Onodera Yasuyuki, Haag Jeremy R, Ream Thomas, Costa Nunes Pedro, Pontes Olga, Pikaard Craig S
Biology Department, Washington University, 1 Brookings Drive, St. Louis, MO 63130, USA.
Cell. 2005 Mar 11;120(5):613-22. doi: 10.1016/j.cell.2005.02.007.
All eukaryotes have three nuclear DNA-dependent RNA polymerases, namely, Pol I, II, and III. Interestingly, plants have catalytic subunits for a fourth nuclear polymerase, Pol IV. Genetic and biochemical evidence indicates that Pol IV does not functionally overlap with Pol I, II, or III and is nonessential for viability. However, disruption of the Pol IV catalytic subunit genes NRPD1 or NRPD2 inhibits heterochromatin association into chromocenters, coincident with losses in cytosine methylation at pericentromeric 5S gene clusters and AtSN1 retroelements. Loss of CG, CNG, and CNN methylation in Pol IV mutants implicates a partnership between Pol IV and the methyltransferase responsible for RNA-directed de novo methylation. Consistent with this hypothesis, 5S gene and AtSN1 siRNAs are essentially eliminated in Pol IV mutants. The data suggest that Pol IV helps produce siRNAs that target de novo cytosine methylation events required for facultative heterochromatin formation and higher-order heterochromatin associations.
所有真核生物都有三种依赖于核DNA的RNA聚合酶，即聚合酶I、II和III。有趣的是，植物有一种第四种核聚合酶（聚合酶IV）的催化亚基。遗传和生化证据表明，聚合酶IV在功能上不与聚合酶I、II或III重叠，且对生存力并非必不可少。然而，聚合酶IV催化亚基基因NRPD1或NRPD2的破坏会抑制异染色质缔合形成染色中心，这与着丝粒周围5S基因簇和AtSN1逆转录元件处胞嘧啶甲基化的丧失同时发生。聚合酶IV突变体中CG、CNG和CNN甲基化的丧失表明聚合酶IV与负责RNA指导的从头甲基化的甲基转移酶之间存在协同关系。与这一假设一致，5S基因和AtSN1小干扰RNA在聚合酶IV突变体中基本消失。数据表明，聚合酶IV有助于产生小干扰RNA，这些小干扰RNA靶向兼性异染色质形成和高阶异染色质缔合所需的从头胞嘧啶甲基化事件。
