Kelley Richard L, Lee Ok-Kyung, Shim Yoon-Kyung
Department of Molecular and Cellular Biology, Baylor College of Medicine, One Baylor Plaza, Houston, TX 77030, USA.
Mech Dev. 2008 Nov-Dec;125(11-12):1009-19. doi: 10.1016/j.mod.2008.08.003. Epub 2008 Aug 28.
The dosage compensation complex in Drosophila is composed of at least five MSL proteins and two noncoding roX RNAs that bind hundreds of sites along the single male X chromosome. The roX RNAs are transcribed from X-linked genes and their RNA products "paint" the male X. The roX RNAs and bound MSL proteins can spread in cis from sites of roX transcription, but the mechanism controlling spreading is unknown. Here we find that cis spreading from autosomal roX1 transgenes is coupled to the level of roX transcription. Low to moderate transcription favors, and vigorous transcription abolishes local spreading. We constructed a roX1 minigene one third the size of wild type as a starting point for mutagenesis. This allowed us to test which evolutionarily conserved motifs were required for activity. One short repeat element shared between roX1 and roX2 was found to be particularly important. When all copies were deleted, the RNA was inactive and unstable, while extra copies seem to promote local spreading of the MSL complex from sites of roX1 synthesis. We propose that assembly of the MSL proteins onto the extreme 3' region of elongating roX1 transcripts determines whether the MSL complex spreads in cis.
果蝇中的剂量补偿复合体由至少五种MSL蛋白和两种非编码roX RNA组成，它们沿着雄性单条X染色体结合数百个位点。roX RNA从X连锁基因转录而来，其RNA产物“描绘”雄性X染色体。roX RNA和结合的MSL蛋白可从roX转录位点顺式扩散，但控制扩散的机制尚不清楚。在这里，我们发现从常染色体roX1转基因的顺式扩散与roX转录水平相关。低至中等水平的转录有利于扩散，而活跃转录则会消除局部扩散。我们构建了一个大小为野生型三分之一的roX1小基因作为诱变的起点。这使我们能够测试哪些进化保守基序对活性是必需的。我们发现roX1和roX2之间共享的一个短重复元件尤为重要。当所有拷贝都被删除时，RNA无活性且不稳定，而额外的拷贝似乎会促进MSL复合体从roX1合成位点的局部扩散。我们提出，MSL蛋白在延伸的roX1转录本的极端3'区域上的组装决定了MSL复合体是否顺式扩散。
