Regulation of expression and kinetic modeling of substrate interactions of a uracil transporter in Aspergillus nidulans

Early genetic evidence suggested that A. nidulans possesses at least one uracil transporter. A gene, named furD, was recently identified by reverse genetics and in silico approaches, and we confirm here that it encodes a high-affinity, high-capacity, uracil transporter. In this work, we study the regulation of expression of FurD and develop a kinetic model describing transporter-substrate interactions. The furD gene is not expressed in resting conidiospores, is transcriptionally activated and reaches a peak during the isotropic growth phase of conidiospore germination, and stays at a basic low level in mycelium. Transcriptional expression is correlated to uracil transport activity. Expression in a strain blocked in uracil biosynthesis (pyrG(-)) is moderately increased and extended to later stages of germination. The presence of excess uracil in the medium leads to down-regulation of furD expression and FurD activity. A detailed kinetic analysis using a number of pyrimidine and purine analogues showed that FurD is able to recognize with high-affinity uracil (K-m 0.45 mu M), thymine (K-i 3.3 mu M) and several 5-substituted analogues of uracil, and with moderate affinity uric acid and xanthine (K-i 94-99 mu M). Kinetic evidence supports a model in which the positions N-H, =O2, N3-H, =O4, as well as planarity play a central role for the substrate binding. This model, which rationalizes the unique specificity of FurD for uracil, is compared to and found to be very similar to analogous models for protozoan uracil transporters.