Abstract：
RNA interference systems depend on the synthesis of small RNA precursors whose sequences define the target spectrum of these silencing pathways. The Drosophila Heterochromatin Protein 1 (HP1) variant Rhino permits transcription of PIWI-interacting RNA (piRNA) precursors within transposon-rich heterochromatic loci in germline cells. Current models propose that Rhino\'s specific chromatin occupancy at piRNA source loci is determined by histone marks and maternally inherited piRNAs, but also imply the existence of other, undiscovered specificity cues. Here, we identify a member of the diverse family of zinc finger associated domain (ZAD)-C2H2 zinc finger proteins, Kipferl, as critical Rhino cofactor in ovaries. By binding to guanosine-rich DNA motifs and interacting with the Rhino chromodomain, Kipferl recruits Rhino to specific loci and stabilizes it on chromatin. In kipferl mutant flies, Rhino is lost from most of its target chromatin loci and instead accumulates on pericentromeric Satellite arrays, resulting in decreased levels of transposon targeting piRNAs and impaired fertility. Our findings reveal that DNA sequence, in addition to the H3K9me3 mark, determines the identity of piRNA source loci and provide insight into how Rhino might be caught in the crossfire of genetic conflicts.
The genes within our DNA encode the essentials of our body plan and how each task in the body is achieved. However, our genome also contains many repetitive regions of DNA that do not encode functional genes. Some of these regions are genetic parasites known as transposons that try to multiply and spread around the DNA of their host. To prevent transposon DNA from interfering with the way the body operates, humans and other animals have evolved elaborate defense mechanisms to identify transposons and prevent them from multiplying. In one such mechanism, known as the piRNA pathway, the host makes small molecules known as piRNAs that have sequences complementary to those of transposons, and act as guides to silence the transposons. The instructions to make these piRNAs are stored in the form of transposon fragments in dedicated regions of host DNA called piRNA clusters. These clusters thereby act as genetic memory, allowing the host to recognize and silence specific transposons in other locations within the host’s genome. In fruit flies, a protein called Rhino binds to piRNA clusters that are densely packed to allow piRNAs to be made. However, it remained unclear how Rhino is able to identify and bind to piRNA clusters, but not to other similarly densely packed regions of DNA. Baumgartner et al. used a combination of genetic, genomic, and imaging approaches to study how Rhino finds its way in the fruit fly genome. They found that another protein called Kipferl interacts with Rhino and is required for Rhino to bind to nearly all piRNA clusters. Since Kipferl can by itself bind to the sequences that Rhino needs to find, the results suggest that Kipferl acts to recruit and initiate Rhino binding within densely packed piRNA clusters. Further experiments found that, in flies lacking Kipferl, Rhino binds to regions of DNA called Satellite repeats, hinting that these selfish sequences may compete for Rhino for their own benefit. The finding that Kipferl and Rhino work together to define the memory system of the piRNA pathway strongly advances our understanding of how a sequence-specific defense system based on small RNAs can be established.
The genes within our DNA encode the essentials of our body plan and how each task in the body is achieved. However, our genome also contains many repetitive regions of DNA that do not encode functional genes. Some of these regions are genetic parasites known as transposons that try to multiply and spread around the DNA of their host. To prevent transposon DNA from interfering with the way the body operates, humans and other animals have evolved elaborate defense mechanisms to identify transposons and prevent them from multiplying. In one such mechanism, known as the piRNA pathway, the host makes small molecules known as piRNAs that have sequences complementary to those of transposons, and act as guides to silence the transposons. The instructions to make these piRNAs are stored in the form of transposon fragments in dedicated regions of host DNA called piRNA clusters. These clusters thereby act as genetic memory, allowing the host to recognize and silence specific transposons in other locations within the host’s genome. In fruit flies, a protein called Rhino binds to piRNA clusters that are densely packed to allow piRNAs to be made. However, it remained unclear how Rhino is able to identify and bind to piRNA clusters, but not to other similarly densely packed regions of DNA. Baumgartner et al. used a combination of genetic, genomic, and imaging approaches to study how Rhino finds its way in the fruit fly genome. They found that another protein called Kipferl interacts with Rhino and is required for Rhino to bind to nearly all piRNA clusters. Since Kipferl can by itself bind to the sequences that Rhino needs to find, the results suggest that Kipferl acts to recruit and initiate Rhino binding within densely packed piRNA clusters. Further experiments found that, in flies lacking Kipferl, Rhino binds to regions of DNA called Satellite repeats, hinting that these selfish sequences may compete for Rhino for their own benefit. The finding that Kipferl and Rhino work together to define the memory system of the piRNA pathway strongly advances our understanding of how a sequence-specific defense system based on small RNAs can be established.
