Mechanisms of HDA6-mediated rRNA gene silencing: suppression of intergenic Pol II transcription and differential effects on maintenance versus siRNA-directed cytosine methylation
Mechanisms of HDA6-mediated rRNA gene silencing: suppression of intergenic Pol II transcription and differential effects on maintenance versus siRNA-directed cytosine methylation
复制标题
DOI：
10.1101/gad.1914110
复制
发表时间：
2010-06-01
影响因子：
10.5
通讯作者：
Pikaard, Craig S.
中科院分区：
文献类型：
--
作者：
Earley, Keith W.;Pontvianne, Frederic;Pikaard, Craig S.
在遗传杂交种中，组蛋白脱乙酰酶HDA6被用来沉默转基因、转座子和核糖体RNA(RRNA)基因。在非杂交拟南芥中，我们发现一类在发育过程中正常失活的45S rRNA基因变体在HDA6突变体中未能被沉默。在这些突变体中，CG和CHG基序上的对称胞嘧啶甲基化减少，虚假RNA聚合酶II(POL II)转录发生在整个基因间隔区。由此产生的正义和反义间隔区转录本促进了siRNAs的大量过度生产，这反过来又直接导致相应基因序列的从头开始胞嘧啶甲基化。然而，由此产生的从头DNA甲基化在缺乏HDA6活性的情况下未能抑制Pol I或Pol II的转录；相反，典型的活性基因的常染色质组蛋白修饰积累。总而言之，这些数据揭示了在没有HDA6介导组蛋白去乙酰化的情况下，不受调控的转录、siRNA产生和siRNA指导的DNA甲基化的无效循环。我们认为，在HDA6突变体中，Pol II在基因间隔区的虚假转录，加上组蛋白脱乙酰酶活性的丧失和/或DNA甲基化的维持，消除了发育rRNA基因剂量控制所需的抑制性染色质修饰。
The Arabidopsis histone deacetylase HDA6 is required to silence transgenes, transposons, and ribosomal RNA (rRNA) genes subjected to nucleolar dominance in genetic hybrids. In nonhybrid Arabidopsis thaliana, we show that a class of 45S rRNA gene variants that is normally inactivated during development fails to be silenced in hda6 mutants. In these mutants, symmetric cytosine methylation at CG and CHG motifs is reduced, and spurious RNA polymerase II (Pol II) transcription occurs throughout the intergenic spacers. The resulting sense and antisense spacer transcripts facilitate a massive overproduction of siRNAs that, in turn, direct de novo cytosine methylation of corresponding gene sequences. However, the resulting de novo DNA methylation fails to suppress Pol I or Pol II transcription in the absence of HDA6 activity; instead, euchromatic histone modifications typical of active genes accumulate. Collectively, the data reveal a futile cycle of unregulated transcription, siRNA production, and siRNA-directed DNA methylation in the absence of HDA6-mediated histone deacetylation. We propose that spurious Pol II transcription throughout the intergenic spacers in hda6 mutants, combined with losses of histone deacetylase activity and/or maintenance DNA methylation, eliminates repressive chromatin modifications needed for developmental rRNA gene dosage control.