Small RNA profiles in wt and swi6 deletion strains

Proteins of the conserved HP1 family are elementary components of heterochromatin and are generally assumed to play a central role in creating a rigid heterochromatic network that is densely packed and inaccessible to the transcription machinery. In this study we demonstrate that the fission yeast HP1 protein Swi6 exists as a single highly dynamic population and rapidly exchanges in cis and in trans between different heterochromatic regions. Binding to methylated H3K9 or to heterochromatic RNA decelerates Swi6 mobility. We further show that Swi6 is largely dispensable to maintain heterochromatin domains. In contrast, our results disclose an unexpected role of Swi6 in demarcating constitutive heterochromatin from neighboring euchromatin. Our results are consistent with a stochastic model of heterochromatin and imply that heterochromatin is permissive for transcription throughout the cell cycle. Rather than promoting maintenance and spreading of heterochromatin, Swi6 appears to limit these processes to ensure that heterochromatin is appropriately confined. Overall design: Examination of small RNA populations in wt and swi6? cells

保守HP1蛋白家族（HP1 family）是异染色质（heterochromatin）的核心组成成分，既往研究普遍认为其在构建致密包裹、转录机器无法进入的刚性异染色质网络过程中发挥核心作用。本研究证实，裂殖酵母（fission yeast）的HP1蛋白Swi6以单一高度动态的群体形式存在，并可在不同异染色质区域间以顺式（cis）和反式（trans）方式快速交换。与甲基化H3K9（methylated H3K9）或异染色质RNA结合会降低Swi6的迁移速率。我们进一步发现，Swi6在维持异染色质结构域方面基本非必需。与之相反，本研究结果揭示了Swi6一项意料之外的功能：将组成型异染色质（constitutive heterochromatin）与邻近的常染色质（euchromatin）进行边界划分。本研究结果与异染色质的随机模型（stochastic model）相符，并表明异染色质在整个细胞周期（cell cycle）中均允许转录过程发生。Swi6并非促进异染色质的维持与扩散，反而似乎通过限制这些过程，确保异染色质被正确限定在特定区域内。实验整体设计：检测野生型（wt）与swi6?细胞中的小RNA群体。