Hexose uptake in the plant symbiotic ascomycete Tuber borchii Vittadini: biochemical features and expression pattern of the transporter TBHXT1

Here, we report the first evidence of a hexose transporter gene, Tbhxt1, in the ectomycorrhizal ascomycete Tuber borchii Vittadini. The protein encoded by Tbhxt1 functionally complements the hxt-null mutant Saccharomyces cerevisiae EBYVW.4000. TBHXT1 has a strong preference for d-glucose (K(m)=38+/-10 microM) over d-fructose (K(m)=16+/-5mM) and uncoupling experiments indicate that TBHXT1 catalyzes the transport via a proton-symport mechanism. The investigations on the substrate specificity reveal that TBHXT1 also imports d-mannose, and the use of deoxyglucose analogues shows that the hydroxyl groups at C1, C3 and C4 are important for substrate recognition. Tbhxt1 is not regulated by fructose, but it reaches its highest level of expression at 3mM glucose and is repressed by very high glucose concentration. Prolonged carbon starvation condition upregulates Tbhxt1, while its expression remains at basal level in the ectomycorrhizal tissue. The mode of regulation of Tbhxt1 is consistent with its role as a high-affinity d-glucose transporter.