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首页> 外文期刊>Molecular and Cellular Biology >DNA in transcriptionally silent chromatin assumes a distinct topology that is sensitive to cell cycle progression.
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DNA in transcriptionally silent chromatin assumes a distinct topology that is sensitive to cell cycle progression.

机译：转录沉默染色质中的DNA呈现出对细胞周期进程敏感的独特拓扑。

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Transcriptionally silent regions of the Saccharomyces cerevisiae genome, the silent mating type loci and telomeres, represent the yeast equivalent of metazoan heterochromatin. To gain insight into the nature of silenced chromatin structure, we have examined the topology of DNA spanning the HML silent mating type locus by determining the superhelical density of mini-circles excised from HML (HML circles) by site-specific recombination. We observed that HML circles excised in a wild-type (SIR+) strain were more negatively supercoiled upon deproteinization than were the same circles excised in a sir- strain, in which silencing was abolished, even when HML alleles in which neither circle was transcriptionally competent were used. cis-acting sites flanking HML, called silencers, are required in the chromosome for establishment and inheritance of silencing. HML circles excised without silencers from cells arrested at any point in the cell cycle retained SIR-dependent differences in superhelical density. However, progression through the cell cycle converted SIR+ HML circles to a form resembling that of circles from sir- cells. This decay was not observed with circles carrying a silencer. These results establish that (i) DNA in transcriptionally silenced chromatin assumes a distinct topology reflecting a distinct organization of silenced versus active chromatin; (ii) the altered chromatin structure in silenced regions likely results from changes in packaging of individual nucleosomes, rather than changes in nucleosome density; and (iii) cell cycle progression disrupts the silenced chromatin structure, a process that is counteracted by silencers.
机译：酿酒酵母基因组的转录沉默区域，即沉默交配型基因座和端粒，代表了后生异染色质的酵母等效物。为了深入了解沉默的染色质结构的本质，我们通过确定通过位点特异性重组从HML（HML环）中切除的小圆圈的超螺旋密度，研究了跨越HML沉默交配型基因座的DNA拓扑。我们观察到，在野生型（SIR +）菌株中切除的HML环比在Sir-菌株中切除的相同环（在其中消除了沉默的情况下）消除了超螺旋，即使其中没​​有一个环都具有转录能力的HML等位基因也是如此被使用。 HML侧翼的顺式作用位点（称为沉默子）是染色体中建立和遗传沉默所必需的。在细胞周期任何点停滞的细胞中，没有消音器切除的HML环保留了SIR依赖的超螺旋密度差异。但是，整个细胞周期的进程将SIR + HML圆圈转换为类似于Sir-cell圆圈的形式。带有消音器的圆圈未观察到这种衰减。这些结果证明：（i）转录沉默染色质中的DNA呈现出独特的拓扑结构，反映了沉默染色质与活性染色质的不同组织; （ii）沉默区域中染色质结构的改变可能是由于单个核小体的包装变化而不是核小体密度变化引起的; （iii）细胞周期进程破坏了沉默的染色质结构，这一过程被沉默子所抵消。

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