Gram-negative bacteria such as Escherichia coli have two cell membranes. The inner, or cytoplasmic, membrane constitutes the main permeability barrier of the cell. The outer membrane excludes macromolecules, but contains numerous pores that permit small molecules to enter the periplasmic space by passive diffusion.
OmpF and OmpC, the major porin proteins, are similar in structure, but have different sieving properties: OmpF forms a slightly larger pore than OmpC. Although the total number of pores in the outer membrane remains fairly constant in different environments, the relative amounts of OmpF and OmpC vary with medium osmolarity. OmpF predominates at low osmolarity, and OmpC predominates at high osmolarity.
The histidine kinase, EnvZ, and the response regulator, OmpR, a transcriptional regulator, regulate the differential expression of the ompF and ompC genes. In cells grown under low osmotic conditions, EnvZ functions to maintain a basal level of OmpR-phosphate, leading to the preferential expression of OmpF. When the cells are grown under high osmolarity conditions, the level of OmpR-phosphate in the cell increases, which, in turn, stimulates the expression of ompC and the repression of ompF. The ompR and envZ genes constitute an operon, referred to as ompB, in which ompR is promoter-proximal. There is a 100:1 ratio of OmpR to EnvZ in the cell.