The autoregulation of a eukaryotic DNA transposon.
The autoregulation of a eukaryotic DNA transposon.
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DOI：
10.7554/elife.00668
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发表时间：
2013-06-18
期刊：
影响因子：
7.7
通讯作者：
Chalmers R
中科院分区：
文献类型：
--
作者：
Claeys Bouuaert C;Lipkow K;Andrews SS;Liu D;Chalmers R
DNA转座子是如何与宿主和谐相处的？细菌提供了唯一记录的自我调节机制，但这些与真核细胞生物学不相容。在这里，我们发现Hsmar1的自动调节在转座体的组装过程中起作用，并由转座酶的多聚体状态引起，由结合位点的竞争介导。我们利用计算机模型，在体外和体内实验的支持下，探索了基因组入侵的动力学，并表明扩增在开始时加速，但随后达到恒定速率。该比率与基因组大小成正比，与转座酶的表达及其对转座子末端的亲和力成反比。因此水手转座子可能抵抗转录后沉默。因为监管是反应的一种自然属性，所以它能抵抗自私的剥削。远亲真核转座子的行为具有相同的机制，因此可能具有广泛的应用价值。转座子是可移动的DNA区域，可以从基因组的一个位置跳到另一个位置。这对宿主来说是一种遗传负担，因为转座子总是有使细胞基因失活的风险。然而，一个更大的问题是，转位是伴随着转座子拷贝数量的增加。由于每一个新的拷贝都是新的拷贝的来源，转座子的扩增必然是指数的。真核细胞能够耐受DNA转座子的事实表明，存在调节机制来化解不可避免的基因组崩溃。宿主介导的表观遗传修饰和RNA干扰将提供一定程度的保护。然而，它们绝不是完全有效的，一个适应良好的基因组寄生虫，如转座子，可能有它自己的调节机制。现在，Claeys bouwaaert、Lipkow和他们的同事利用计算机模型结合体内和体外实验来寻找这种机制。他们的实验揭示了DNA转座子是如何被自身的转座酶下调的。转座酶是催化“跳跃”或转座的酶。它与转座子两端的特定位点结合，并将它们结合在一起，形成一种称为转座体的核蛋白复合物。反应的化学步骤就是在这个复合体中进行的。当转座子数量增加时，转座酶的浓度也随之增加。Claeys Bouuaert等人的研究表明，在相对较低的转座酶浓度下，结合位点就会饱和，由此产生的竞争会产生负调控。因此，随着转座子数量的增加，转座率降低。他们进一步使用计算机模型来探索转座子的扩增如何受到转座子特异性和细胞特异性因素的影响。Claeys bouwaaert、Lipkow及其同事的研究主要基于Hsmar1转座子的复活副本，该转座子在5000万年前在人类基因组中活跃。然而，他们也测试了两个远亲真核转座子，并观察到它们的行为是相似的，这表明这可能是控制跳跃基因活性的一般机制。他们还指出，它们的竞争机制在概念上类似于免疫学上的“丙氨酸效应”。这是蛋白质化学中一个反复出现的主题，并再次证明了有时少即是多。DOI: http://dx.doi.org/10.7554/eLife.00668.002
How do DNA transposons live in harmony with their hosts? Bacteria provide the only documented mechanisms for autoregulation, but these are incompatible with eukaryotic cell biology. Here we show that autoregulation of Hsmar1 operates during assembly of the transpososome and arises from the multimeric state of the transposase, mediated by a competition for binding sites. We explore the dynamics of a genomic invasion using a computer model, supported by in vitro and in vivo experiments, and show that amplification accelerates at first but then achieves a constant rate. The rate is proportional to the genome size and inversely proportional to transposase expression and its affinity for the transposon ends. Mariner transposons may therefore resist post-transcriptional silencing. Because regulation is an emergent property of the reaction it is resistant to selfish exploitation. The behavior of distantly related eukaryotic transposons is consistent with the same mechanism, which may therefore be widely applicable. DOI: http://dx.doi.org/10.7554/eLife.00668.001 Transposons are regions of mobile DNA that can jump from one location in the genome to another. This represents a genetic burden to the host because there is always the risk that the transposon will inactivate a cellular gene. However, a greater problem is that transposition is accompanied by an increase in the number of copies of the transposon. Since each new copy will be a source of further new copies, amplification of transposons is necessarily exponential. The fact that eukaryotic cells are able to tolerate DNA transposons suggests the existence of regulatory mechanisms to defuse the inevitable genomic melt-down. Host-mediated epigenetic modifications and RNA interference will provide some level of protection. However, they are by no means completely effective and a well-adapted genomic parasite, such as a transposon, might be expected to have its own mechanism of regulation. Now, Claeys Bouuaert, Lipkow and colleagues have used a computer model in combination with in vivo and in vitro experiments to search for this mechanism. Their experiments reveal how a DNA transposon is down-regulated by its own transposase. The transposase is the enzyme that catalyzes the ‘jump’ or transposition. It binds to specific sites at either end of the transposon and brings these together to make up a nucleoprotein complex called the transpososome. It is within this complex that the chemical steps of the reaction take place. When the number of transposons increases, so does the concentration of transposase. Claeys Bouuaert et al. show that the binding sites become saturated at a relatively low transposase concentration and that negative regulation arises from the resulting competition. Thus, the rate of transposition decreases as the number of transposons increases. They further use the computer model to explore how the amplification of the transposon is affected by transposon-specific and cellular-specific factors. Claeys Bouuaert, Lipkow and colleagues based their study predominantly on a resurrected copy of the Hsmar1 transposon, which was active in the human genome 50 million years ago. However, they also tested two distantly related eukaryotic transposons and observed that their behavior was similar, which suggests that this could be a general mechanism that controls the activity of jumping genes. They also note that their competition mechanism is conceptually similar to the immunological ‘prozone effect’. This is a recurrent theme in protein chemistry and demonstrates once again that less is in fact sometimes more. DOI: http://dx.doi.org/10.7554/eLife.00668.002