RNA Pol IV and V in gene silencing: Rebel polymerases evolving away from Pol II's rules

Noncoding RNAs regulate gene expression at both the transcriptional and post-transcriptional levels, and play critical roles in development, imprinting and the maintenance of genome integrity in eukaryotic organisms [1, 2, 3]. Therefore, it is important to understand how the production of such RNAs are controlled. In addition to the three canonical DNA dependent RNA polymerases (Pol) Pol I, II and III, two non-redundant plant-specific RNA polymerases, Pol IV and Pol V, have been identified and shown to generate noncoding RNAs that are required for transcriptional gene silencing via the RNA-directed DNA methylation (RdDM) pathway. Thus, somewhat paradoxically, transcription is required for gene silencing. This paradox extends beyond plants, as silencing pathways in yeast, fungi, flies, worms, and mammals also require transcriptional machinery [4, 5]. As plants have evolved specialized RNA polymerases to carry out gene silencing in a manner that is separate from the essential roles of Pol II, their characterization offers unique insights into how RNA polymerases facilitate gene silencing. In this review, we focus on the mechanisms of Pol IV and Pol V function, including their compositions, their transcripts, and their modes of recruitment to chromatin.