Plant Multisubunit RNA Polymerases IV and V

Plants are unique among eukaryotes in having five nuclear multisubunit RNA polymerases. These include RNA polymerases I, II, and III (Pols I, II, and III), which are ubiquitous among eukaryotes, plus two plant-specific RNA polymerases, Pol IV and Pol V, that are 12-subunit enzymes that evolved as specialized forms of Pol II. Pols IV and V are nonessential for viability but play important roles in RNA-mediated gene silencing pathways that tame transposons, defend against invading viruses, mediate cross talk among alleles, and influence development. Numerous amino acids that are invariant in the catalytic subunits of Pols I, II, and III are substituted, or even absent, in Pols IV or V, implying that Pols IV and V have fewer functional constraints on their evolution than other multisubunit RNA polymerases. In vitro, Pol IV and Pol V can extend an RNA primer hybridized to a DNA template, but the templates transcribed by Pol IV and Pol V in vivo are unclear. Likewise, the boundaries of Pol IV and Pol V transcription units and the characteristics of their primary transcripts remain undefined. In this chapter, the state of our understanding of Pol IV and Pol V subunit compositions and functions is discussed.