RNA-based regulation of transposon expression.

Throughout the domains of life, transposon activity represents a serious threat to genome integrity and evolution has realized different molecular mechanisms that aim to inhibit the transposition of mobile DNA. Small noncoding RNAs that function as guides for Argonaute effector proteins represent a key feature of so-called RNA interference (RNAi) pathways and specialized RNAi pathways exist to repress transposon activity on the transcriptional and posttranscriptional level. Transposon transcription can be diminished by targeted DNA methylation or chromatin remodeling via repressive Histone modifications. Posttranscriptional transposon silencing bases on degradation of transposon transcripts to prevent either reverse transcription followed by genomic reintegration or translation into proteins that mediate the transposition process. In plants, Argonaute-like proteins guided by short interfering RNAs (siRNAs) are essential for transposon repression on the epigenetic and posttranscriptional level. In the germline of animals, these tasks are often assumed by a second subclass of Argonaute proteins referred to as Piwi-like proteins, which bind a distinct class of small noncoding RNAs named piwi-interacting RNAs (piRNAs). Though the principals of RNAi pathways are essentially the same in all eukaryotic organisms, remarkable differences can be observed even in closely related species reflecting the astonishing plasticity and diversity of these pathways.