Characterization and kinetics of the major purine transporters in Aspergillus fumigatus

Three genes encoding putative purine transporters have been identified in silico in the genome of Aspergillus fumigatus by their very close similarity of their translation products to well-studied homologues in A. nidulans. Two of these transporters, called AfUapC and AfAzgA, were found responsible for bulk uptake of purines and studied in detail herein. Genetic knock-out analysis, regulation of transcription, direct purine uptake assays and heterologous expression in A. nidulans have unequivocally shown that AfUapC and AfAzgA are high-affinity, high-capacity, purine/H+ symporters, the first being specific for xanthine, uric acid and oxypurinol, whereas the second for adenine, hypoxanthine, guanine and purine. The expression of these transporters is primarily controlled at the level of transcription. Transcription of both genes is purine-inducible, albeit with different efficiencies, whereas AfuapC is also ammonium-repressible. We characterised in detail the kinetics of the AfUapC and AfAzgA transporters, both in A. fumigatus and in A. nidulans, using a plethora of possible purine substrates. This analysis led us to propose kinetic models describing the molecular interactions of AfUapC and AfAzgA with purines. These models are discussed comparatively with analogous models from other purine transporters from fungi, bacteria and humans, and within the frame of a systematic development of novel purine-related antifungals.