Bayer C A, Holley B, Fristrom J W
Department of Molecular and Cell Biology, University of California, Berkeley, California, 94720-3200, USA.
Dev Biol. 1996 Jul 10;177(1):1-14. doi: 10.1006/dbio.1996.0140.
The Broad-Complex (BR-C) is a key member of the 20-hydroxyecdysone regulatory hierarchy that coordinates changes in gene expression during Drosophila metamorphosis. The family of transcription factors encoded by the BR-C share a common amino-terminal domain which is fused by alternative splicing to one of four pairs of C2H2 zinc-finger domains (Z1, Z2, Z3, and Z4). In this study, we examine the temporal expression of transcripts encoding each BR-C zinc-finger isoform-including the newly discovered fourth zinc-finger domain-during the metamorphosis of imaginal discs which form the integumental structures of the adult head and thorax. We find that all BR-C zinc-finger RNA isoforms are induced as a primary response to 20-hydroxyecdysone. However, induced BR-C RNA isoforms exhibit two divergent expression profiles. The Z2, Z3, and Z4 RNA isoforms accumulate to high levels at the beginning of the ecdysone response and abruptly disappear after several hours. In contrast, the Z1 RNA isoform continues to accumulate while the others decline, resulting in a switch in relative isoform levels. Using probes specific to different regions of the BR-C, we show that the switch in BR-C RNA isoform expression appears to be posttranscriptionally regulated, presumably by ecdysone-responsive factors. We propose that this switch results from a change in splice acceptor site choice. Finally, we present a model describing how this temporal switch in isoform expression could mediate changes in BR-C function, from transcriptional activation to repression and vice versa, that are critical for coordinate downstream target gene expression.
泛素复合体（BR-C）是20-羟基蜕皮激素调控层级中的关键成员，它在果蝇变态发育过程中协调基因表达的变化。由BR-C编码的转录因子家族共享一个共同的氨基末端结构域，该结构域通过可变剪接与四对C2H2锌指结构域（Z1、Z2、Z3和Z4）之一融合。在本研究中，我们检测了在形成成虫头部和胸部体表结构的成虫盘变态发育过程中，编码每种BR-C锌指异构体（包括新发现的第四锌指结构域）的转录本的时间表达情况。我们发现，所有BR-C锌指RNA异构体都是作为对20-羟基蜕皮激素的主要反应而被诱导产生的。然而，诱导产生的BR-C RNA异构体表现出两种不同的表达模式。Z2、Z3和Z4 RNA异构体在蜕皮激素反应开始时积累到高水平，并在数小时后突然消失。相比之下，Z1 RNA异构体在其他异构体下降时继续积累，导致异构体相对水平发生转变。使用针对BR-C不同区域的探针，我们表明BR-C RNA异构体表达的转变似乎是在转录后受到调控的，推测是由蜕皮激素反应因子介导的。我们提出这种转变是由于剪接受体位点选择的变化所致。最后，我们提出了一个模型，描述了异构体表达的这种时间转变如何介导BR-C功能的变化，从转录激活到抑制，反之亦然，这对于协调下游靶基因的表达至关重要。
