Activation of the ompC gene by the OmpR protein in Escherichia coli. The cis-acting upstream sequence can function in both orientations with respect to the canonical promoter.

Expression of the ompC gene coding for an outer membrane protein of Escherichia coli is regulated by a transcriptional activation mechanism that requires the ompR gene product that acts on nucleotide sequences located upstream of the -35 and -10 regions of the ompC promoter. Using an ompC-lacZ fusion gene, the orientation of the cis-acting upstream sequence (OmpR-binding site) with respect to the canonical -35 and -10 regions (RNA polymerase-binding site) was changed. The ompC gene was activated in an OmpR-dependent manner even when the upstream sequence was in the reverse orientation with respect to the -35 and -10 regions, providing that these two DNA elements were aligned stereospecifically on the DNA helix. Evidence is presented that the upstream sequence can increase the transcription efficiency of the ompC promoter in a manner relatively independent of its orientation with respect to the -35 and -10 regions.