DNA METHYLATION AND GENE SILENCING IN ARABIDOPSIS

胞嘧啶甲基化是在真核生物基因组中发现的DNA修饰，所述真核生物包括哺乳动物、高等动物和哺乳动物。植物和丝状真菌。甲基化与基因表达的丧失（沉默）和同源重组的抑制有关。维持正确的甲基化模式对发育至关重要。例如，在人类中，某些基因组区域中甲基化模式的改变导致许多遗传疾病，包括精神发育迟滞综合征和恶性肿瘤的易感性。在小鼠中，胞嘧啶甲基转移酶敲除突变导致胚胎致死。在高等植物模式生物拟南芥中，甲基化缺陷突变体显示出许多异常的形态，但仍然是可行的和可育的。因此，拟南芥为从分子和遗传学的角度研究甲基化和沉默的机制提供了一个非常容易操作的系统。我们的研究主要集中在拟南芥中的一个甲基化内源基因家族，该家族由两个单态基因和一个在三个不连锁的基因组位置上的尾对尾反向重复基因重复组成。先前的工作表明，反向重复基因座触发了基因组中其他地方相同序列的甲基化。有趣的是，虽然甲基化导致单态基因座沉默，但甲基化反向重复序列中的基因被表达。这个提议的目的是了解反向重复基因座逃避甲基化的抑制作用的机制。这个问题将探讨通过表征染色质结构，顺式作用的DNA序列，和反式作用的因素，控制甲基化和表达的沉默的单线态基因座与表达的反向重复基因座。

Cytosine methylation is a DNA modification found in the genomes of eukaryotes including mammals, higher. plants, and filamentous fungi. Methylation is associated with loss of gene expression (silencing) and suppression of homologous recombination. Maintenance of correct methylation patterns is critically important for development. For example, in humans, alteration of methylation patterns in certain genomic regions leads to a number of genetic diseases including mental retardation syndromes and pre-disposition to malignancies. In mice, a cytosine methyltransferase knock-out mutation results in embryo-lethality. In the higher plant model organism Arabidopsis thaliana, methylation-deficient mutants display a number of aberrant morphologies but are nonetheless viable and fertile. Therefore, Arabidopsis provides an extremely tractable system to study the mechanisms of methylation and silencing with molecular and genetic approaches. Our research focuses on a family of methylated endogenous genes in Arabidopsis that consists of two singlet genes plus a tail-to-tail inverted repeat gene duplication at three unlinked genomic positions. Previous work showed that the inverted repeat locus triggers methylation of the identical sequences elsewhere in the genome. Paradoxically, although methylation causes silencing of a singlet locus, a gene in the methylated inverted repeat is expressed. The goal of this proposal is to understand the mechanisms by which the inverted repeat locus escapes the normally repressive effects of methylation. This question will be explored by characterizing the chromatin structure, cis-acting DNA sequences, and trans-acting factors that control methylation and expression of the silenced singlet locus versus the expressed inverted repeat locus.