The chromatin structure at the Drosophila hsp26 promoter in vivo is 
characterized by two DNase I-hypersensitive (DH) sites harboring regulatory 
elements. Proximal and distal DH sites are separated by a positioned nucleosome. 
To study the contribution of transcription factors to the establishment of this 
specific chromatin configuration we assembled nucleosomes on the hsp26 promoter 
using a cell-free reconstitution system derived from fly embryos. Both DH sites 
were readily reconstituted from extract components. They were separated by a 
nucleosome which was less strictly positioned than its in vivo counterpart. The 
interactions of GAGA factor and heat shock factor with their binding sites in 
chromatin occurred in two modes. Their interaction with binding sites in the 
nucleosome-free regions did not require ATP. In the presence of ATP both factors 
interacted also with nucleosomal binding sites, causing nucleosome 
rearrangements and a refinement of nucleosome positions. While chromatin 
remodeling upon transcription factor interaction has previously been interpreted 
to involve nucleosome disruption, the data suggest energy-dependent nucleosome 
sliding as main principle of chromatin reorganization.