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

has a DNA topoisomerase I with a unique C-terminal domain (CTD) homologous to eukaryotic SWIB domains. This study focused on determining the function of the SWIB domain-containing TopA from (CtTopA). We demonstrated that, despite the lack of sequence similarity at the CTDs between CtTopA and TopA from (EcTopA), full-length CtTopA removed negative DNA supercoils and complemented the growth defect of a mutant of . 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, 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 . Overexpressing a mutant allele lacking the SWIB domain in resulted in slow growth when host protein synthesis was inhibited. These data suggest that productive infection of requires functional SWIB domain-containing CtTopA and host protein synthesis. Because SWIB domain-containing CtTopAs are not found in prokaryotes beyond spp., our work suggests an important function of the SWIB domain on CtTopA activity during infection.IMPORTANCE is a medically important bacterial pathogen that is responsible for the most prevalent bacterial sexually transmitted infection. Bioinformatics, genetics, and biochemical analyses have established that the presence of a SWIB domain in CtTopA is relevant to chlamydial physiology. Our findings also underline the mechanistic diversity among the family of TopAs that are likely driven by pathogen-specific adaptations.