RNA-DIRECTED DNA METHYLATION PREVENTS RAPID AND HERITABLE REVERSAL OF TRANSPOSON UNDER HEAT STRESS IN ZEA MAYS
RNA-directed DNA methylation (RdDM) is a process by which epigenetic silencing is maintained at the boundary between genes and flanking transposable elements. In maize, RdDM is dependent on Mediator of Paramutation 1 (Mop1), a putative RNA dependent RNA polymerase. Here I show that although RdDM is essential for the maintenance of DNA methylation of a silenced MuDR transposon in maize, a loss of that methylation does not result in a restoration of activity of that element. Instead, heritable maintenance of silencing is maintained by histone modifications. At one terminal inverted repeat (TIR) of the element, heritable silencing is mediated via H3K9 and H3K27 dimethylation, even in the absence of DNA methylation. At the second TIR, heritable silencing is mediated by H3K27 trimethylation, a mark normally associated with somatically inherited gene silencing. I find that a brief exposure of high temperature in a mop1 mutant rapidly reverses both of these modifications in conjunction with a loss of transcriptional silencing. These reversals are heritable, even in mop1 wild type progeny in which methylation is restored at both TIRs. These observations suggest that DNA methylation is neither necessary to maintain silencing, nor is it sufficient to initiate silencing once it has been reversed. To leverage the specificity of our observations made at bench, I also performed a transcriptome analysis in mop1 mutants under heat. I found that a substantial number of genes as well as a subset of TEs are reactivated in mop1 mutants under heat, which is consistent with the effects I observed on MuDR. Interestingly, I found that mop1-specific reactivation of TEs is closely correlated with changes in expression of nearby genes, most of which are involved in metabolic transportation and sensing. This suggests that one function of MOP1 is to prevent inappropriate expression of genes in this pathway when they are close to TEs. Taken together, my work will provide an opportunity to better understand the causes and consequences of TE silencing and reactivation, as well as the effects of TEs on gene regulation under stress conditions.
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