Summary: A region encoding ORFs with homology to known lipopolysaccharide (LPS) biosynthesis genes was isolated from two strains of One of the strains produces LPS, but the second strain is reported to produce only lipooligosaccharide (LOS) and therefore lacks the O-chain. The two strains shared six predicted ORFs, but an additional ORF, of unknown function was identified in the LOS-producing strain. Mutation of the shared () homologue (orfF) or deletion of five of the seven genes reduced core reactivity with specific antiserum without affecting O-chain production. Mutation of either the homologue () or the unique had no detectable effect on LOS or LPS production. The presence or absence of in 36 isolates of did not correlate with LOS/LPS phenotype or serotype. However, after insertion of into a LPS-producing -negative strain the O-chain ladder was no longer detectable on Western blots. We were not able to disrupt the () homologue () in .


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