Chemical footprinting was used to study the spatial structure of bacteriophage T7 promoter D upon formation of the transcriptionally active complex with Escherichia coli RNA polymerase. Enzyme binding was shown to induce conformational changes in sites located at positions 43 and 57, several helix turns away from the transcription start. This was the first finding of a structural deformation induced by assembly of the transcription complex. The deformation was associated with specific features of the promoter nucleotide sequence, and suggested high cooperativity in the organization of the transcription complex and substantial energy perturbations caused by the enzyme.