A UAS site substitution approach

The specific gene transcription patterns formed during development of metazoans result from the activity of the cisregulatory modules that control gene promoters. Even when these cis-regulatory modules are responsible for elementary transcription patterns, as for example in a single tissue during a short period of development, they are always composed of multiple target sites for transcription factors (Arnone and Davidson, 1997; Kirchhamer et al., 1996). How the activities of transcription factors are integrated at the level of cisregulatory modules to produce highly specific regulatory outputs remains a central issue in developmental biology. Hormone response units (HRU) in nuclear receptorregulated promoters constitute a particular class of cisregulatory modules. They are composed of an assembly of binding sites for a variety of transcription factors, including hormone receptor-binding sites, and determine in which tissue(s) and during which developmental period(s) a gene will respond to the hormone (Lucas and Granner, 1992). Drosophila melanogaster is a choice animal for the study of HRUs in the context of a developing organism. At the end of the third larval instar, a pulse of the steroid hormone 20hydroxyecdysone (hereafter referred to as ecdysone) activates the ecdysone receptor, which is composed of a heterodimer between the nuclear receptors EcR and USP (Koelle et al., 1991; Thomas et al., 1993; Yao et al., 1993). This receptor in turn differentially regulates a number of primary ecdysone response genes in different target tissues (Andres and Thummel, 1992). This hormonally controlled genetic program triggers puparium formation and initiates metamorphosis. We are interested in understanding the molecular mechanisms whereby ecdysone response units (EcRUs) integrate multiple regulatory inputs to mediate distinct tissueand time-specific transcriptional responses to circulating ecdysone. Our model gene Fat body protein 1 (Fbp1), which encodes a receptor mediating the uptake of hexamerins from the hemolymph by the larval fat body (Burmester et al., 1999), is transcribed 2593 Development 128, 2593-2602 (2001) Printed in Great Britain © The Company of Biologists Limited 2001 DEV5926