The moulting hormone ecdysone is able to recognize target elements composed of direct repeats

In Drosophila melanogaster, three temporally distinct ecdysone-responsive puff sets, the so-called intermoult, early and late puffs, have been described on the salivary gland polytene chromosomes. We have analyzed in detail a DNA segment of the 3C polytene region, from which originates one of the most prominent intermoult puffs, with the aim of identifying ecdysone response elements (EcREs). Here we report that two putative EcREs of identical sequence are located at this puff site. Interestingly, these elements display a novel structural feature, being composed of directly repeated half-sites. Our results show that the EcR/USP heterodimer known to constitute the ecdysone functional receptor complex is able to bind to and transactivate through target elements composed of directly repeated half-sites. In addition, we show that these elements are also able to bind efficiently USP alone, suggesting that USP and EcR/USP could compete for their binding to DNA.