Cleard Fabienne, Wolle Daniel, Taverner Andrew M, Aoki Tsutomu, Deshpande Girish, Andolfatto Peter, Karch Francois, Schedl Paul
Department of Genetics and Evolution, University of Geneva, Geneva 4, Switzerland.
Department of Molecular Biology, Princeton University, New Jersey 08544.
Genetics. 2017 Feb;205(2):589-603. doi: 10.1534/genetics.116.195586. Epub 2016 Dec 22.
Chromatin boundary elements subdivide chromosomes in multicellular organisms into physically independent domains. In addition to this architectural function, these elements also play a critical role in gene regulation. Here we investigated the evolution of a Drosophila Bithorax complex boundary element called Fab-7, which is required for the proper parasegment specific expression of the homeotic Abd-B gene. Using a "gene" replacement strategy, we show that Fab-7 boundaries from two closely related species, D. erecta and D. yakuba, and a more distant species, D. pseudoobscura, are able to substitute for the melanogaster boundary. Consistent with this functional conservation, the two known Fab-7 boundary factors, Elba and LBC, have recognition sequences in the boundaries from all species. However, the strategies used for maintaining binding and function in the face of sequence divergence is different. The first is conventional, and depends upon conservation of the 8 bp Elba recognition sequence. The second is unconventional, and takes advantage of the unusually large and flexible sequence recognition properties of the LBC boundary factor, and the deployment of multiple LBC recognition elements in each boundary. In the former case, binding is lost when the recognition sequence is altered. In the latter case, sequence divergence is accompanied by changes in the number, relative affinity, and location of the LBC recognition elements.
染色质边界元件将多细胞生物中的染色体细分为物理上独立的结构域。除了这种结构功能外，这些元件在基因调控中也起着关键作用。在这里，我们研究了果蝇双胸复合体边界元件Fab-7的进化，该元件是同源异型Abd-B基因在副节特异性正确表达所必需的。使用“基因”替换策略，我们表明来自两个近缘物种（直翅果蝇和雅库布果蝇）以及一个较远物种（拟暗果蝇）的Fab-7边界能够替代黑腹果蝇的边界。与这种功能保守性一致，两个已知的Fab-7边界因子Elba和LBC在所有物种的边界中都有识别序列。然而，面对序列差异时用于维持结合和功能的策略是不同的。第一种是常规策略，依赖于8bp的Elba识别序列的保守性。第二种是非常规策略，利用了LBC边界因子异常大且灵活的序列识别特性，以及每个边界中多个LBC识别元件的部署。在前一种情况下，当识别序列改变时结合就会丧失。在后一种情况下，序列差异伴随着LBC识别元件数量、相对亲和力和位置的变化。
