Scanning mutagenesis of the –35 probe to characterize the binding site for nuclear proteins. EMSA assays using p5424 nuclear extracts and the -35 probe, with wild-type or indicated mutant competitors. The wild-type p35 sequence is indicated at the top of the figure, with the respective mutations indicated by the arrows. p35 and p35m are wild-type and mutant -35 probe, respectively; BS4 is a probe with a consensus Ikaros binding site.