Summary: is a nosocomial pathogen with a high incidence of β-lactam resistance. Reduced amounts of outer-membrane porins have been correlated with increased resistance to β-lactams but only one porin, OmpC, has been characterized at the molecular level. In this study we present the molecular characterization of a second porin, OmpF, and an analysis of the expression of porins in response to various environmental changes. Two porins were isolated from the outer membrane using urea-SDS-PAGE and the relative amounts were shown to be influenced by the osmolarity of the medium and the presence of salicylate. From a genomic DNA library an 8 kb RI fragment was isolated that hybridized with an oligonucleotide encoding the published N-terminal amino acid sequence of the 41 kDa porin. A 41 kDa protein was detected in the outer membrane of NM522 carrying the cloned DNA. The cloned gene was sequenced and shown to code for a protein that shared 60-70% identity with other known OmpF and OmpC sequences. The upstream DNA sequence of the gene was similar to the corresponding sequence; however, a regulatory element important in repression of at high osmolarity was absent. The cloned OmpF in increased in expression in conditions of high osmolarity. The potential involvement of micF in the observed osmoregulation of porins is discussed.


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