An RNA-dependent RNA polymerase is required for paramutation in maize

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Abstract
Paramutation is an allele-dependent transfer of epigenetic information, which results in the heritable silencing of one allele by another1. Paramutation at the b1 locus in maize is mediated by unique tandem repeats that communicate in trans to establish and maintain meiotically heritable transcriptional silencing2. The mop1 (mediator of paramutation1) gene is required for paramutation3, and mop1 mutations reactivate silenced Mutator elements4. Plants carrying mutations in the mop1 gene also stochastically exhibit pleiotropic developmental phenotypes3. Here we report the map-based cloning of mop1, an RNA-dependent RNA polymerase gene (RDRP), most similar to the RDRP in plants that is associated with the production of short interfering RNA (siRNA) targeting chromatin5,6. Nuclear run-on assays reveal that the tandem repeats required for b1 paramutation are transcribed from both strands, but siRNAs were not detected. We propose that the mop1 RDRP is required to maintain a threshold level of repeat RNA, which functions in trans to establish and maintain the heritable chromatin states associated with paramutation.