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Originally published as Genetics Published Articles Ahead of Print on May 15, 2006.
Genetics, Vol. 173, 1637-1647, July 2006, Copyright © 2006
doi:10.1534/genetics.106.058669
Transcriptionally Silenced Transgenes in Maize Are Activated by Three Mutations Defective in Paramutation
Karen M. McGinnis, Catherine Springer1, Yan Lin2, Charles C. Carey3 and Vicki Chandler4
Department of Plant Sciences, University of Arizona, Tucson, Arizona 85721
4 Corresponding author: University of Arizona, Department of Plant Sciences, 303 Forbes Hall, Tucson, AZ 85721.
E-mail: chandler{at}ag.arizona.edu
Plants with mutations in one of three maize genes, mop1, rmr1, and rmr2, are defective in paramutation, an allele-specific interaction that leads to meiotically heritable chromatin changes. Experiments reported here demonstrate that these genes are required to maintain the transcriptional silencing of two different transgenes, suggesting that paramutation and transcriptional silencing of transgenes share mechanisms. We hypothesize that the transgenes are silenced through an RNA-directed chromatin mechanism, because mop1 encodes an RNA-dependent RNA polymerase. In all the mutants, DNA methylation was reduced in the active transgenes relative to the silent transgenes at all of the CNG sites monitored within the transgene promoter. However, asymmetrical methylation persisted at one site within the reactivated transgene in the rmr1-1 mutant. With that one mutant, rmr1-1, the transgene was efficiently resilenced upon outcrossing to reintroduce the wild-type protein. In contrast, with the mop1-1 and rmr2-1 mutants, the transgene remained active in a subset of progeny even after the wild-type proteins were reintroduced by outcrossing. Interestingly, this immunity to silencing increased as the generations progressed, consistent with a heritable chromatin state being formed at the transgene in plants carrying the mop1-1 and rmr2-1 mutations that becomes more resistant to silencing in subsequent generations.
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