Abstract
Bacterial RNA polymerase-promoter open complexes can exist in a range of states in which the leading edge of the enzyme moves but the trailing edge does not, a phenomenon we refer to as "open complex scrunching." Here we describe how open complex scrunching can determine the position of the transcription start site for some promoters, modulate the level of expression, and potentially could be targeted by factors to regulate transcription. We suggest that open complex scrunching at the extraordinarily active ribosomal RNA promoters might have evolved to initiate transcription at an unusual position relative to the core promoter elements in order to maximize the rate of promoter escape.