Preferential synthesis of heptaasyl lipopolysaccharide by the ssc permeability mutant of Salmonella typhimurium

In Salmonella typhimurium, a chromosomal gene termed ssc has been shown to cause an antibiotic‐supersensitive phenotype. We studied the effect of the ssc gene on the chemical composition of the lipopolysaccharide component, using a thermosensitive ssc1 mutant (SH7622) that grows poorly at 42°C. Analysis of the lipopolysaccharide by various techniques including fast‐atom‐bombardment mass spectrometry of lipid A, and determination of the type of linkage of fatty acids, revealed a profound temperature‐dependent effect associated with the ssc1 mutation. At the non‐permissive temperature, SH7622 contained hexadecanoic acid in the majority of lipid A molecules, resulting in the exclusive presence of heptaacyl lipopolysaccharide. This effect was largely reversed by the introduction of the cloned wild‐type ssc gene to SH7622 and much reduced by growth of SH7622 at 37°C.