Regulatory mechanism of gene expression by Escherichio coli nucleoid protein H-NS

大肠杆菌H-NS蛋白是类核结构的主要成分之一。这种蛋白不仅与类核结构的紧密组织有关，而且与基因表达的全球调控有关。在本研究中，对HNS的系统突变分析发现，在H-NS中发现了三个不同的功能结构域，分别负责DNA结合、转录抑制和二聚化。C-末端结构域的突变导致其DNA结合能力的丧失，表明该结构域直接参与其与DNA的结合。N-末端结构域被认为参与了抑制转录的能力。H-NS相对中心的部分参与蛋白质间的相互作用，特别是二聚化作用。并阐明了每个结构域的功能意义。此外，BGL操纵子的表达被发现只受HNS突变的子集以高度等位基因特异性的方式影响。这一发现还涉及到BGL操纵子的一种独特的调控机制(即沉默)，这部分由H-N介导。大肠杆菌BGL操纵子之所以引起人们的兴趣，是因为它的表达是沉默的(表型为Bgl^-)，至少在标准的实验室条件下是这样。为了确定一个可能与bgl调节相关的反式作用因子(S)，我筛选了表现bgl+表型的突变体。通过对mini-Tn10进行随机插入诱变，获得了一种新型突变。在这种情况下，插入突变被发现正好在编码一种可能的LysR样DNA结合蛋白的leuO基因的前面。遗传分析表明，野生型细胞中LeuO的过量生产导致了Bgl^+的表型。

The Escherichia coil H-NS protein is one of the major constituents of the nucleoid structure. This protein has been implicated not only in the compact organization of the nucleoid structure, but also in the global regulation of gene expression. In this study, on systematic mutational analysis of hns, three distinct functional domains were found in H-NS, which appear to be responsible for DNA-binding, transcriptional repression and dimerization, respectively. Mutations in the C-terminal domain resulted in a loss of its DNA-binding ability, suggesting that this domain is directly involved in its binding to DNA.The N-terminal domain was suggested to be involved in the ability to repress transcription. The relatively central portion of H-NS was found to be involved in protein-protein interaction, especially dimerization. The functional significance for each domain was also clarified. In addition, expression of the bgl operon was found to be affected by only a subset of hns mutations in a highly allele-specific manner. This finding is also addressed with regard to a unique regulatory mechanism (i.e. silencing) for the bgl operon, which is partly mediated by H-NS.The Escherichia coli bgl operon is of interest since its expression is silent (phenotypically Bgl^-), at least under standard laboratory conditions. To identify a trans-acting factor(s) that is presumably relevant to the regulation of bgl, I screened mutants exhibiting Bgl+ phenotype. By a random insertion mutagenesis with mini-Tn10, a novel type of mutation was obtained. In this case, the insertion mutation was found to be just in front of the leuO gene encoding a putative LysR-like DNA-binding protein. Genetic analyses revealed that an overproduction of LeuO in the wild-type cells causes the phenotype of Bgl^+.