DIFFERENTIAL EXPRESSION OF BROAD-COMPLEX TRANSCRIPTION FACTORS MAY FORECAST TISSUE-SPECIFIC DEVELOPMENTAL FATES DURING DROSOPHILA METAMORPHOSIS

The steroid hormone ecdysone initiates metamorphosis in Drosophila mel anogaster by activating a cascade of gene activity that includes prima ry response transcriptional regulators and secondary response structur al genes. The Broad-Complex (BR-C) primary response gene is composed o f several distinct genetic functions and encodes a family of related t ranscription factor isoforms, Our objective was to determine whether B R-C isoforms were components of the primary ecdysone response in all t issues and whether tissue-specific isoform expression is associated wi th tissue-specific metamorphic outcomes, We used specific antibody rea gents that recognize and distinguish among the Z1, Z2 and Z3 BR-C prot ein isoforms to study protein expression patterns during the initial s tages of metamorphosis, Western blot analyses demonstrated that BR-C i soforms are induced at the onset of metamorphosis, each with unique ki netics of induction and repression, Whole-mount immunostaining showed that the BR-C proteins accumulate in the nuclei of all larval and imag inal tissues indicating that the BR-C is induced as a primary response in many tissues, Several tissues express different levels and combina tions of the BR-C isoforms suggesting that the BR-C is important in de termining the tissue-specific outcome of many parallel ecdysone respon se cascades, For example, prepupal salivary glands (destined for histo lysis during metamorphosis) express Z1 isoforms while imaginal discs ( destined for cell differentiation and morphogenesis) shift from the sy nthesis of Z2 isoforms to the synthesis of Z1 isoforms. The prepupal c entral nervous system (destined for tissue remodeling) expresses all i soforms, with Z3 predominating, Salivary gland chromosome immunostaini ng indicated that BR-C proteins interact directly with numerous loci i n the polytene genome, Finally, western blot analyses showed that dist inct BR-C genetic functions can be correlated with single and specific BR-C protein isoforms.