Fig. 4.

Loss of Fis binding and loss of Fis production biases fimS towards phase OFF. (a) Electrophoresis of the fimS DNA fragments from the wild-type strain (VL386) and from its derivative (VL386fimS-dist) in which the Fis binding site in fimS is disrupted, following BstUI digestion of the PCR-amplified fimS genetic element. (b) Electrophoresis of the fimS DNA fragments from the wild-type strain (VL386), its lrp knockout mutant derivative and the derivative with knockout mutations in both the lrp and fis genes, following BstUI digestion of the PCR-amplified fimS genetic element. The red arrowhead indicates the 539 bp phase OFF diagnostic band and the green arrowhead shows the 433 bp ON diagnostic band. In (a) and (b), the cultures have been treated with novobiocin at the concentrations given above each gel lane. The intensities of the DNA bands in each lane corresponding to the ON and OFF orientations of fimS in each lane were determined by densitometry and are reported as percentages below the lane. The experiment was performed three times and typical data are presented.