Molybdenum-containing enzymes of the hydroxylase class (such as xanthine dehydrogenase, aldehyde oxidase and nicotinate dehydrogenase) require a terminal sulphur atom attached to the molybdenum to hydroxylate their specific substrates. The transulphurylation reaction is carried out in Drosophila melanogaster by the product of the ma-I gene. In Aspergillus nidulans, the activity of the isofunctional and homologous HxB protein is needed in at least two different metabolic contexts, when the organism grows on purines and when it grows on nicotinate as nitrogen sources. We show here that the expression 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 mutations 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 nitrogen metabolite repression and highly dependent on the AreA GATA factor.