We have previously described a new K-12 mutant, LH530, which has a defective outer membrane. LH530 is very sensitive to hydrophobic antibiotics, does not grow at 42 ° and synthesizes reduced amounts of lipid A. Phenotypically LH530 is very similar to the known lipid A biosynthesis mutants of and Its genetic defect is not known, but the defect is suppressed by multiple copies of ORF195. Here we show that at 37 ° LH530 contains a reduced amount of the OmpF porin and that it leaks periplasmic °-lactamase at 37 °. and 42 °. We further show that ORF195, when present at low copy number, restores the antibiotic resistance and lipid A biosynthesis of LH530 at 28 °, but not at higher temperatures. In contrast, OmpF expression is restored at 37 °.


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