Mutants that overproduce TraTp, a plasmid-specified major outer membrane protein of Escherichia coli

The isolation of a series of plasmid mutant derivatives that overproduce the traT outer membrane protein, TraTp, is described. Some of the mutants directed the synthesis of 10-fold more TraTp (200,000 copies/cell) than did the parental plasmid (20,000 copies/cell). The proteins specified by all mutant plasmids except one were correctly inserted into the outer membrane and exposed on the cell surface. The TraTp that was not correctly inserted did not mediate the expected levels of surface exclusion and serum resistance, suggesting that surface localization is a requirement of TraTp function. The overproduction of TraTp was deleterious to bacterial growth, particularly that of minicell mutants of E. coli K-12.