Isolation of Mutants Conditionally Blocked in the Biosynthesis of the 3‐Deoxy‐d‐manno‐octulosonic‐acid—Lipid‐A Part of Lipopolysaccharides Derived from Salmonella typhimurium

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Abstract
A procedure is described for the selection of conditional 3‐deoxy‐d‐manno‐octulosonic‐acid—Lipid A mutants which depends on temperature sensitivity for both synthesis of complete lipopolysaccharide and for growth. Using this procedure new types of mutants were isolated which cease growth and accumulate lipid A precursors following a shift to nonpermissive temperatures. All precursor molecules differ in their charge as judged by DEAE‐cellulose chromatography. While they all contain glucosamine, phosphate and 3‐hydroxymyristic acid, they lack detectable 3‐deoxy‐d‐manno‐octulosonic acid (dOclA) as well as the nonhydroxylated fatty acids of the complete lipid A structure.Three mutants proved to be conditionally defective in dOclA metabolism, whereas one seems to be blocked at a relatively early step in lipid A synthesis. The phenotypes of all these mutants appear to be due to single mutations by reversion analysis and by characterization of the temperature‐resistant revertants.Studies of these mutants may shed light on the essential role of the complete dOclA—lipid A part of lipopolysaccharides in membrane function.