摘要：
RNA干扰系统依赖于小RNA前体的合成,所述小RNA前体的序列定义了这些沉默途径的目标谱。果蝇异染色质蛋白1（HP1）变体Rhino允许在种系细胞中富含转座子的异色基因座内转录PIWI相互作用RNA（piRNA）前体。目前的模型表明，犀牛在piRNA来源基因座上的特定染色质占有率是由组蛋白标记和母系遗传的piRNAs决定的。但也暗示了其他人的存在，未发现的特异性线索。这里,我们鉴定了锌指相关结构域（ZAD）-C2H2锌指蛋白的不同家族的成员，Kipferl,作为卵巢中关键的犀牛辅助因子。通过与富含鸟苷的DNA基序结合并与犀牛色域相互作用，Kipferl将犀牛招募到特定基因座并使其稳定在染色质上。在kipferl突变果蝇中，犀牛从其大多数目标染色质基因座中丢失，而是在着丝粒卫星阵列上积累，导致靶向piRNAs的转座子水平降低和生育力受损。我们的发现揭示了DNA序列,除了H3K9me3标志，确定piRNA源基因座的身份，并提供有关犀牛如何陷入遗传冲突的交火的见解。
我们DNA中的基因编码我们身体计划的要素，以及身体中的每项任务是如何实现的。然而,我们的基因组还包含许多不编码功能基因的DNA重复区域。这些区域中的一些是被称为转座子的遗传寄生虫，它们试图在宿主的DNA周围繁殖和传播。为了防止转座子DNA干扰身体的运作方式，人类和其他动物已经进化出复杂的防御机制来识别转座子并防止它们繁殖。在一个这样的机制中，称为piRNA通路，宿主制造称为piRNAs的小分子，其序列与转座子的序列互补，并充当引导使转座子沉默。制备这些piRNA的指令以转座子片段的形式存储在宿主DNA的专用区域中，称为piRNA簇。这些集群因此充当遗传记忆，允许宿主识别并沉默宿主基因组中其他位置的特定转座子。在果蝇中，一种叫做Rhino的蛋白质与piRNA簇结合，这些簇被密集地包装以允许piRNA被制造出来。然而,目前还不清楚犀牛是如何识别和结合piRNA簇的，但不是其他类似密集的DNA区域。Baumgartner等人。使用了遗传的组合，基因组，和成像方法来研究犀牛如何在果蝇基因组中找到自己的方式。他们发现另一种称为Kipferl的蛋白质与Rhino相互作用，并且Rhino需要与几乎所有的piRNA簇结合。由于Kipferl本身可以与Rhino需要找到的序列结合，结果表明，Kipferl可以在密集的piRNA簇中招募和启动Rhino结合。进一步的实验发现,在缺乏Kipferl的苍蝇中，犀牛与称为卫星重复的DNA区域结合，暗示这些自私序列可能会为了自己的利益而争夺犀牛。Kipferl和Rhino共同定义piRNA途径的记忆系统的发现极大地促进了我们对如何建立基于小RNA的序列特异性防御系统的理解。
我们DNA中的基因编码我们身体计划的要素，以及身体中的每项任务是如何实现的。然而,我们的基因组还包含许多不编码功能基因的DNA重复区域。这些区域中的一些是被称为转座子的遗传寄生虫，它们试图在宿主的DNA周围繁殖和传播。为了防止转座子DNA干扰身体的运作方式，人类和其他动物已经进化出复杂的防御机制来识别转座子并防止它们繁殖。在一个这样的机制中，称为piRNA通路，宿主制造称为piRNAs的小分子，其序列与转座子的序列互补，并充当引导使转座子沉默。制备这些piRNA的指令以转座子片段的形式存储在宿主DNA的专用区域中，称为piRNA簇。这些集群因此充当遗传记忆，允许宿主识别并沉默宿主基因组中其他位置的特定转座子。在果蝇中，一种叫做Rhino的蛋白质与piRNA簇结合，这些簇被密集地包装以允许piRNA被制造出来。然而,目前还不清楚犀牛是如何识别和结合piRNA簇的，但不是其他类似密集的DNA区域。Baumgartner等人。使用了遗传的组合，基因组，和成像方法来研究犀牛如何在果蝇基因组中找到自己的方式。他们发现另一种称为Kipferl的蛋白质与Rhino相互作用，并且Rhino需要与几乎所有的piRNA簇结合。由于Kipferl本身可以与Rhino需要找到的序列结合，结果表明，Kipferl可以在密集的piRNA簇中招募和启动Rhino结合。进一步的实验发现,在缺乏Kipferl的苍蝇中，犀牛与称为卫星重复的DNA区域结合，暗示这些自私序列可能会为了自己的利益而争夺犀牛。Kipferl和Rhino共同定义piRNA途径的记忆系统的发现极大地促进了我们对如何建立基于小RNA的序列特异性防御系统的理解。
