Feedback regulation of glucose transporter gene transcription in Kluyveromyces lactis by glucose uptake

In the respirofermentative yeast Kluyveromyces lactis, only a single geneti c locus encodes glucose transporters that can support fermentative growth. This locus is polymorphic in wild-type isolates carrying either KHT1 and KH T2, two tandemly arranged HXT-like genes, or RAG1, a low-affinity transport er gene that arose by recombination between KHT1 and KHT2. Here we show tha t KHT1 is a glucose-induced gene encoding a low-affinity transporter very s imilar to Rag1p. Kht2p has a lower K-m (3.7 mM) and a more complex regulati on. Transcription is high in the absence of glucose, further induced by low glucose concentrations, and repressed at higher glucose concentrations. Th e response of KHT1 and KHT2 gene regulation to high but not to low concentr ations of glucose depends on glucose transport. The function of either Kht1 p or Kht2p is sufficient to mediate the characteristic response to high glu cose, which is impaired in a kht1 kht2 deletion mutant. Thus, the KHT genes are subject to mutual feedback regulation. Moreover, glucose repression of the endogenous beta -galactosidase (LAC4) promoter and glucose induction o f pyruvate decarboxylase were abolished in the kht1 kht2 mutant. These phen otypes could be partially restored by HZT gene family members from Saccharo myces cerevisiae. The results indicate that the specific responses to high but not to low glucose concentrations require a high rate of glucose uptake .