FIG. 4.

Open complex formation at the icsB promoter does not depend on VirB and is repressed by H-NS. Potassium permanganate footprinting was used to detect single-stranded regions in icsB promoter DNA that were indicative of open transcription complex formation. Experiments were performed to examine the effects of VirB (A) and H-NS (B) on open complex formation. Arrows indicate reactive thymine bases in the region of the Pribnow box of the icsB promoter that reacted strongly with KMnO₄. Plus and minus signs above the lanes indicate the presence or absence, respectively, of the reaction mixture components listed at the right. These components were added at the following concentrations: RNA polymerase (RNAP), 500 nM; NTPs, 30 μM; VirB, 500 nM (lanes 3, 5, and 10) and 1 mM (lanes 4, 6, and 11); H-NS, 100 nM (lanes 3, 5, and 10) and 250 nM (lanes 4, 6, and 11).