The two-component regulatory system, OmpR and EnvZ, in Escherichia coli controls the differential expression of ompF and ompC in response to medium osmolarity. Previous studies suggest that EnvZ functions as a membrane sensor relaying information to the DNA-binding protein, OmpR, which in turn activates expression of the appropriate promoter. A strategy has been devised to isolate and characterize a collection of missense mutations in ompR that alter, but do not abolish protein function. Mutants were isolated using strains that contain the ompR and envZ genes in separate chromosomal locations yet maintain the production of both regulatory proteins at physiological levels. Such an arrangement facilitates ompR diploid analysis and tests of epistasis with known envZ mutations. The data obtained indicate that OmpR works in both a positive and negative fashion to control the transcription of ompF and this result forms the basis of a model for porin regulation that explains the switch from OmpF to OmpC production in response to increasing medium osmolarity.