Model of multigenerational target gene silencing by piRNAs and downstream 22G-RNAs.

<p>Left: piRNAs against target sites (blue) initiate localized 22G-RNA (blue) production that involves RNA-dependent RNA Polymerases (RdRPs) and Mutator proteins (Muts). 3′ to 5′ spreading of 22G-RNAs along the target gene (e.g. GFP, green) requires the nuclear RNAi pathway. This induces gene silencing that can be maintained over subsequent generations. Middle: Tertiary 22G-RNAs against a target (e.g. GFP, green) are able to silence genes with sequence similarity <i>in trans</i>. This leads to further generation of tertiary 22G-RNAs along the trans-silenced target (e.g. the <i>operon</i>/mCherry, red) by nuclear RNAi factors. Silencing by tertiary 22G-RNAs can become trigger-independent. Right: Paramutation by tertiary 22G-RNAs against mCherry (red) can be stably maintained in the absence of the original trigger(s).