The SWIB domain-containing DNA topoisomerase I of mediates DNA relaxation.

The obligate intracellular bacterial pathogen, (Ct), has a distinct DNA topoisomerase I (TopA) with a C-terminal domain (CTD) homologous to eukaryotic SWIB domains. Despite the lack of sequence similarity at the CTDs between TopA (CtTopA) and TopA (EcTopA), full-length CtTopA removed negative DNA supercoils and complemented the growth defect of an mutant. We demonstrated that CtTopA is less processive in DNA relaxation than EcTopA in dose-response and time course studies. An antibody generated against the SWIB domain of CtTopA specifically recognized CtTopA but not EcTopA or TopA (MtTopA), consistent with the sequence differences in their CTDs. The endogenous CtTopA protein is expressed at a relatively high level during the middle and late developmental stages of . Conditional knockdown of expression using CRISPRi in resulted in not only a developmental defect but also in the downregulation of genes linked to nucleotide acquisition from the host cells. Because SWIB-containing proteins are not found in prokaryotes beyond spp., these results imply a significant function for the SWIB-containing CtTopA in facilitating the energy metabolism of for its unique intracellular growth.