Retrotransposons are endogenous elements that have the ability to mobilise their DNA between different locations in the host genome. The Ty3 retrotransposon integrates with an exquisite specificity in a narrow window upstream of RNA Polymerase (Pol) III-transcribed genes, representing a paradigm for harmless targeted integration. Here we present the cryo-EM reconstruction at 4.0 Å of an active Ty3 strand transfer complex bound to TFIIIB transcription factor and a tRNA gene. The structure unravels the molecular mechanisms underlying Ty3 targeting specificity at Pol III-transcribed genes and sheds light into the architecture of retrotransposon machinery during integration. Ty3 intasome contacts a region of TBP, a subunit of TFIIIB, which is blocked by NC2 transcription regulator in RNA Pol II-transcribed genes. A newly-identified chromodomain on Ty3 integrase interacts with TFIIIB and the tRNA gene, defining with extreme precision the integration site position.

逆转录转座子是内源性元件，能够在宿主基因组的不同位置之间调动其 DNA。 Ty3 逆转录转座子在 RNA 聚合酶 (Pol) III 转录基因上游的狭窄窗口中以精致的特异性进行整合，代表了无害靶向整合的范例。在这里，我们展示了与 TFIIIB 转录因子和 tRNA 基因结合的活性 Ty3 链转移复合物在 4.0 Å 处的冷冻电镜重建。该结构揭示了 Ty3 靶向 Pol III 转录基因特异性的分子机制，并揭示了整合过程中逆转录转座子机制的结构。 Ty3 嵌体接触 TBP 的一个区域，TBP 是 TFIIIB 的一个亚基，该区域被 RNA Pol II 转录基因中的 NC2 转录调节因子阻断。 Ty3 整合酶上新鉴定的染色结构域与 TFIIIB 和 tRNA 基因相互作用，极其精确地定义整合位点位置。