Opening promoters for m-RNA transcription

Approximately 40 years ago it was found that the natural form of DNA was double-stranded. The basis for recognition between the two strands was base-pairing and this led to a potential paradox regarding transcription. If the formation of m-RNA used the same base-pairing patterns to match RNA to DNA then that pairing could not occur with DNA in its double-stranded form. The problem was resolved by proposing that the DNA would be opened locally and transiently during the act of transcription. This would allow the DNA to remain largely double-stranded but would allow local sequences to be copied into RNA. Because the double-stranded form of DNA is more stable than the single-stranded form, it was recognized that this transient opening of the DNA would require energy. Energy-requiring steps tend to occur slowly and thus it was thought that DNA opening might be the critical rate-limiting step in transcription. We now know that this is often, but by no means always, true for bacterial transcription (McClure, 1985; Sasse-Dwight and Gralla, 1989). In the case of mammalian m-RNA formation, the open complex was identified only very recently and so it is too soon to generalize (Wang et al., 1992a). The purpose of this article is to report on recent progress concerning how open transcription complexes form.