Genetic analysis of the genes involved in synthesis of the lipopolysaccharide core in Escherichia coli K-12: three operons in the rfa locus.

The region of the Escherichia coli K-12 chromosome encoding the enzymes responsible for the synthesis of responsible for the synthesis of the lipopolysaccharide (LPS) core has been cloned in vivo by using a mini-Mu vector. This region, formerly known as the rfa locus, comprises 18 kb of DNA between the markers tdh and rpmBG. Results of in vitro mutagenesis of this region with MudII1734 indicate the presence of at least 17 open reading frames or genes, a number considerably higher than expected on the basis of genetic and biochemical studies. Specific insertions in different genes have been recombined into the chromosome, and the mutations have been phenotypically characterized. Complementation analysis indicates that these genes are arranged in three different operons transcribed in opposite directions. A detailed physical map of this region has been constructed on the basis of complementation analysis, fusion protein data, and phenotypic characterizations. Additionally, the role of some genes in the synthesis of LPS has been defined by complementation analysis with known Salmonella typhimurium LPS mutants. The genetic organization of this locus seems to be identical in E. coli K-12 and S. typhimurium.