Figure 1.
The genetic switch regulating the timing and expression of R–M genes. The -35 (green) and -10 (red) transcriptional signals are indicated upstream of the C-gene (blue) and the R-gene (not shown). C-protein dimers are shown in pink and the sigma subunit of RNA polymerase in yellow. Low-level C-protein expression occurs from a weak C-independent promoter (not shown). Occupation of the high affinity OL site by C-protein dimers stimulates transcription of the C-gene via recruitment of RNA polymerase sigma subunit to the -35 site; at higher concentrations of C-protein, occupation of the OR site displaces the subunit and down-regulates the C- and R-genes.