Genes for TDP-rhamnose synthesis affect the pattern of lipopolysaccharide heterogeneity in Escherichia coli K-12.

The rough lipopolysaccharide (LPS) of commonly used strains of Escherichia coli K-12 has two distinctly different band patterns when analyzed by high-resolution polyacrylamide gel electrophoresis. The LPS of ancestral strains such as W1485F- consists primarily of a single broad gel band. In contrast, the LPS of strains derived from strain Y10 such as AB1133 or C600 gives three sharp gel bands. Complementation studies using DNA fragments from the rfb gene cluster of Shigella dysenteriae 1 indicated that the difference between the two gel patterns is due to a mutation in the gene encoding the TDP-rhamnose synthetase, the final enzyme involved in TDP-rhamnose biosynthesis. This mutation arose during the construction of strain Y10, and not in strain 679-680 as previously thought. The requirement for the rfaS gene for synthesis of the broad major band seen in W1485F- LPS and the shift in gel migration of a component of this band when an rfaQ mutation was introduced indicated that this broad band contained the unique form of rough E. coli LPS which has been termed lipooligosaccharide. This finding indicates that lipooligosaccharide is likely to contain rhamnose and suggests a model in which one of the functions of partial substituents such as rhamnose may be to direct core synthesis into different pathways to produce alternative forms of LPS.