表观遗传记忆在多细胞生物体中细胞身份的建立和维持中起着至关重要的作用。Polycomb和trithorax组(PcG和TrxG)蛋白负责表观遗传记忆,在苍蝇中,它们被招募到称为多梳反应元件(PRES)的专门DNA调控元件。以前的转基因研究表明,PRES可以在正常环境之外沉默报告基因,通常通过配对敏感(PSS)机制;然而,它们的沉默活性是非自主的,取决于周围的染色质环境。不知道为什么PRE活动取决于当地环境或哪些外部因素可以引起沉默。
在果蝇中使用attP系统,我们发现所谓的中性染色质环境在支持特征明确的bxdPRE沉默活性的能力上有很大差异.在难治性染色体环境中,PcG沉默所需的因素无法获得PRE。可以通过将bxdPRE连接到边界元素(绝缘体)来挽救沉默活动。当放在PRE旁边时,边界诱导染色质结构的改变,使PcG沉默获得bxdPRE至关重要的因素。当放置在远离bxdPRE时,边界通过将bxdPRE带到每个同源物附近来诱导PSS。
这项概念验证研究表明,附近的边界元素可以诱导或增强PRE的抑制活性。
Epigenetic memory plays a critical role in the establishment and maintenance of cell identities in multicellular organisms. Polycomb and trithorax group (PcG and TrxG) proteins are responsible for epigenetic memory, and in flies, they are recruited to specialized DNA regulatory elements termed polycomb response elements (PREs). Previous transgene studies have shown that PREs can silence reporter genes outside of their normal context, often by pairing sensitive (PSS) mechanism; however, their silencing activity is non-autonomous and depends upon the surrounding chromatin context. It is not known why PRE activity depends on the local environment or what outside factors can induce silencing.
Using an attP system in Drosophila, we find that the so-called neutral chromatin environments vary substantially in their ability to support the silencing activity of the well-characterized bxdPRE. In refractory chromosomal contexts, factors required for PcG-silencing are unable to gain access to the PRE. Silencing activity can be rescued by linking the bxdPRE to a boundary element (insulator). When placed next to the PRE, the boundaries induce an alteration in chromatin structure enabling factors critical for PcG silencing to gain access to the bxdPRE. When placed at a distance from the bxdPRE, boundaries induce PSS by bringing the bxdPREs on each homolog in close proximity.
This proof-of-concept study demonstrates that the repressing activity of PREs can be induced or enhanced by nearby boundary elements.