Effects of Global and Specific DNA-Binding Proteins on Transcriptional Regulation of the Operon

Using reporter gene () transcriptional fusions we examined the transcriptional dependencies of the promoter (P) and the entire operon regulatory region (P-) on eight transcription factors as well as the inducer, salicin, and an IS5 insertion upstream of P. Crp-cAMP is the primary activator of both P and the operon, while H-NS is a strong dominant operon repressor but only a weak repressor of P. H-NS may exert its repressive effect by looping the DNA at two binding sites. StpA is a relatively weak repressor in the absence of H-NS, while Fis also has a weak repressive effect. Salicin has no effect on P activity but causes a 30-fold induction of operon expression. Induction depends on the activity of the BglF transporter/kinase. IS5 insertion has only a moderate effect on P but causes a much greater activation of the operon expression by preventing the full repressive effects of H-NS and StpA. While several other transcription factors (BglJ, RcsB, and LeuO) have been reported to influence operon transcription when overexpressed, they had little or no effect when present at wild type levels. These results indicate the important transcriptional regulatory mechanisms operative on the operon in .