Hypoxia and Glucose Regulate Transcription of the Low-affinity Glucose Transporter Gene RAG1 in Kluyveromyces lactis

Glucose signaling and glycolysis regulates the expression of the low-affinity glucose transporter gene RAG1 in Kluyveromyces lactis. Glucose signaling acts through cascades involving the glucose sensor Rag4 and downstream proteins. A major role is exerted by the casein kinase Rag8 and the repressor proteins Sms1 and KlRgt1. Another pathway involves the chromatin remodeler KlSnf2 and Sck1 and also signals from glycolysis are involved. We have found that transcription of RAG1 is also induced by hypoxia and this induction requires the presence of glucose. Molecular dissection and structural analysis of the RAG1 promoter allowed to identify the region essential for the induction. Transcription analysis of RAG1 in various mutant strains of the glucose regulation allowed to identify Sck1 as the possible element of connection between glucose and oxygen signaling. Dependence of Sck1 expression on hypoxia and binding of Sck1 to the RAG1 promoter has been also investigated. Interestingly, the level of RAG1 transcription, but not the hypoxic induction, depended on the presence of the hypoxic transcription regulator KlMga2. Our results show that the expression of the glucose transporter gene RAG1 is synergistically regulated by high glucose and low oxygen and the glucose regulator Sck1 and the hypoxic regulator KlMga2 might cooperate in this mechanism. NuSA analysis of the promoter in the pertinent physiological conditions and mutant strains, indicates the role of chromatin organization in RAG1 transcriptional regulation.