Interaction of a transcriptional activator, OmpR, with reciprocally osmoregulated genes, ompF and ompC, of Escherichia coli.

The ompB locus, comprised of the genes ompR and envZ, regulates the expression of the genes ompF and ompC that encode the major porin proteins in the outer membrane of Escherichia coli K-12. OmpR is believed to activate transcription of ompF and ompC in a reciprocal manner depending upon the osmolarity of the culture medium. We were able to purify OmpR to homogeneity and to characterize its interaction with the porin gene promoters. The purified OmpR was shown to bind specifically to promoter fragments of both ompF and ompC. Deoxyribonuclease I footprinting was carried out in order to determine binding sites of OmpR in the promoter regions of the ompF and ompC. OmpR was found to protect the regions -105 to -60 of ompF and -102 to -78 of ompC, respectively. Two consensus sequences were found in these protected sites and are likely to play an important role in OmpR recognition of these promoter regions. The purified OmpR was also shown to be functionally active in transcription in vitro; OmpR activates the expression of ompF and inhibits the transcription of ompC from the most distal of its three tandem promoters, P3.