Poster abstracts

Poster number 53 submitted by Meredith Sigman

How many polymerases does it take to silence a transposable element?

Meredith J Sigman (OSU Molecular Genetics), Dalen Fultz (OSU Molecular Genetics), Diego Cuerda Gil (OSU Molecular Genetics), R. Keith Slotkin (OSU Molecular Genetics)

Abstract:
Transposable elements (TEs) are pieces of DNA capable of jumping from one location within the genome to another. The insertion of TEs into new locations is inherently mutagenic as they can interrupt genes or regulatory regions. Therefore it is in an organism’s best interest to shut down or silence TEs to prevent mutagenesis. In order for TEs to be silenced, an organism must be able recognize them as non-self DNA and trigger silencing through the deposition of repressive epigenetic marks over the TE promoter. These epigenetic marks come in the form of cytosine methylation and histone H3K9 methylation. Plants, such as Arabidopsis thaliana, possess two RNA Polymerase II-related protein complexes called RNA Polymerase IV (Pol IV) and RNA Polymerase V (Pol V). In contrast to Pol II, which transcribes active regions of the genome into protein-coding mRNAs, Pol IV and Pol V operate at TE loci to ultimately recruit the methyltransferases responsible for cytosine and H3K9 methylation(1). The conundrum is that Pol IV and Pol V are only known to be recruited to a locus for transcription by existing cytosine methylation (Pol V)(2,3) and H3K9 histone methylation (Pol IV)(4,5). The field does not understand whether Pol II can coordinate the original rounds of TE chromatin modifications, or whether there are alternative mechanisms, besides preexisting chromatin modification, responsible for Pol IV and Pol V recruitment. The key unanswered question of my research is, therefore, how does the initial or de novo methylation of a TE locus occur?
Thus far, evidence points to the involvement of RNA Polymerase II in de novo cytosine methylation of TEs in Arabidopsis (1,6,7). Published and unpublished data from our lab provide three key pieces of data: 1) Pol IV is not required for TE de novo methylation, 2) TEs must possess a promoter and actively transcribe in order to be de novo silenced, and 3) we find that de novo TE silencing is absent in pol V mutant plants. Combined, these results suggest that a TE is required to produce an mRNA transcript before it can be recognized and silenced. Yet, if Pol V is also required, it must be recruited to the unmethylated TE by an undefined, methylation-independent mechanism. My work will address the question of Pol V’s recruitment and examine the mechanism of de novo methylation at unmethylated TEs.

References:
1. You, W et al. Interplay among RNA polymerases II, IV and V in RNA-directed DNA methylation… Plant molecular biology (2013).
2. Liu, ZW et al. The SET domain proteins SUVH2 and SUVH9 are required for Pol V occupancy… PLoS Genet (2014).
3. Johnson, LM et al. SRA-and SET-domain-containing proteins link RNA polymerase V occupancy to DNA methylation. Nature (2014).
4. Law, JA et al. Polymerase IV occupancy at RNA-directed DNA methylation sites requires SHH1. Nature (2013).
5. Zhang, H. et al. DTF1 is a core component of RNA-directed DNA methylation and may assist in the recruitment of Pol IV. Proc. Natl. Acad. Sci. U.S.A. (2013).
6. Nuthikattu, S. et al. The initiation of epigenetic silencing of active transposable elements is triggered… Plant Physiol. (2013).
7. Zheng, B. et al. Intergenic transcription by RNA polymerase II coordinates Pol IV and Pol V … Genes Dev. (2009).

Keywords: Transposable Element, Silencing, Methylation