Summary: Using transposon insertion mutagenesis with Tn or Tn, we obtained mutant strains that showed either diminished serum resistance (five mutants) or diminished mouse lethality (two mutants). Detailed restriction cleavage analysis to determine the single sites of transposon insertion in an 80 kb plasmid (pTE800) indicated that a region for serum resistance was located within a 3·0 kb region of the I cleavage fragment 5 and the dIII fragment 1. while the region for mouse lethality was within a 6·0 kb region of the I fragment 2 and the dIII fragment 1. When the Tn-containing I fragment 5 was reconverted, by homologous recombination, to the original I fragment 5 (9·6 kb), serum resistance was recovered to the same level as that of a parent strain 52401. Moreover, the change in the serum resistance correlated with changes in the neutral sugar composition of the LPS. The mutation in the plasmid in strain TE4-55 that gave diminished mouse lethality was also reversed by recombination with the cloned I fragment 2 (15·0 kb), with concomitant recovery of mouse lethality. These results indicate that the genetic region for serum resistance is different from that for mouse lethality, and that the gene for serum resistance is closely involved with the expression of the neutral sugar composition of the LPS of .


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