Specific but interdependent functions for Arabidopsis AGO4 and AGO6 in RNA-directed DNA methylation.

Argonaute (AGO) family proteins are conserved key components of small RNA-induced silencing pathways. In the RNA-directed DNA methylation (RdDM) pathway in Arabidopsis, AGO6 is generally considered to be redundant with AGO4. In this report, our comprehensive, genomewide analyses of AGO4- and AGO6-dependent DNA methylation revealed that redundancy is unexpectedly negligible in the genetic interactions between AGO4 and AGO6. Immunofluorescence revealed that AGO4 and AGO6 differ in their subnuclear co-localization with RNA polymerases required for RdDM. Pol II and AGO6 are absent from perinucleolar foci, where Pol V and AGO4 are co-localized. In the nucleoplasm, AGO4 displays a strong co-localization with Pol II, whereas AGO6 co-localizes with Pol V. These patterns suggest that RdDM is mediated by distinct, spatially regulated combinations of AGO proteins and RNA polymerases. Consistently, Pol II physically interacts with AGO4 but not AGO6, and the levels of Pol V-dependent scaffold RNAs and Pol V chromatin occupancy are strongly correlated with AGO6 but not AGO4. Our results suggest that AGO4 and AGO6 mainly act sequentially in mediating small RNA-directed DNA methylation.