GEBF-I activates the Drosophila Sgs3 gene enhancer by altering a positioned nucleosomal core particle

The enhancer region of the Drosophila melanogaster ecdysone-regulated glue gene, Sgs3, shows dramatic modifications of chromatin structure in strict correlation with changes in gene expression during development. We show that there is a positioned nucleosomal core particle over the enhancer which is displaced or disrupted during gene activation. This transition is prevented in Drosophila larvae mutated in the ecdysone-dependent 2B5 locus, in which Sgs3 is inactive and GEBF-I, a Glue Enhancer Binding Factor, is missing. We have defined the GEBF-I binding sites in vitro and shown that mutation of these sequences abolishes the enhancer activity in vivo. This combined in vitro and in vivo approach reveals new aspects of the dynamic organization of a regulatory element during development and highlights the potential of this model for studies of the relation between chromatin structure and gene activity.