The nadA gene of Aspergillus nidulans, encoding adenine deaminase, is subject to a unique regulatory pattern.

The adenine deaminase of A. nidulans, encoded by nadA, can be considered both as a catabolic and a purine salvage enzyme. We show that its transcriptional regulation reflects this double metabolic role. As all other genes involved in purine utilisation it is induced by uric acid, and this induction is mediated by the UaY transcription factor. However, it is also independently and synergistically induced by adenosine by a UaY-independent mechanism. At variance with all other enzymes of purine catabolism it is not repressed but induced by ammonium. This is at least partly due to the ammonium responsive GATA factor, AreA, acting in the nadA promoter as a competitor rather than in synergy with UaY. The adB gene, encoding adenylo-succinate synthetase, which can be considered both a biosynthetic and a salvage pathway enzyme, shares with nadA both ammonium and adenosine induction.

Sub-families of alpha/beta barrel enzymes: a new adenine deaminase family.

No gene coding for an adenine deaminase has been described in eukaryotes. However, physiological and genetical evidence indicates that adenine deaminases are present in the ascomycetes. We have cloned and characterised the genes coding for the adenine deaminases of Aspergillus nidulans, Saccharomyces cerevisiae and Schizosaccharomyces pombe. The A.nidulans gene was expressed in Escherichia coli and the purified enzyme shows adenine but not adenosine deaminase activity. The open reading frames coded by the three genes are very similar and obviously related to the bacterial and eukaryotic adenosine deaminases rather than to the bacterial adenine deaminases. The latter are related to allantoinases, ureases and dihydroorotases. The fungal adenine deaminases and the homologous adenosine deaminases differ in a number of residues, some of these being clearly involved in substrate specificity. Other prokaryotic enzymes in the database, while clearly related to the above, do not fit into either sub-class, and may even have a different specificity. These results imply that adenine deaminases have appeared twice in the course of evolution, from different ancestral enzymes constructed both around the alpha/beta barrel scaffold.