Glucose transport in the yeastKluyveromyces lactis

Summary

In most strains ofKluyveromyces 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⁻ henotype). This phenotype is due to an allelic variation of the chromosomal geneRAG1. The sensitive variants have a recessive allelerag1. TheRAG1 gene has been cloned by complementation of arag1 strain from a genomic bank derived from a Rag⁺ strain. The nucleotide sequence of the cloned gene indicated that theRAG1 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 theGAL2 (galactose permease) andHXT2 (a high-affinity glucose transporter) proteins ofSaccharomyces cerevisiae. In arag1 null mutant, as in the naturalrag1 variant, uptake of glucose at high external glucose concentrations was impaired. TheRAG1 protein appears to correspond to a low-affinity glucose transporter. Transcription of theRAG1 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.