THE LEUCINE-RESPONSIVE REGULATORY PROTEIN OF ESCHERICHIA-COLI NEGATIVELY REGULATES TRANSCRIPTION OF OMPC AND MICF AND POSITIVELY REGULATES TRANSLATION OF OMPF

The two major porins of Escherichia coli K-12 strains, OmpC and OmpF, are inversely regulated with respect to one another. The expression of OmpC and OmpP has been shown to be influenced by the leucine-responsi ve regulatory protein (Lrp): two dimensional gel electrophoresis of pr oteins from strains with and strains without a functional Lrp protein revealed that OmpC expression is increased in an lrp strain, while Omp F expression is decreased. In agreement,vith these findings, we now pr esent evidence that transcriptional (operon) fusions of lacZ(+) to omp C and micF are negatively regulated by Lrp. Lrp binds specifically to the intergenic region between micF and ompC, as indicated by mobility shift assays and by DNase I footprinting. The expression of an ompF-la cZ(+) gene (translational) fusion is increased 3.7-fold in an lrp(+) b ackground compared with an lrp background, but expression of an ompF-l acZ(+) operon fusion is not. Studies of in vivo expression of the oute r membrane porins during growth on glucose minimal medium showed that the OmpF/OmpC ratio is higher in lrp(+) strains than it is in isogenic lrp strains. The effect of Lrp was not seen in a strain containing a deletion of micF. Our studies suggest that the positive effect of Lrp on OmpF expression stems from a negative effect of Lrp on the expressi on of micF, an antisense RNA that inhibits ompF translation.