Glucose transport in the yeast Kluyveromyces lactis. I. Properties of an inducible low-affinity glucose transporter gene.

In most strains of Kluyveromyces lactis, respiratory function is not required for growth on glucose. However, some natural variant strains are unable to grow when respiration is blocked by specific inhibitors (Rag- phenotype). This phenotype is due to an allelic variation of the chromosomal gene RAG1. The sensitive variants have a recessive allele rag1. The RAG1 gene has been cloned by complementation of a rag1 strain from a genomic bank derived from a Rag+ strain. The nucleotide sequence of the cloned gene indicated that the RAG1 product was a sugar transporter protein. The amino acid sequence deduced from the gene structure contained the 12 hydrophobic segments typical of a transmembrane protein, and showed a high degree of homology with the GAL2 (galactose permease) and HXT2 (a high-affinity glucose transporter) proteins of Saccharomyces cerevisiae. In a rag1 null mutant, as in the natural rag1 variant, uptake of glucose at high external glucose concentrations was impaired. The RAG1 protein appears to correspond to a low-affinity glucose transporter. Transcription of the RAG1 gene, which was undetectable when cells were grown in glycerol, was induced by glucose. It is concluded that respiration-dependent growth on glucose of the Rag- variant strains is due to a defect in this inducible glucose transport system.