SEQUENTIAL GENE ACTIVATION BY ECDYSONE IN POLYTENE CHROMOSOMES OF DROSOPHILA-MELANOGASTER .3. CONSEQUENCES OF ECDYSONE WITHDRAWAL

Ecdysone induces a sequence of changes in puffing activity of the polytene salivary gland chromosomes of D. melanogaster. In the continuous presence of ecdysone this sequence takes about 12 h to complete. Salivary glands were incubated in the presence of ecdysone for various times and then transferred to hormone-free medium to study the effects of hormone removal on the sequence of puffing activities. The early ecdysone-induced puffs immediately regressed on removal of hormone. They could be fully reinduced if exposed to hormone again, but only if the initial period of exposure to the hormone was .ltoreq. 2 h; with longer periods of initial exposure the early puffs became progressively refractory to reinduction. This process was inhibited by cycloheximide. A group of the late puffs could be induced prematurely by removal of the hormone; the extent of their premature induction was correlated with the size of the early puffs during the period of ecdysone exposure. Ecdysone inhibited the premature induction of these late puffs.