Second messenger nucleotides, such as guanosine penta- or tetra-phosphate, commonly referred to as (p)ppGpp, are powerful signaling molecules, used by all bacteria to fine-tune cellular metabolism in response to nutrient availability. Indeed, under nutritional starvation, accumulation of (p)ppGpp reduces cell growth, inhibits stable RNAs synthesis, and selectively up- or down- regulates the expression of a large number of genes. Here, we show that the E. coli hns promoter responds to intracellular level of (p)ppGpp. hns encodes the DNA binding protein H-NS, one of the major components of bacterial nucleoid. Currently, H-NS is viewed as a global regulator of transcription in an environment-dependent mode. Combining results from relA (ppGpp synthetase) and spoT (ppGpp synthetase/hydrolase) null mutants with those from an inducible plasmid encoded RelA system, we have found that hns expression is inversely correlated with the intracellular concentration of (p)ppGpp, particularly in exponential phase of growth. Furthermore, we have reproduced in an in vitro system the observed in vivo (p)ppGpp-mediated transcriptional repression of hns promoter. Electrophoretic mobility shift assays clearly demonstrated that this unusual nucleotide negatively affects the stability of RNA polymerase-hns promoter complex. Hence, these findings demonstrate that the hns promoter is subjected to an RNA polymerase-mediated down-regulation by increased intracellular levels of (p)ppGpp.

中文翻译：

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大肠杆菌的hns基因在转录上受（p）ppGpp调控

第二信使核苷酸，例如鸟嘌呤五磷酸或四磷酸鸟嘌呤，通常称为（p）ppGpp，是强大的信号分子，所有细菌均可利用它来调节细胞的新陈代谢，以响应养分的可利用性。实际上，在营养匮乏的情况下，（p）ppGpp的积累会降低细胞生长，抑制稳定的RNA合成，并选择性地上调或下调大量基因的表达。在这里，我们显示了大肠杆菌 hns启动子对（p）ppGpp的细胞内水平有反应。hns编码DNA结合蛋白H-NS，它是细菌核苷的主要成分之一。当前，H-NS被视为环境依赖性模式下转录的全局调节剂。合并relA的结果（ppGpp合成酶）和s poT（ppGpp合成酶/水解酶）无效突变体与来自可诱导质粒编码的RelA系统的突变体，我们发现hns表达与（p）ppGpp的细胞内浓度成反比，特别是在生长的指数阶段。此外，我们已经在体外系统中重现了在体内（p）ppGpp介导的hns启动子转录抑制。电泳迁移率变动分析清楚地表明，这种不寻常的核苷酸会对RNA聚合酶-hns启动子复合物的稳定性产生负面影响。因此，这些发现表明，hns启动子通过增加（p）ppGpp的细胞内水平而受到RNA聚合酶介导的下调。