Puffs and gene regulation--molecular insights into the Drosophila ecdysone regulatory hierarchy

Sixteen years ago, Michael Ashburner and his colleagues proposed a hierarchical model for the genetic control of polytene chromosome puffing by the steroid hormone ecdysone. The recent molecular isolation and characterization of three early ecdysone-inducible genes has confirmed many aspects of this model--these genes are directly induced by ecdysone, repressed by ecdysone-induced proteins, and appear to encode DNA binding regulatory proteins. The three early genes are also remarkably similar in structure. They are all unusually long and complex, with multiple transcripts that direct the synthesis of several related proteins from each locus. Proteins encoded by two of the early genes bind to both early and late ecdysone-induced puffs, implying that they are key regulators in the hierarchy.