THE MOLTING HORMONE ECDYSONE IS ABLE TO RECOGNIZE TARGET ELEMENTS COMPOSED OF DIRECT REPEATS

In Drosophila melanogaster, three temporally distinct ecdysone-respons ive puff sets, the so-called intermoult, early and late puffs, have be en described on the salivary gland polytene chromosomes. We have analy zed in detail a DNA segment of the 3C polytene region, from which orig inates one of the most prominent intermoult puffs, with the aim of ide ntifying ecdysone response elements (EcREs). Here we report that two p utative EcREs of identical sequence are located at this puff site. Int erestingly, these elements display a novel structural feature, being c omposed 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 c omposed of directly repeated half-sites. In addition, we show that the se elements are also able to bind efficiently USP alone, suggesting th at USP and EcR/USP could compete for their binding to DNA.