Nolan Tony, Braccini Laura, Azzalin Gianluca, De Toni Arianna, Macino Giuseppe, Cogoni Carlo
Dipartimento di Biotecnologie Cellulari ed Ematologia, Sezione di Genetica Molecolare, Universita' di Roma La Sapienza 00161 Rome, Italy.
Nucleic Acids Res. 2005 Mar 14;33(5):1564-73. doi: 10.1093/nar/gki300. Print 2005.
Post-transcriptional gene silencing (PTGS) involving small interfering RNA (siRNA)-directed degradation of RNA transcripts and transcriptional silencing via DNA methylation have each been proposed as mechanisms of genome defence against invading nucleic acids, such as transposons and viruses. Furthermore, recent data from plants indicates that many transposons are silenced via a combination of the two mechanisms, and siRNAs can direct methylation of transposon sequences. We investigated the contribution of DNA methylation and the PTGS pathway to transposon control in the filamentous fungus Neurospora crassa. We found that repression of the LINE1-like transposon, Tad, requires the Argonaute protein QDE2 and Dicer, each of which are required for transgene-induced PTGS (quelling) in N.crassa. Interestingly, unlike quelling, the RNA-dependent RNA polymerase QDE1 and the RecQ DNA helicase QDE3 were not required for Tad control, suggesting the existence of specialized silencing pathways for diverse kinds of repetitive elements. In contrast, Tad elements were not significantly methylated and the DIM2 DNA methyltransferase, responsible for all known DNA methylation in Neurospora, had no effect on Tad control. Thus, an RNAi-related transposon silencing mechanism operates during the vegetative phase of N.crassa that is independent of DNA methylation, highlighting a major difference between this organism and other methylation-proficient species.
转录后基因沉默（PTGS）涉及小干扰RNA（siRNA）介导的RNA转录本降解，以及通过DNA甲基化实现的转录沉默，这两种机制均被认为是基因组抵御转座子和病毒等入侵核酸的防御机制。此外，来自植物的最新数据表明，许多转座子是通过这两种机制的组合被沉默的，并且siRNA可以引导转座子序列的甲基化。我们研究了DNA甲基化和PTGS途径在丝状真菌粗糙脉孢菌中转座子控制中的作用。我们发现，类似LINE1的转座子Tad的抑制需要Argonaute蛋白QDE2和Dicer，它们在粗糙脉孢菌中是转基因诱导的PTGS（抑制作用）所必需的。有趣的是，与抑制作用不同，Tad控制不需要RNA依赖性RNA聚合酶QDE1和RecQ DNA解旋酶QDE3，这表明存在针对不同类型重复元件的专门沉默途径。相比之下，Tad元件没有明显的甲基化，负责粗糙脉孢菌中所有已知DNA甲基化的DIM2 DNA甲基转移酶对Tad控制没有影响。因此，在粗糙脉孢菌的营养生长阶段存在一种与RNAi相关的转座子沉默机制，该机制独立于DNA甲基化，这突出了该生物体与其他具有甲基化能力的物种之间的主要差异。
