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Abstract：
Transfer RNA (tRNA) genes and other RNA polymerase III transcription units are dispersed in high copy throughout nuclear genomes, and can antagonize RNA polymerase II transcription in their immediate chromosomal locus. Previous work in Saccharomyces cerevisiae found that this local silencing required subnuclear clustering of the tRNA genes near the nucleolus. Here we show that the silencing also requires nucleosome participation, though the nature of the nucleosome interaction appears distinct from other forms of transcriptional silencing. Analysis of an extensive library of histone amino acid substitutions finds a large number of residues that affect the silencing, both in the histone N-terminal tails and on the nucleosome disk surface. The residues on the disk surfaces involved are largely distinct from those affecting other regulatory phenomena. Consistent with the large number of histone residues affecting tgm silencing, survey of chromatin modification mutations shows that several enzymes known to affect nucleosome modification and positioning are also required. The enzymes include an Rpd3 deacetylase complex, Hos1 deacetylase, Glc7 phosphatase, and the RSC nucleosome remodeling activity, but not multiple other activities required for other silencing forms or boundary element function at tRNA gene loci. Models for communication between the tRNA gene transcription complexes and local chromatin are discussed.
摘要：
转移RNA（tRNA）基因和其他RNA聚合酶III转录单位分散在整个核基因组中的高拷贝，并且可以拮抗RNA聚合酶II在其直接染色体位点的转录。先前在酿酒酵母中的工作发现，这种局部沉默需要靠近核仁的tRNA基因的亚核聚类。在这里，我们表明沉默也需要核小体的参与，尽管核小体相互作用的性质似乎与其他形式的转录沉默不同。对组蛋白氨基酸取代的广泛文库的分析发现了影响沉默的大量残基，在组蛋白N末端尾巴和核小体盘表面。所涉及的磁盘表面上的残留物与影响其他调节现象的残留物大不相同。与影响tgm沉默的大量组蛋白残基一致，对染色质修饰突变的调查表明，还需要几种已知影响核小体修饰和定位的酶。这些酶包括Rpd3脱乙酰酶复合物，Hos1脱乙酰酶，Glc7磷酸酶，和RSC核小体重塑活性，但不是其他沉默形式或tRNA基因位点的边界元件功能所需的多种其他活性。讨论了tRNA基因转录复合物与局部染色质之间的通讯模型。
