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J. Bacteriol., 01 1996, 240-247, Vol 178, No. 1
S Lukomski, RA Hull and SI Hull
Computer analysis of the O4 polysaccharide gene cluster of Escherichia coli
revealed the presence of two open reading frames (ORFs) encoding strongly
hydrophobic polypeptides. O antigen polymerase, which is encoded by the rfc
gene, is a potential membrane protein and therefore should be hydrophobic.
To identify the rfc gene, these two ORFs were subjected to insertional
mutagenesis. A chloramphenicol resistance cassette was designed which, when
properly inserted, does not cause a polar effect in downstream genes. Each
of two ORFs, cloned into a plasmid vector, was inactivated with this
cassette. Two types of mutants bearing chromosomal insertions of the
cassettes in each ORF were constructed by homologous recombination. These
mutants were characterized by PCR, Southern blotting, and
transverse-alternating- field electrophoresis. Only one class of mutants
exhibited the expected O polymerase-deficient phenotype; they produced
O4-specific, semirough lipopolysaccharide. Therefore, this ORF was
identified as the rfc gene. The chromosomal rfc mutation was complemented
in trans by the rfc gene expressed from a plasmid vector.
Copyright © 1996, American Society for Microbiology
Identification of the O antigen polymerase (rfc) gene in Escherichia coli O4 by insertional mutagenesis using a nonpolar chloramphenicol resistance cassette
Department of Microbiology and Immunology, Baylor College of Medicine, Houston, Texas 77030, USA.
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