Osmoregulatory expression of porin genes in Escherichia coli: a comparative study on strains B and K-12.

Escherichia coli K-12 produces both the OmpF and OmpC porins, the relative amounts of which in the outer membrane are affected in a reciprocal manner by the osmolarity of the growth medium. In contrast, E. coli B produces only the OmpF porin, regardless of the medium osmolarity. In this study, it was revealed that there is an extensive deletion within the ompC locus of the E. coli B chromosome. Cloning and nucleotide sequencing of the regulatory gene, ompR, of E. coli B revealed that there are two amino acid alterations (Lys-6 to Asn and Ala-130 to Thr) in the amino acid sequence of the OmpR protein, as compared with that of E. coli K-12. It is suggested that these particular amino acid alterations are responsible for the constitutive expression of the ompF gene observed in E. coli B.