Hale Christopher J, Erhard Karl F, Lisch Damon, Hollick Jay B
Department of Plant and Microbial Biology, University of California Berkeley, Berkeley, CA, USA.
PLoS Genet. 2009 Aug;5(8):e1000598. doi: 10.1371/journal.pgen.1000598. Epub 2009 Aug 14.
Mutations affecting the maintenance of heritable epigenetic states in maize identify multiple RNA-directed DNA methylation (RdDM) factors including RMR1, a novel member of a plant-specific clade of Snf2-related proteins. Here we show that RMR1 is necessary for the accumulation of a majority of 24 nt small RNAs, including those derived from Long-Terminal Repeat (LTR) retrotransposons, the most common repetitive feature in the maize genome. A genetic analysis of DNA transposon repression indicates that RMR1 acts upstream of the RNA-dependent RNA polymerase, RDR2 (MOP1). Surprisingly, we show that non-polyadenylated transcripts from a sampling of LTR retrotransposons are lost in both rmr1 and rdr2 mutants. In contrast, plants deficient for RNA Polymerase IV (Pol IV) function show an increase in polyadenylated LTR RNA transcripts. These findings support a model in which Pol IV functions independently of the small RNA accumulation facilitated by RMR1 and RDR2 and support that a loss of Pol IV leads to RNA Polymerase II-based transcription. Additionally, the lack of changes in general genome homeostasis in rmr1 mutants, despite the global loss of 24 nt small RNAs, challenges the perceived roles of siRNAs in maintaining functional heterochromatin in the genomes of outcrossing grass species.
影响玉米中可遗传表观遗传状态维持的突变鉴定出多个RNA介导的DNA甲基化（RdDM）因子，包括RMR1，它是植物特异性Snf2相关蛋白进化枝中的一个新成员。我们在此表明，RMR1对于大多数24 nt小RNA的积累是必需的，包括那些源自长末端重复序列（LTR）反转录转座子的小RNA，LTR反转录转座子是玉米基因组中最常见的重复特征。对DNA转座子抑制的遗传分析表明，RMR1在RNA依赖的RNA聚合酶RDR2（MOP1）的上游起作用。令人惊讶的是，我们发现，在rmr1和rdr2突变体中，来自LTR反转录转座子样本的非聚腺苷酸化转录本都缺失了。相反，缺乏RNA聚合酶IV（Pol IV）功能的植物中，聚腺苷酸化的LTR RNA转录本增加。这些发现支持了一个模型，即Pol IV独立于由RMR1和RDR2促进的小RNA积累发挥作用，并支持Pol IV的缺失导致基于RNA聚合酶II的转录。此外，尽管rmr1突变体中24 nt小RNA整体缺失，但一般基因组稳态没有变化，这对异源杂交禾本科物种基因组中siRNA在维持功能性异染色质方面的既定作用提出了挑战。
