Eukaryotic ribonucleotide reductases are iron-dependent enzymes that catalyze the rate-limiting step in the de novo synthesis of deoxyribonucleotides. Multiple mechanisms regulate the activity of ribonucleotide reductases in response to genotoxic stresses and iron deficiency. Upon iron starvation, the Saccharomyces cerevisiae Aft1 transcription factor specifically binds to iron-responsive cis elements within the promoter of a group of genes, known as the iron regulon, activating their transcription. Members of the iron regulon participate in iron acquisition, mobilization and recycling, and trigger a genome-wide metabolic remodeling of iron-dependent pathways. Here, we describe a mechanism that optimizes the activity of yeast ribonucleotide reductase when iron is scarce. We demonstrate that Aft1 and the DNA-binding protein Ixr1 enhance the expression of the gene encoding for its catalytic subunit, RNR1, in response to iron limitation, leading to an increase in both mRNA and protein levels. By mutagenesis of the Aft1-binding sites within RNR1 promoter, we conclude that RNR1 activation by iron depletion is important for Rnr1 protein and deoxyribonucleotide synthesis. Remarkably, Aft1 also activates the expression of IXR1 upon iron scarcity through an iron-responsive element located within its promoter. These results provide a novel mechanism for the direct activation of ribonucleotide reductase function by the iron-regulated Aft1 transcription factor.

中文翻译：

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酵母Aft1转录因子响应铁缺乏而激活核糖核苷酸还原酶催化亚基RNR1。

真核生物核糖核苷酸还原酶是铁依赖性酶，可催化脱氧核糖核苷酸从头合成中的限速步骤。多种机制调节对遗传毒性胁迫和铁缺乏症的核糖核苷酸还原酶的活性。缺铁时，酿酒酵母Aft1转录因子与一组基因（称为铁调节子）的启动子内的铁响应顺式元件特异性结合，从而激活其转录。铁调节子的成员参与铁的获取，动员和回收，并触发铁依赖性途径的全基因组代谢重塑。在这里，我们描述了一种在铁缺乏时优化酵母核糖核苷酸还原酶活性的机制。我们证明Aft1和DNA结合蛋白Ixr1响应铁的限制，增强编码其催化亚基RNR1的基因的表达，从而导致mRNA和蛋白质水平的增加。通过诱变RNR1启动子内的Aft1结合位点，我们得出结论，通过铁耗竭RNR1激活对于Rnr1蛋白和脱氧核糖核苷酸合成很重要。值得注意的是，铁缺乏时，Aft1还通过位于其启动子内的铁响应元件来激活IXR1的表达。这些结果提供了铁调节的Aft1转录因子直接激活核糖核苷酸还原酶功能的新机制。通过诱变RNR1启动子内的Aft1结合位点，我们得出结论，通过铁耗竭RNR1激活对于Rnr1蛋白和脱氧核糖核苷酸合成很重要。值得注意的是，铁缺乏时，Aft1还通过位于其启动子内的铁响应元件来激活IXR1的表达。这些结果提供了铁调节的Aft1转录因子直接激活核糖核苷酸还原酶功能的新机制。通过诱变RNR1启动子内的Aft1结合位点，我们得出结论，通过铁耗竭RNR1激活对于Rnr1蛋白和脱氧核糖核苷酸合成很重要。值得注意的是，铁缺乏时，Aft1还通过位于其启动子内的铁响应元件来激活IXR1的表达。这些结果提供了铁调节的Aft1转录因子直接激活核糖核苷酸还原酶功能的新机制。