The chapter analyzes the results available in the process of nitrate assimilation as it occurs in two filamentous ascomycetes—Aspergillus nidulans and Neurospora crassa (N. crassa). Not only are these results in general representative of observations made on other fungi and yeasts, but they provide for the most concise and comprehensive description of nitrate assimilation and its regulation. It is a vagary of scientific fortune that the genetics of nitrate assimilation is best understood for A. nidulans, whereas the enzymology of nitrate assimilation is better characterized in N. crassa, the organism more often subjected to biochemical investigation. The assimilatory reduction of nitrate to ammonia is achieved by a metabolic pathway composed of just two enzymes—namely, nitrate reductase and nitrite reductase, which act in this sequence. Recently both have been purified to homogeneity and detailed characterizations of each are being made. Regulation of nitrate assimilation in fungi is eminently simple and straightforward. Ammonia, the end product of the nitrate assimilatory pathway, strongly represses its expression. This phenomenon is the dominant aspect in regulation of nitrate assimilation. © 1978, Academic Press Inc. (London) Ltd.
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