Karim F D, Guild G M, Thummel C S
Howard Hughes Medical Institute, University of Utah, Salt Lake City 84112.
Development. 1993 Jul;118(3):977-88. doi: 10.1242/dev.118.3.977.
During Drosophila third instar larval development, one or more pulses of the steroid hormone ecdysone activate three temporally distinct sets of genes in the salivary glands, represented by puffs in the polytene chromosomes. The intermolt genes are induced first, in mid-third instar larvae; these genes encode a protein glue used by the animal to adhere itself to a solid substrate for metamorphosis. The intermolt genes are repressed at puparium formation as a high titer ecdysone pulse directly induces a small set of early regulatory genes. The early genes both repress their own expression and activate more than 100 late secondary-response genes. The Broad-Complex (BR-C) is an early ecdysone-inducible gene that encodes a family of DNA binding proteins defined by at least three lethal complementation groups: br, rbp, and l(1)2Bc. We have found that the BR-C is critical for the appropriate regulation of all three classes of ecdysone-inducible genes. Both rbp and l(1)2Bc are required for glue gene induction in mid-third instar larvae. In addition, the l(1)2Bc function is required for glue gene repression in prepupae; in l(1)2Bc mutants the glue genes are re-induced by the late prepupal ecdysone pulse, recapitulating a mid-third instar regulatory response at an inappropriate stage in development. The l(1)2Bc function is also required for the complete ecdysone induction of some early mRNAs (E74A, E75A, and BR-C) and efficient repression of most early mRNAs in prepupae. Like the intermolt secondary-response genes, the late secondary-response genes are absolutely dependent on rbp for their induction. An effect of l(1)2Bc mutations on late gene activity can also be detected, but is most likely a secondary consequence of the submaximal ecdysone-induction of a subset of early regulatory products. Our results indicate that the BR-C plays a key role in dictating the stage-specificity of the ecdysone response. In addition, the ecdysone-receptor protein complex alone is not sufficient for appropriate induction of the early primary-response genes, but requires the prior expression of BR-C proteins. These studies define the BR-C as a key regulator of gene activity at the onset of metamorphosis in Drosophila.
在果蝇三龄幼虫发育过程中，类固醇激素蜕皮激素的一个或多个脉冲激活唾液腺中三组时间上不同的基因，这些基因在多线染色体上表现为胀泡。蜕皮间期基因首先在三龄幼虫中期被诱导；这些基因编码一种蛋白质胶水，动物用它在变态时附着在固体基质上。在化蛹时，蜕皮间期基因受到抑制，因为高滴度的蜕皮激素脉冲直接诱导一小套早期调控基因。早期基因既抑制自身表达，又激活100多个晚期二级反应基因。泛素复合体（BR-C）是一个早期蜕皮激素诱导基因，它编码一个由至少三个致死互补群定义的DNA结合蛋白家族：br、rbp和l(1)2Bc。我们发现BR-C对于所有三类蜕皮激素诱导基因的适当调控至关重要。rbp和l(1)2Bc都是三龄幼虫中期胶水基因诱导所必需的。此外，l(1)2Bc功能是蛹前期胶水基因抑制所必需的；在l(1)2Bc突变体中，胶水基因在蛹前期晚期蜕皮激素脉冲作用下被重新诱导，在发育的不适当阶段重现了三龄幼虫中期的调控反应。l(1)2Bc功能对于某些早期mRNA（E74A、E75A和BR-C）的完全蜕皮激素诱导以及蛹前期大多数早期mRNA的有效抑制也是必需的。与蜕皮间期二级反应基因一样，晚期二级反应基因的诱导绝对依赖于rbp。也可以检测到l(1)2Bc突变对晚期基因活性的影响，但这很可能是早期调控产物子集的次最大蜕皮激素诱导的次要结果。我们的结果表明，BR-C在决定蜕皮激素反应的阶段特异性方面起关键作用。此外，单独的蜕皮激素受体蛋白复合体不足以适当诱导早期初级反应基因，而是需要BR-C蛋白的预先表达。这些研究将BR-C定义为果蝇变态开始时基因活性的关键调节因子。
