ENHANCER AND SILENCER ELEMENTS WITHIN THE 1ST INTRON MEDIATE THE TRANSCRIPTIONAL REGULATION OF THE BETA-3 TUBULIN GENE BY 20-HYDROXYECDYSONE IN DROSOPHILA KC-CELLS

We have studied the transcriptional regulation of the beta3 tubulin ge ne by the steroid hormone 20-hydroxyecdysone (20-E) in Drosophila Kc c ells. A series of hybrid genes, with different fragments of the beta3 tubulin gene driving the bacterial chloramphenicol acetyl transferase (CAT) gene were constructed. The promoter activity was assayed after t ransient expression in Kc cells, in the presence and the absence of 20 -E. Constructs with 0.91 kb upstream from the transcription start site and 360 bp from the first large intron allowed the hormonal regulatio n, i.e. a repression in the absence of 20-E and a derepression-activat ion in the presence of the hormone. This 360 bp fragment contains seve ral enhancers and silencer(s) sequences. The regulation of the express ion of the beta3 tubulin gene results from the combined activity of al l the positive and negative regulatory sequences of the first intron, and a dialogue with the promoter sequences. The nucleotide sequence of this intronic regulatory-fragment has been established and we have id entified several EcRE (ecdysone responsive element) consensus sequence s.