Functional dissection of the Pol V large subunit CTD in RNA-directed DNA methylation

Plant multisubunit RNA Polymerase V transcription recruits Argonaute siRNA complexes that specify sites of RNA-directed DNA methylation (RdDM) for gene silencing. Pol V9s largest subunit, NRPE1, evolved from the largest subunit of Pol II but has a distinctive carboxyl-terminal domain (CTD). We show that the Pol V CTD is dispensable for catalytic activity in vitro, yet essential in vivo. One CTD subdomain (DeCL) is required for Pol V function at virtually all loci. Other CTD subdomains have locus-specific effects. In a yeast two-hybrid screen, the 3-prime to 5-prime exoribonuclease, RRP6L1 was identified as an interactor with the DeCL and glutamine-serine-rich (QS) subdomains, located downstream from an Argonaute-binding repeat subdomain. Experimental evidence indicates that RRP6L1 trims the 3-prime ends of Pol V transcripts sliced by ARGONAUTE 4 (AGO4), suggesting a model whereby the CTD enables the spatial and temporal coordination of AGO4 and RRP6L1 RNA processing activities.