A SWITCH IN BROAD-COMPLEX ZINC-FINGER ISOFORM EXPRESSION IS REGULATEDPOSTTRANSCRIPTIONALLY DURING THE METAMORPHOSIS OF DROSOPHILA IMAGINALDISCS

The Broad-Complex (BR-C) is a key member of the 20-hydroxyecdysone reg ulatory hierarchy that coordinates changes in gene expression during D rosophila metamorphosis. The family of transcription factors encoded b y the BR-C share a common amino-terminal domain which is fused by alte rnative 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 new ly discovered fourth zinc-finger domain--during the metamorphosis of i maginal discs which form the integumental structures of the adult head and thorax. We find that all BR-C zinc-finger RNA isoforms are induce d as a primary response to 20-hydroxyecdysone. However, induced BR-C R NA isoforms exhibit two divergent expression profiles. The Z2, Z3, and Z4 RNA isoforms accumulate to high levels at the beginning of the ecd ysone 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 specif ic to different regions of the BR-C, we show that the switch in BR-C R NA isoform expression appears to be posttranscriptionally regulated, p resumably by ecdysone-responsive factors. We propose that this switch results from a change in splice acceptor site choice. Finally, we pres ent a model describing how this temporal switch in isoform expression could mediate changes in BR-C function, from transcriptional activatio n to repression and vice versa, that are critical for coordinate downs tream target gene expression. (C) 1996 Academic Press, Inc.