We report here the cloning of the genomic topoisomerase I gene () by use of PCR and subsequent hybridization. The predicted protein sequence shared 58.8% identity with the topoisomerase I and 30-50% identity with other eukaryotic topoisomerase I proteins. A conditional gene disruption strain (CWJ477) was constructed so that one copy of was deleted and the other copy of was placed under a regulatable promoter. Under repressed conditions, cells grew slowly and cell morphology was abnormal. The virulence of CWJ477 was markedly reduced in a mouse model system, and that of the single gene knockout strain was slightly attenuated, indicating that might play a role in the infection of in mice in a dose-dependent manner. Despite the reduced virulence of both the single and double knockout strains, viable cells of the pathogen were recovered from the kidneys as late as 22 d post-infection.


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