Ecdysteroids control molting and metamorphosis in insects via a heterodimeric complex of two nuclear receptors, the ecdysone receptor (EcR) and ultraspiracle protein (Usp). We used fluorescence resonance energy transfer (FRET) to study the topology of the natural pseudopalindromic element from the hsp27 gene (hsp27pal) in complex with the DNA-binding domains of Usp and EcR (UspDBD and EcRDBD, respectively). Steady-state data revealed shortening of the end-to-end distance of the hsp27pal-derived probe. For the 70.8 +/- 0.6 A distance obtained for the UspDBD-complexed DNA a bend of about 23.1 +/- 2.9 degrees was measured. Nearly the same value (23.0 +/- 3.4 degrees) was obtained for the DNA complexed with the UspDBD/EcRDBD heterodimer. The respective bend angles estimated using fluorescence decay measurements were 19.0 +/- 2.1 degrees and 20.9 +/- 3.6 degrees . Thus, the FRET data suggest for the first time that the UspDBD defines the architecture of the UspDBD/EcRDBD heterocomplex due to the significant deformation of the hsp27pal. This suggestion has been further reinforced using gel retardation experiments, which, in conjunction with high-resolution DNase I footprinting, indicate that the main contribution to the observed bend is given by the UspDBD itself, while binding of the EcRDBD molecule brings on a slight additional change of the preformed structure.