FIG. 6.

DNase I protection experiments of the xsa promoter by the AraR protein. Each strand of a 339-bp DNA fragment carrying the xsa promoter region was end labeled with γ-³²P in separate experiments. AraR concentrations were calculated considering a pure dimeric protein. Lane 1, no protein; lane 2, 25 nM AraR; lane 3, 50 nM AraR; lane 4, 100 nM AraR; lane 5, 150 nM AraR; lane 6, 200 nM AraR; lane 7, 250 nM AraR; lane 8, 250 nM AraR plus 0.02% (wt/vol) l-arabinose. Protected regions, termed OR_X1 and OR_X2, are indicated in the autoradiograms by brackets. The sites of enhanced (black arrows) and diminished (open triangles) DNase I cleavage outside of the protected regions are both indicated in the autoradiograms. The size of the arrow reflects the intensity of enhanced cleavage by DNase I. Total repressor concentration at which half-maximal site occupancy is achieved (a value that represents K_app, the apparent affinity of AraR to each site) is indicated within parentheses for each operator and was calculated from a single experiment.