Multiple Inducers of the Drosophila Heat Shock

The Drosophila hsroJ locus produces one of the largest and most active heat shock puffs, yet it does not encode a heat shock protein. Instead, this locus produces a distinctive set of three transcripts, all from the same start site. The largest transcript, c01, is limited to the nucleus and appears to have a role there. A second nuclear transcript, o~2, is produced by alternative termination and contains the sequence found in the 5' 20-25% of ~01 (depending on the Drosophila species). The cytoplasmic transcript, oo3, is produced by removal of a 700-bp intron from o~2. All three hsrw RNAs are produced constitutively and production is enhanced by heat shock. In addition to being a member of the set of heat shock puffs, the hsro~ puff is induced by agents that do not affect other heat shock loci, suggesting that hsro~ is more sensitive to environmental changes than other loci. We report here that agents that induce puffing of hsro~ loci in polytene nuclei also lead to an increase in hsro~ transcripts in diploid cells. We also show that the relative levels of o~ 1 and o~3 can be modulated independently by several agents. All drugs that inhibit translation, either initiation or elongation, stabilize the oo3 transcript, which normally turns over within minutes in control cells. Drugs (such as benzamide and colchicine) that induce puffing of hsro~, but not other heat shock loci, lead to large increases in o~1. Although the constitutive level of 601 is relatively stable, the drug-induced excess is lost rapidly when the drug is withdrawn. The relative levels of hsro~ transcripts may reflect different states in cellular metab-