Nitrogen Metabolism in Filamentous Fungi

Among the filamentous fungi, the genetic basis of nitrogen metabolism has been most intensively studied in the model ascomycetes Aspergillus nidulans and Neurospora crassa by utilizing the excellent classical and molecular genetic systems provided by these species. Much of one's current knowledge is based on classical genetic analysis of mutants affected in specific aspects of the enzymology or the regulation of nitrogen metabolism. There are also instances where significant differences across species provide fascinating insights into the evolutionary divergence of nitrogen metabolism within the filamentous fungi. In this chapter, the molecular genetics of the ammonium assimilatory pathways is considered as the starting point for the biosynthesis of complex nitrogenous macromolecules. The switch from anabolism to catabolism requires the relief of nitrogen metabolite repression, a global control system that modulates the expression of large sets of nitrogen-catabolic enzymes. Recent studies suggest some diversity in the complex molecular mechanisms underlying this regulation among different fungal groups. Details of several nitrogen-catabolic systems are reviewed to illustrate the metabolic and regulatory strategies employed by fungi in the acquisition of nitrogen metabolites. The catabolism of certain amino acids, such as proline and arginine, provides a good source of nitrogen metabolites and supports strong growth in A. nidulans, whereas other amino acids, such as histidine and leucine, are very poor sources of nitrogen for the wild-type organism.