Preferential synthesis of heptaacyl 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 degrees 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 degrees C.