INTRONIC AND 5' FLANKING SEQUENCES OF THE DROSOPHILA-BETA-3 TUBULIN GENE ARE ESSENTIAL TO CONFER ECDYSONE RESPONSIVENESS

The expression of the beta3 tubulin gene is regulated, at the transcri ptional level, by the steroid hormone ecdysone, in Drosophila Kc cells . Using a transient expression assay, we show that 360 bp from the fir st intron of the beta3 tubulin gene, associated with the 5' flanking s equences, are essential to confer ecdysone inducibility on a minimum p romoter driving the chloramphenicol acetyl transferase (CAT) gene. The 5' flanking region contains ecdysone-independent cis-positive element s located in proximity to the promoter. Deletion analysis of the 360 b p intronic region reveals that a fragment of 57 bp is crucial for the ecdysone response of the beta3 tubulin gene. This fragment contains 5' -TGA(A/C)C-3' motifs homologous to ecdysone responsive elements (EcRE) half sites. Band shift assays show that this 57-bp fragment is bound by three specific complexes. One of them appears to be involved in the level of the ecdysone response.