Fungi employ different
carbohydrate uptake systems to adapt to certain environmental conditions and to different
carbon source concentrations. The hydrolysis of polymeric
carbohydrates and the subsequent uptake of monomeric forms may also play a role in development. Aspergillus nidulans accumulates cell wall components during vegetative growth and degrades them during sexual development. We have identified the hxtA (high affinity
hexose transporter) gene in a differential library, which was enriched for sexual-specific genes. The hxtA gene is disrupted by 6 introns and predicted to encode a 531
amino acid protein with high similarity to major facilitator superfamily members including the high affinity
hexose transporter Gtt1 from Trichoderma harzianum. A. nidulans HxtA contains the 12 predicted transmembrane domains characteristic for this family. Deletion of hxtA did not impair growth of A. nidulans on a variety of
carbon sources nor did it inhibit sexual development suggesting redundant
sugar uptake systems. We found at least 17 putative
hexose transporters in the genome of A. nidulans. Despite the high similarity of HxtA to fungal high affinity
glucose transporters, the hxtA gene did not restore growth on
glucose of a Saccharomyces cerevisiae mutant, in which all
hexose transporters were deleted. Northern blot analysis revealed that the A. nidulans hxtA gene was repressed under high
glucose conditions and expressed in vegetative hyphae upon
carbon starvation and during sexual development. We found hxtA(p)::sgfp expression in developing cleistothecia specifically in ascogenous hyphae and propose that HxtA is a high affinity
glucose transporter involved in
sugar metabolism during sexual development.