SEVEN-UP INHIBITS ULTRASPIRACLE-BASED SIGNALING PATHWAYS IN-VITRO ANDIN-VIVO

Seven-up (Svp), the Drosophila homolog of the chicken ovalbumin upstre am transcription factor (COUP-TF); Ultraspiracle (Usp), the Drosophila homolog of the retinoid X receptor; and the ecdysone receptor are all members of the nuclear/steroid receptor superfamily. COUP-TF negative ly regulates hormonal signaling involving retinoid X receptor in tissu e culture systems. Here we demonstrate that Svp, like COUP-TF, can mod ulate Ultraspiracle-based hormonal signaling both in vitro and in vivo . Transfection assays in CV-1 cells demonstrate that Seven-up can inhi bit ecdysone-dependent transactivation by the ecdysone receptor comple x, a heterodimeric complex of Usp and ecdysone receptor. This repressi on depends on the dose of Svp and occurs with two different Drosophila ecdysone response elements. Ectopic expression of Svp in vivo induces lethality during early metamorphosis, the time of maximal ecdysone re sponsiveness. Concomitant overexpression of Usp rescues the larvae fro m the lethal effects of Svp. DNA binding studies show that Svp can bin d to various direct repeats of the sequence AGGTCA but cannot bind to one of the ecdysone response elements used in the transient transfecti on assays. Our results suggest that Svp-mediated repression can occur by both DNA binding competition and protein-protein interactions.