Morozov I Y, Galbis-Martinez M, Jones M G, Caddick M X
Plant Science and Fungal Molecular Biology Research Group, School of Biological Sciences, Donnan Labs, The University of Liverpool, Liverpool L69 7ZD, UK.
Mol Microbiol. 2001 Oct;42(1):269-77. doi: 10.1046/j.1365-2958.2001.02636.x.
AreA is the principal transcription factor involved in determining nitrogen utilization in Aspergillus nidulans. NH4+ and Gln are utilized preferentially but in their absence, AreA acts to facilitate the expression of genes involved in metabolizing alternative nitrogen sources. It is crucial to the function of AreA that its expression is tightly modulated by the quality and availability of nitrogen sources. One signalling mechanism involves regulated degradation of the areA transcript in response to NH4+ and Gln, which provides the first direct means of monitoring nitrogen signalling in this fungus. Here we assess the specificity of the transcript degradation response, determining that it responds qualitatively to a variety of additional nitrogen sources including Asn. Furthermore, the response to Gln and NH4+ requires the same discrete region of the areA 3'-UTR but both NH4+ and Asn need to be metabolized to Gln before they are effective as a signal. However, NH4+ signalling is independent of AreA activity, unlike Gln and Asn signalling. A mutation in the structural gene for NADP-linked glutamate dehydrogenase, gdhA, which disrupts metabolism of NH4+ to Glu, is additive with mutations in two distinct regions of areA that disrupt the previously identified signalling mechanisms. The triple mutant is both strongly derepressed and expresses very high levels of nitrate reductase activity. These data suggest nitrogen metabolism in A. nidulans is in part regulated in response to the intracellular levels of Gln via the regulated degradation of areA mRNA, but the intracellular Gln level is not the sole determinant of nitrogen metabolite repression.
AreA是构巢曲霉中参与决定氮利用的主要转录因子。优先利用NH₄⁺和谷氨酰胺（Gln），但在缺乏它们时，AreA会促进参与代谢替代氮源的基因的表达。AreA的表达受到氮源质量和可用性的严格调控，这对其功能至关重要。一种信号传导机制涉及响应NH₄⁺和Gln对areA转录本的调控降解，这提供了在这种真菌中监测氮信号传导的首个直接手段。在此，我们评估了转录本降解反应的特异性，确定其对包括天冬酰胺（Asn）在内的多种其他氮源有定性反应。此外，对Gln和NH₄⁺的反应需要areA 3'-非翻译区的相同离散区域，但NH₄⁺和Asn在作为信号有效之前都需要代谢为Gln。然而，与Gln和Asn信号传导不同，NH₄⁺信号传导独立于AreA活性。烟酰胺腺嘌呤二核苷酸磷酸（NADP）连接的谷氨酸脱氢酶（gdhA）结构基因中的突变会破坏NH₄⁺向谷氨酸（Glu）的代谢，该突变与areA两个不同区域的突变具有累加效应，后两个区域的突变会破坏先前确定的信号传导机制。三重突变体既强烈去阻遏，又表达非常高水平的硝酸还原酶活性。这些数据表明，构巢曲霉中的氮代谢部分是通过areA mRNA的调控降解来响应细胞内Gln水平进行调控的，但细胞内Gln水平并非氮代谢物阻遏的唯一决定因素。
