The bacteriophage T7 elongation complex is an excellent model system in which to characterize the fundamental steps of transcription. We have formed functional elongation complexes, by mixing preassembled and RNA-primed DNA "bubble" constructs with T7 RNA polymerase and by initiating transcription at promoters, and have monitored the low-energy CD and fluorescence spectra of pairs of 2-aminopurine residues that have been inserted at defined sites within the DNA and RNA scaffold of the complex. In this way, we have been able to probe specific changes in the local conformations of the bases and base-pairs at these positions as the elongation complex goes through the various steps of the nucleotide addition cycle. The advantage of using pairs of 2-aminopurine residues, inserted at defined nucleic acid positions, as probes, is that the rest of the complex is spectrally "transparent" at wavelengths >300 nm. Thus, by combining CD and fluorescence measurements we obtain both structural and dynamic information that applies uniquely at each position within the functioning complex. In this way, we have mapped the details of steps central to transcription, including the formation and translocation of the transcription bubble, the formation and unwinding of the RNA-DNA hybrid, the passage of the nascent RNA through the exit channel of the polymerase, and the events of the template-controlled NTP selection process that controls transcriptional fidelity. This approach defines specific structural aspects of the elongation process under physiological conditions, and can be extended to examine other key aspects of transcriptional regulation, such as termination, editing, pausing, etc., that involve conformational rearrangements within the nucleic acid framework of the transcription complex.