Correlation between DNA bending and transcriptional activation at a plasmid promoter

Plasmid pLS1 replicates in both Gram-positive and Gram-negative bacteria. One of the elements controlling plasmid replication is an antisense RNA (ctRNA II), transcribed from promoter PctII, which should be expressed in all the hosts where the plasmid replicates. We show here that expression from PctII and the presence of curved DNA regions located upstream of this promoter are related. DNA fragments containing these upstream regions exhibit anomalous electrophoretic mobility, and their presence increases in vivo and in vitro transcription, apparently with an independence of any activator protein. RNA polymerase is able to bend a DNA fragment containing PctII, suggesting that an increase in the RNA polymerase-DNA contacts is an important step in transcription initiation. Furthermore, the upstream activating region could be substituted by targets of unrelated DNA-bending proteins. This finding supports the role of curved DNA as a transcriptional modulator.