveromyces lactis codes for an inducible lactose permease. We have determined the nucleotide sequence of a DNA fragment which includes the complete LAC12 gene. The 4.7-kilobase (kb) mRNA carrying LAC12 contained two open reading frames, ORFI (1761 bases) and ORFII (1266 bases), separated by a 573-base pair noncoding region. Mung bean and exonuclease VII mapping showed that there was no splicing of the 4.7-kb transcript and thus no intron between the two open reading frames. Chromosomal disruption of ORFI with the URA3 gene destroyed lactose transport activity, suggesting that ORFI codes for a component of the permease. Disruption of ORFII and the noncoding region between the two open reading frames did not affect the lactose permease function, indicating that they do not comprise a part of the permease. We do not know if ORFII is translated, but in either case, the structure of the 4.7-kb mRNA is unusual. We discuss possible origins for it. The peptide predicted from ORFI is hydrophobic as would be expected for a membrane-bound protein. Compared with other membrane proteins, LAC12 (ORFI) protein showed sequence similarity to the human glucose and the Escherichia coli xylose-H+ and arabinose-H+ transporters. No obvious amino acid sequence similarity was found with the lactose permease of E. coli.