FIG. 1.

Mutagenesis of soxR and selection-screening for activation-defective mutant proteins. The soxR gene was mutagenized in a mutD5 strain, and activation-defective soxR mutants were selected in strain MC9, where they did not stimulate transcription from a soxS promoter-sacB fusion and were therefore viable on plates containing both PQ and sucrose; MC9 cells expressing wild-type (WT) SoxR were inviable. These activation-defective proteins were also screened for their ability to bind to DNA in the same strain; proteins that retained this property repressed expression from a soxR promoter-lacZ fusion, and the Lac⁻ cells were pink on EMB indicator plates. Details of the mutagenesis and selection-screen are described in Materials and Methods and in Results.