Jakowitsch J, Papp I, Moscone E A, van der Winden J, Matzke M, Matzke A J
Institute of Molecular Biology, Austrian Academy of Sciences, Salzburg, Austria.
Plant J. 1999 Jan;17(2):131-40. doi: 10.1046/j.1365-313x.1999.00357.x.
One type of homology-dependent gene silencing in transgenic plants involves a silencing locus that is able to transcriptionally inactivate and methylate an unlinked target locus with which it shares sequence identity in promoter regions. In a manner resembling paramutation of endogenous genes, the target locus reactivates and loses methylation progressively over several generations after segregating away from the silencing locus, which autonomously acquires stable methylation. To investigate the origins of trans-silencing ability and susceptibility, we have analyzed the structures, flanking DNA sequences and chromosomal locations of a nopaline synthase promoter silencing locus, H2, and a sensitive target locus, K81. A partially resistant target locus, K alpha has been characterized molecularly. The complex and scrambled H2 locus comprises six copies of the nopaline synthase promoter, two of which are collinear with prokaryotic non-T-DNA sequences, and is integrated close to a region of intercalary heterochromatin. These features probably contribute collectively to the silencing ability because H2 subclones reintroduced into random locations in the K81 genome did not frequently induce silencing. Both the K81 and K alpha loci have simple structures, although the former contains non-T-DNA prokaryotic sequences that are also present at H2, and they are flanked by low copy plant DNA. H2 and K81 might interact effectively because they are present on morphologically similar chromosomes from the T subgenome of allotetraploid tobacco.
转基因植物中一种同源依赖性基因沉默涉及一个沉默位点，该位点能够转录失活并甲基化一个与之在启动子区域具有序列同源性的非连锁靶位点。以类似于内源基因副突变的方式，靶位点在与沉默位点分离后的几代中逐渐重新激活并失去甲基化，而沉默位点则自主获得稳定的甲基化。为了研究反式沉默能力和敏感性的起源，我们分析了胭脂碱合酶启动子沉默位点H2和敏感靶位点K81的结构、侧翼DNA序列和染色体位置。已对部分抗性靶位点Kα进行了分子表征。复杂且混乱的H2位点包含六个胭脂碱合酶启动子拷贝，其中两个与原核非T-DNA序列共线，并整合在一个居间异染色质区域附近。这些特征可能共同促成了沉默能力，因为重新导入K81基因组随机位置的H2亚克隆并不经常诱导沉默。K81和Kα位点都具有简单的结构，尽管前者包含也存在于H2的非T-DNA原核序列，并且它们侧翼是低拷贝植物DNA。H2和K81可能有效相互作用，因为它们存在于异源四倍体烟草T亚基因组形态相似的染色体上。
