Nellesen D T, Lai E C, Posakony J W
Department of Biology and Center for Molecular Genetics, University of California, San Diego, 9500 Gilman Drive, La Jolla, California 92093-0349, USA.
Dev Biol. 1999 Sep 1;213(1):33-53. doi: 10.1006/dbio.1999.9324.
In Drosophila, genes of the Enhancer of split Complex [E(spl)-C] are important components of the Notch (N) cell-cell signaling pathway, which is utilized in imaginal discs to effect a series of cell fate decisions during adult peripheral nervous system development. Seven genes in the complex encode basic helix-loop-helix (bHLH) transcriptional repressors, while 4 others encode members of the Bearded family of small proteins. A striking diversity is observed in the imaginal disc expression patterns of the various E(spl)-C genes, suggestive of a diversity of function, but the mechanistic basis of this variety has not been elucidated. Here we present strong evidence from promoter-reporter transgene experiments that regulation at the transcriptional level is primarily responsible. Certain E(spl)-C genes were known previously to be direct targets of transcriptional activation both by the N-signal-dependent activator Suppressor of Hairless [Su(H)] and by the proneural bHLH proteins achaete and scute. Our extensive sequence analysis of the promoter-proximal upstream regions of 12 transcription units in the E(spl)-C reveals that such dual transcriptional activation is likely to be the rule for at least 10 of the 12 genes. We next show that the very different wing imaginal disc expression patterns of E(spl)m4 and E(spl)mgamma are a property of small (200-300 bp), evolutionarily conserved transcriptional enhancer elements, which can confer these distinct patterns on a heterologous promoter despite their considerable structural similarity [each having three Su(H) and two proneural protein binding sites]. We also demonstrate that the characteristic inactivity of the E(spl)mgamma enhancer in the notum and margin territories of the wing disc can be overcome by elevated activity of the N receptor. We conclude that the distinctive expression patterns of E(spl)-C genes in imaginal tissues depend to a significant degree on the capacity of their transcriptional cis-regulatory apparatus to respond selectively to direct proneural- and Su(H)-mediated activation, often in only a subset of the territories and cells in which these modes of regulation are operative.
在果蝇中，分裂增强子复合体[E(spl)-C]的基因是Notch（N）细胞间信号通路的重要组成部分，该信号通路在成虫盘（imaginal discs）中发挥作用，以在成虫外周神经系统发育过程中影响一系列细胞命运决定。该复合体中的七个基因编码基本螺旋-环-螺旋（bHLH）转录抑制因子，而另外四个基因编码小蛋白的Bearded家族成员。在不同的E(spl)-C基因的成虫盘表达模式中观察到显著的多样性，这暗示了功能的多样性，但这种多样性的机制基础尚未阐明。在这里，我们通过启动子-报告基因转基因实验提供了有力证据，表明转录水平的调控起主要作用。某些E(spl)-C基因先前已知是N信号依赖性激活因子无毛抑制因子[Su(H)]以及原神经bHLH蛋白achaete和scute转录激活的直接靶标。我们对E(spl)-C中12个转录单元的启动子近端上游区域进行的广泛序列分析表明，这种双重转录激活可能是12个基因中至少10个基因的规律。接下来，我们表明E(spl)m4和E(spl)mgamma在翅成虫盘中非常不同的表达模式是小的（200-300 bp）、进化保守的转录增强子元件的特性，尽管它们在结构上有相当大的相似性（每个都有三个Su(H)和两个原神经蛋白结合位点），但这些元件仍可将这些不同的模式赋予异源启动子。我们还证明，通过提高N受体的活性，可以克服E(spl)mgamma增强子在翅盘背板和边缘区域的特征性无活性。我们得出结论，E(spl)-C基因在成虫组织中的独特表达模式在很大程度上取决于其转录顺式调控装置选择性响应直接的原神经和Su(H)介导的激活的能力，通常仅在这些调控模式起作用的部分区域和细胞中起作用。
