The azgA gene of Aspergillus nidulans encodes a hypoxanthine-adenine-guanine 
transporter. It has been cloned by a novel transposon methodology. The null 
phenotype of azgA was defined by a number of mutations, including a large 
deletion. In mycelia, the azgA gene is, like other genes of purine catabolism, 
induced by uric acid and repressed by ammonium. Its transcription depends on the 
pathway-specific UaY zinc binuclear cluster protein and the broad domain AreA 
GATA factor. AzgA is not closely related to any other characterized membrane 
protein, but many close homologues of unknown function are present in fungi, 
plants, and prokaryotes but not metazoa. Two of three data bases and the 
phylogeny presented in this article places proteins of this family in a cluster 
clearly separated (but perhaps phylogenetically related) from the NAT family 
that includes other eukaryotic and prokaryotic nucleobase transporters. Thus 
AzgA is the first characterized member of this family or subfamily of membrane 
proteins.