Identification of the O antigen polymerase (rfc) gene in Escherichia coli O4 by insertional mutagenesis using a nonpolar chloramphenicol resistance cassette

The rfc gene in Escherichia coli O4 was identified using insertional mutagenesis performed on two open reading frames found in the O4 polysaccharide gene cluster. A chloramphenicol resistance cassette was constructed and inserted in both ORFs. Each ORF was then placed in two mutant types. Only one mutant produced O4-specific, semirough polysaccharide, indicating that the ORF it contained was the rfc gene.