Arnosti D N, Barolo S, Levine M, Small S
Department of Biology, UCSD, La Jolla 92093-0347, USA.
Development. 1996 Jan;122(1):205-14. doi: 10.1242/dev.122.1.205.
Previous studies have provided a detailed model for the regulation of even-skipped (eve) stripe 2 expression in the Drosophila embryo. The bicoid (bcd) regulatory gradient triggers the expression of hunchback (hb); these work synergistically to activate the stripe in the anterior half of the embryo, bcd also coordinates the expression of two repressors, giant (gt) and Kruppel (Kr), which define the anterior and posterior borders of the stripe, respectively. Here, we report the findings of extensive cis- and trans- complementation analyses using a series of defective stripe 2 enhancers in transgenic embryos. This study reaches two primary conclusions. First, the strip 2 enhancer is inherently 'sensitized' for repression by gt. We propose that gt specifies the sharp anterior stripe border by blocking two tiers of transcriptional synergy, cooperative binding to DNA and cooperative contact of bound activators with the transcription complex. Second, we find that the synergistic activity of hb and bcd is 'promiscuous'. For example, a maternally expressed Gal4-Sp1 fusion protein can functionally replace hb in the stripe 2 enhancer. This finding challenges previous proposals for dedicated hb and bcd interactions in the segmentation process.
先前的研究为果蝇胚胎中偶数缺口（eve）条纹2表达的调控提供了详细模型。双尾（bcd）调控梯度触发驼背（hb）的表达；它们协同作用以激活胚胎前半部分的条纹，bcd还协调两种阻遏物巨人（gt）和克鲁佩尔（Kr）的表达，它们分别界定了条纹的前后边界。在此，我们报告了在转基因胚胎中使用一系列有缺陷的条纹2增强子进行广泛的顺式和反式互补分析的结果。这项研究得出两个主要结论。首先，条纹2增强子对gt的抑制具有内在的“敏感性”。我们提出，gt通过阻断两层转录协同作用来确定条纹的尖锐前边界，即与DNA的协同结合以及结合的激活剂与转录复合物的协同接触。其次，我们发现hb和bcd的协同活性是“混杂的”。例如，母源表达的Gal4-Sp1融合蛋白在条纹2增强子中可以在功能上替代hb。这一发现挑战了先前关于hb和bcd在分割过程中特异性相互作用的观点。
