The Regulation of Nitrate Assimilation

点击翻译按钮获取中文摘要

Nitrate assimilation is the predominant pathway within the biosphere for acquisition of inorganic nitrogen. In the lower eukaryotic Neurospora crassa, nitrate assimilation is an inducible metabolic pathway consisting of two enzymes, nitrate reductase and nitrite reductase. Biochemical studies reveal that the expression of both enzymes is induced by nitrate, and both are repressed by the end product, ammonium, of primary products of ammonium assimilation such as glutamine. This latter effect, termed nitrogen metabolite repression, is dominant in that nitrate assimilation is essentially precluded by ammonium. To date, nine genes are implicated in nitrate assimilation. Two of them, nit-3 and nit-6, are coordinately controlled, unlinked structural genes encoding the enzymes, nitrate reductase and nitrite reductase, respectively. Four genes, nit-1,7, 8 and 9, function in producing the molybdenum cofactor essential to nitrate reductase. Three genes serve regulatory roles: nit-4 is required for nitrate induction of the nitrate assimilatory pathway, whereas nit-2 and nmr-1 have pleiotropic effects controlling many secondary pathways of nitrogen acquisition, e.g. amino acid oxidation or purine catabolism, which like nitrate assimilation, provide for nitrogen nutrition in the absence of available ammonium. Genes nit-2,3 and 4, as well nmr-1, have been cloned. This research undertakes cloning and analysis of nit-6, the nitrite reductase structural gene, and comparison of its sequence organization to nit-3, its coordinately regulated partner. Further, a molecular strategy has revealed additional nitrate regulated genes for genetic and biochemical characterization.