CROSS-TALKING AMONG DROSOPHILA NUCLEAR RECEPTORS AT THE PROMISCUOUS RESPONSE ELEMENT OF THE NG-1 AND NG-2 INTERMOLT GENES

In Drosophila, peaks of the titer of the steroid hormone ecdysone act as molecular signals that trigger all the major developmental transiti ons occurring along the life cycle. The EcR/USP heterodimer, known to constitute the functional ecdysone receptor, binds with high affinity to specific target sequences, the ecdysone response elements (EcREs), whose repertoire still remains to be fully characterized at both the m olecular and functional levels. Ln order to investigate the properties of EcREs composed of directly repeated half-sites (DRs), we have anal ysed the binding properties of the ng-EcRE, a DR element located withi n the coding region of ng-1 and ng-2, two highly homologous genes mapp ing at the ecdysone-regulated 3C intermolt puff. We report here that t he ng-EcRE contacts the ecdysone receptor through its directly repeate d half-sites spaced by 12 bp, and that this element may interact effic iently with at least three Drosophila orphan receptors, namely DHR38, DHR39 and beta FTZ-F1. Interestingly, DHR38 is bound alone or in combi nation with USP, providing the first evidence that the EcR-USP and DHR 38-USP may directly compete for binding to a common response element. These results suggest that EcREs composed of widely spaced DRs may con tribute to the establishment of extensive nuclear receptors cross-talk ing along the development, a mechanism that might play a relevant role in determining the temporal and spatial specificty of the ecdysone re sponse. Finally, we show that the ng-EcRE can promote functional inter actions in vitro as well as in vivo, acting as a transcriptional enhan cer able to confer a specific developmental expression profile to a mi nimal promoter in transgenic flies. (C) 1998 Academic Press Limited.