It is becoming clearer that genetic activity is closely associated with the intracellular energy state. However, the mechanisms of this association are still unclear. In this study, we focused on large-scale changes in the structure of DNA to examine the effect of the NTP concentration on the transcription reaction with T7 RNA polymerase and compared the results with long duplex DNA to those with a short persistent-length(1) fragment. The transcriptional activity dramatically changed only for long duplex DNA within a narrow range of NTP concentrations associated with changes in the large-scale structure of DNA. This result suggests that the energy state may play an essential role in regulating ON/OFF switching on transcriptional activity.