The Regulation of Nitrate Assimilation

硝酸盐同化是主要的途径， 生物圈获取无机氮。 下 真核粗糙脉孢菌，硝酸盐同化是一种 由两种酶组成的诱导代谢途径，硝酸盐 还原酶和亚硝酸盐还原酶。 生化研究表明， 这两种酶的表达都是由硝酸盐诱导的， 被初级产品的终产物铵所抑制 如谷氨酰胺。 后一种效果， 称为氮代谢抑制，是占主导地位的， 硝酸盐同化基本上被铵所阻止。 到 迄今为止，有九个基因与硝酸盐同化有关。 两 它们，nit-3和nit-6，是协调控制的， 结构基因编码的酶，硝酸还原酶和 亚硝酸盐还原酶。 四个基因，nit-1，7，8和9， 在生产钼辅因子中的作用， 硝酸还原酶 三个基因起调节作用：nit-4是 所需的硝酸盐诱导的硝酸盐同化 而nit-2和nmr-1具有多效性 控制氮素获取的许多次级途径，例如， 氨基酸氧化或嘌呤催化剂，如硝酸盐 同化，在缺乏氮素营养的情况下提供氮素营养。 可用铵。 基因nit-2，3和4，以及nmr-1， 被克隆了 这项研究进行克隆和分析， nit-6，亚硝酸盐还原酶结构基因，以及 其序列组织与nit-3，其协调调节 搭档 此外，分子策略揭示了额外的 硝酸盐调节基因的遗传和生化 特征化

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.