Poster abstracts

Poster number 100 submitted by Jessica Storer

The roles of RNA polymerases II, IV, and V in defense against geminiviruses

Jessica M. Storer (Molecular Genetics, OSU), Jamie N. Jackel (Molecular Genetics, OSU), Tami Coursey (Molecular Genetics, OSU), David M. Bisaro (Molecular Genetics, OSU)

Abstract:
Epigenetic modifications are influential in organizing and controlling gene expression. An example of epigenetic control is repressive cytosine and histone methylation that leads to transcriptional gene silencing (TGS) of potentially harmful DNA, including transposable elements and DNA viruses. Geminiviruses have circular ssDNA genomes that replicate via dsDNA intermediates that associate with histones to form minichromosomes, which can be repressed by epigenetic modifications leading to TGS. Arabidopsis encodes two plant-specific RNA polymerases, Pol IV and Pol V, which carry out non-coding transcription that results in cytosine methylation. We are using geminiviruses as de novo methylation models to more precisely define the roles of these polymerases in chromatin methylation. Results indicate that Pol IV and V are not required to initiate viral DNA methylation, but are necessary for the hypermethylation of viral promoters that accompanies host recovery from infection. Additionally, virus-specific siRNAs are produced in pol IV and pol V mutants, albeit at reduced levels. Thus we hypothesize that another polymerase, likely Pol II, can perform non-coding transcription leading to methylation initiation, while Pol IV and V are needed for its amplification and spread. However, Pol IV and V are individually required for the establishment of a repressive mark, dimethyl histone 3 lysine 9 (H3K9me2), on viral chromatin. Interestingly, Pol IV and V are multi-subunit enzymes that are related to, and share subunits with, Pol II. In addition to transcribing mRNA, Pol II orchestrates the formation of heterochromatin and transcriptional gene silencing in fission yeast. Experiments with hypomorphic Arabidopsis Pol II mutants are in progress to examine its role in methylation-mediated defense. Pol II mutants are hypersusceptible to geminivirus infection and also are unable to deposit H3K9me2 on viral chromatin, suggesting that this enzyme may recruit Pol IV and V to amplify repressive cytosine methylation and establish H3K9me2 marks.

Keywords: geminivivirus, rddm, pol ii