Stanne Tara M, Kushwaha Manish, Wand Matthew, Taylor Jesse E, Rudenko Gloria
Division of Cell and Molecular Biology, Sir Alexander Fleming Building, Imperial College London-South Kensington, Imperial College Road, London, United Kingdom.
Eukaryot Cell. 2011 Jul;10(7):964-76. doi: 10.1128/EC.05048-11. Epub 2011 May 13.
The unicellular eukaryote Trypanosoma brucei is unusual in having very little transcriptional control. The bulk of the T. brucei genome is constitutively transcribed by RNA polymerase II (Pol II) as extensive polycistronic transcription units. Exceptions to this rule include several RNA Pol I transcription units such as the VSG expression sites (ESs), which are mono-allelically expressed. TbISWI, a member of the SWI2/SNF2 related chromatin remodeling ATPases, plays a role in repression of Pol I-transcribed ESs in both bloodstream- and procyclic-form T. brucei. We show that TbISWI binds both active and silent ESs but is depleted from the ES promoters themselves. TbISWI knockdown results in an increase in VSG transcripts from the silent VSG ESs. In addition to its role in the repression of the silent ESs, TbISWI also contributes to the downregulation of the Pol I-transcribed procyclin loci, as well as nontranscribed VSG basic copy arrays and minichromosomes. We also show that TbISWI is enriched at a number of strand switch regions which form the boundaries between Pol II transcription units. These strand switch regions are the presumed sites of Pol II transcription initiation and termination and are enriched in modified histones and histone variants. Our results indicate that TbISWI is a versatile chromatin remodeler that regulates transcription at multiple Pol I loci and is particularly abundant at many Pol II transcription boundaries in T. brucei.
单细胞真核生物布氏锥虫在转录控制方面非常特殊，几乎没有转录调控。布氏锥虫基因组的大部分由RNA聚合酶II（Pol II）组成型转录，形成广泛的多顺反子转录单元。该规则的例外情况包括几个RNA Pol I转录单元，如VSG表达位点（ESs），它们是单等位基因表达的。TbISWI是SWI2/SNF2相关染色质重塑ATP酶家族的成员，在血流型和前循环型布氏锥虫中对Pol I转录的ESs的抑制起作用。我们发现TbISWI与活跃和沉默的ESs都结合，但在ES启动子本身中却不存在。敲低TbISWI会导致沉默的VSG ESs的VSG转录本增加。除了在沉默ESs的抑制中发挥作用外，TbISWI还参与了Pol I转录的前环蛋白基因座以及非转录的VSG基本拷贝阵列和微型染色体的下调。我们还表明TbISWI在许多形成Pol II转录单元边界的链交换区域富集。这些链交换区域被认为是Pol II转录起始和终止的位点，并且富含修饰的组蛋白和组蛋白变体。我们的结果表明，TbISWI是一种多功能的染色质重塑因子，可调节多个Pol I基因座的转录，并且在布氏锥虫的许多Pol II转录边界处特别丰富。
