神秘“副突变”缺不了RNA酶

　　

生物通报道：副突变（paramutation）是对孟德尔定理的一个挑战。它指的是一个等位基因可以使其同源基因的转录产生稳定可遗传变化，即一个等位基因被另外一个等位基因在转录水平上所沉默且这种能力可遗传。虽然研究者们曾怀疑这里可能包含有表观遗传性质的变化，但是自从1950s这个问题被提出以来，科学家们就一直期待能在分子水平上解释这个奇怪的现象。

副突变的现象存在于几种植物中，这个现象在玉米的几个基因中很明显。在这种现象中，必须是特异而正常的等位基因才被称为能产生副突变的等位基因，当在相同的基因组中存在另一种特殊的致副突变的等位基因时它可发生不可逆变化。

玉米中b1基因座在被独特的回纹重复（tandem repeat）所调节。Mop1(副突变调节因子1)基因是副突变必须的，并且mop1的突变会恢复被沉默的Mutator元件。携带mop1基因突变的植株还会随机地表现出多向性发育显型。

在7月20日的《自然》杂志上，来自亚利桑那州大学和Duquesne大学的研究人员公布了mop1基因（一种RNA依赖型RNA聚合酶基因）的定位克隆结果，它与植物中与siRNA的生产有关的RDRP非常相似。

细胞核分析结果揭示出b1副突变所需的回纹结构由双链转录得来，但是检测不到siRNA。研究人员推测mop1 RDRP是维持重复RNA阈值水平所需的。（生物通记者杨遥）

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

Mary Alleman^1,2, Lyudmila Sidorenko¹, Karen McGinnis¹, Vishwas Seshadri^1,2, Jane E. Dorweiler^1,2, Joshua White^1,2, Kristin Sikkink¹ and Vicki L. Chandler¹

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 chromatin^5,6. 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.