The hxB gene, necessary for the post-translational activation of purine hydroxylases in Aspergillus nidulans, is independently controlled by the purine utilization and the nicotinate utilization transcriptional activating systems

Molybdenum-containing enzymes of the hydroxylase class (such as xanthine de hydrogenase, aldehyde oxidase and nicotinate dehydrogenase) require a termi nal sulphur atom attached to the molybdenum to hydroxylate their specific s ubstrates, The transulphurylation reaction is carried out in Drosophila mel anogaster by the product of the ma-I gene. In Aspergillus nidulans, the act ivity of the isofunctional and homologous HxB protein is needed in at least two different metabolic contexts, when the organism grows on purines and w hen it grows on nicotinate as nitrogen sources. We show here that the expre ssion of the hxB gene is not constitutive. It is induced independently and additively by the inducers of the purine and of the nicotinate utilization pathways. Each of these induction pathways is affected independently by mut ations in their cognate genes, uric acid induction by mutations in the UaY protein and nicotinate and 6-nicotinate induction by those in the hxnR/aplA complex. It is, in both metabolic contexts, exquisitely sensitive to nitro gen metabolite repression and highly dependent on the AreA GATA factor.