All eukaryotes use three essential DNA-dependent RNA polymerases to decode genetic information stored in DNA, namely RNA Polymerases I, II and III. Remarkably, plants have two more RNA polymerases, abbreviated as Pol IV and Pol V, that evolved from Pol II to specialize in the synthesis of RNAs that guide gene silencing, an important process in all living organisms for controlling retrotransposons, viruses or genes important for development. Pols IV and V are key to a complicated RNA-directed DNA methylation (RdDM) pathway. The pathway is initiated by Pol IV, acting in partnership and physical association with a RNA- dependent RNA polymerase, RDR2 to transcribe DNA into short double-stranded RNAs (dsRNAs). These dsRNAs are then trimmed, from either end, by the DICER endonuclease, DCL3, yielding 24 nt short interfering RNAs (siRNAs). The siRNAs, loaded into ARGONAUTE 4 (AGO4), guide the siRNA-AGO complexes to sites of Pol V transcription, where they bind to Pol V transcripts as well as to the C-terminal domain of the Pol V largest subunit. The DNA methyltransferase, DRM2 (the ortholog of human DNMT3) is recruited, methylating cytosines within the Pol V-transcribed DNA. Resulting heterochromatin formation is refractive to transcription by Pols I, II or III. However, Pols IV and V are not repressed in this chromatin environment. Instead, helper proteins that recognize cytosine methylation or repressive histone modifications are thought to recruit Pols IV and V, taking the place of transcription factors and dispensing with conventional promoter elements. There is much that we do not understand about the RdDM process. What unwinds DNA for Pols IV and V to gain access to template strands? How are Pol IV and RDR2 activities coupled for dsRNA synthesis? Where do Pol V transcripts begin and end, and how many siRNA-AGO complexes can bind them ? What do the presumed Pol IV and Pol V helper proteins actually do? By devising new biochemical assays combined with genetic, genomic and structural studies, our goal is to answer these questions in mechanistic detail. In eukaryotes as diverse as humans, flies, worms and fission yeast, noncoding RNAs guide chromatin modifications important for centromere function, transposon silencing, X-chromosome inactivation or imprinting of paternal or maternal alleles. Of special relevance to our studies is the piRNA pathway that directs the silencing of transposons in the human germline, thereby serving the same purpose, and using the same DNA methylation machinery, as the RdDM pathway of plants. Controlled DNA methylation is critical, such that aberrant DNA methylation and chromatin modification is implicated in the pathology of Rett, ICF, Prader-Willi, Beckwith-Wiedemann and Fragile X syndromes, and in most forms of cancer. By understanding the biogenesis and targeting mechanisms of noncoding RNAs in DNA methylation and gene silencing, our studies will contribute new understanding of fundamental processes important for human development and disease.

Public Health Relevance

Noncoding RNAs direct gene silencing to equalize X-chromosome dosage in males and females (X inactivation), to selectively inactivate maternal or paternal imprinted genes, to silence transposons and viruses, and to direct the formation of heterochromatin states necessary for centromere function and proper chromosome segregation. Numerous human diseases and genetic disorders, including cancer, involve changes in DNA methylation, chromatin modification and/or noncoding RNA profiles. Our studies of the enzymes responsible for RNA-directed DNA methylation will explore the generation, processing and transmission of silencing signals encompassed by long and short non- coding RNAs, yielding new insights significant to understanding human development, genetic disorders and disease.

National Institute of Health (NIH)
National Institute of General Medical Sciences (NIGMS)
Research Project (R01)
Project #
Application #
Study Section
Molecular Genetics B Study Section (MGB)
Program Officer
Carter, Anthony D
Project Start
Project End
Budget Start
Budget End
Support Year
Fiscal Year
Total Cost
Indirect Cost
Indiana University Bloomington
Schools of Arts and Sciences
United States
Zip Code
Wendte, Jered M; Pikaard, Craig S (2017) The RNAs of RNA-directed DNA methylation. Biochim Biophys Acta Gene Regul Mech 1860:140-148
Blevins, Todd; Wang, Jing; Pflieger, David et al. (2017) Hybrid incompatibility caused by an epiallele. Proc Natl Acad Sci U S A 114:3702-3707
Wang, Jing; Blevins, Todd; Podicheti, Ram et al. (2017) Mutation of Arabidopsis SMC4 identifies condensin as a corepressor of pericentromeric transposons and conditionally expressed genes. Genes Dev 31:1601-1614
Marasco, Michelle; Li, Weiyi; Lynch, Michael et al. (2017) Catalytic properties of RNA polymerases IV and V: accuracy, nucleotide incorporation and rNTP/dNTP discrimination. Nucleic Acids Res 45:11315-11326
Wendte, Jered M; Haag, Jeremy R; Singh, Jasleen et al. (2017) Functional Dissection of the Pol V Largest Subunit CTD in RNA-Directed DNA Methylation. Cell Rep 19:2796-2808
Wendte, Jered M; Pikaard, Craig S (2016) Targeting Argonaute to chromatin. Genes Dev 30:2649-2650
Blevins, Todd; Podicheti, Ram; Mishra, Vibhor et al. (2015) Identification of Pol IV and RDR2-dependent precursors of 24 nt siRNAs guiding de novo DNA methylation in Arabidopsis. Elife 4:e09591
Ream, Thomas S; Haag, Jeremy R; Pontvianne, Frederic et al. (2015) Subunit compositions of Arabidopsis RNA polymerases I and III reveal Pol I- and Pol III-specific forms of the AC40 subunit and alternative forms of the C53 subunit. Nucleic Acids Res 43:4163-78
Blevins, Todd; Pontvianne, Frédéric; Cocklin, Ross et al. (2014) A two-step process for epigenetic inheritance in Arabidopsis. Mol Cell 54:30-42
Pikaard, Craig S; Mittelsten Scheid, Ortrun (2014) Epigenetic regulation in plants. Cold Spring Harb Perspect Biol 6:a019315

Showing the most recent 10 out of 41 publications