Relationships between Protein Isoforms and Genetic Functions Demonstrate Functional Redundancy at theBroad-ComplexduringDrosophilaMetamorphosis

Metamorphosis in holometabolous insects is an ecdysone-dependent process by which the larval form is replaced by a reproductive, adult form. At the onset of metamorphosis ecdysone induces a set of early genes which coordinate tissue-specific responses to hormone. TheBroad-Complex(BR-C) early gene, which acts as a global regulator of tissue-specific responses to ecdysone, encodes a family of zinc-finger DNA binding proteins known as Z1, Z2, Z3, and Z4. Genetically theBR-Cencodes three complementing functions,br, rbp,and2Bc,and a class ofnpr1alleles that fail to complement any of the other genetic functions. The effects ofBR-Cmutations on metamorphic development are highly pleiotropic, yet little is known about the roles of individualBR-Cproteins in directing the required responses to ecdysone. Because theBR-Cis a vital regulator of metamorphosis it is essential to establish the relationships betweenBR-Cgenetic functions and protein products. We present here the first general and definitive study of these relationships. Using heat-inducible transgenes we have rescued lethality associated with each of the complementing genetic functions and have restored transcriptional activity of tissue-specificBR-C+-dependent target genes. Our data lead us to conclude thatbr+function is only provided by the Z2 isoform. We find that Z1 transgenes provide fullrbp+function, while Z4 provides partial function. Likewise, while Z3 provides full2Bc+function, Z2 also provides partial function. These results indicate possible functional redundancy or regulatory dependence (via autoregulation) associated with therbp+and2Bc+functions. The establishment of these relationships betweenBR-Cgenetic functions and protein isoforms is an important step toward understanding the roles ofBR-Cproteins in directing metamorphic responses to ecdysone.