GENETIC AND MOLECULAR CHARACTERIZATION OF A GENE ENCODING A WIDE SPECIFICITY PURINE PERMEASE OF ASPERGILLUS-NIDULANS REVEALS A NOVEL FAMILYOF TRANSPORTERS CONSERVED IN PROKARYOTES AND EUKARYOTES

In Aspergillus nidulans, loss of-function mutations in the uapA and az gA genes, encoding the major uric acid-xanthine and hypoxanthine-adeni ne-guanine permeases, respectively, result in impaired utilization of these purines as sole nitrogen sources. The residual growth of the mut ant strains is due to the activity of a broad specificity purine perme ase. We have identified uapC, the gene coding for this third permease through the isolation of both gain-of-function and loss-of-function mu tations. Uptake studies with wild-type and mutant strains confirmed th e genetic analysis and showed that the UapC protein contributes 30% an d 8-10% to uric acid and hypoxanthine transport rates, respectively. T he uapC gene was cloned, its expression studied, its sequence and tran script map established, and the sequence of its putative product analy zed. uapC message accumulation is: (i) weakly induced by 2-thiouric ac id; (ii) repressed by ammonium; (iii) dependent on functional uaY and areA regulatory gene products (mediating uric acid induction and nitro gen metabolite repression, respectively); (iv) increased by uapC gain- of-function mutations which specifically, but partially, suppress a le ucine to valine mutation in the zinc finger of the protein coded by th e areA gene. The putative uapC gene product is a highly hydrophobic pr otein of 580 amino acids (M(r) = 61,251) including 12-14 putative tran smembrane segments. The UapC protein is highly similar (58% identity) to the UapA permease and significantly similar (23-34% identity) to a number of bacterial transporters. Comparisons of the sequences and hyd ropathy profiles of members of this novel family of transporters yield insights into their structure, functionally important residues, and p ossible evolutionary relationships.