The SWIB domain-containing DNA topoisomerase I of <i>Chlamydia trachomatis</i> mediates DNA relaxation

<i>Chlamydia trachomatis</i> 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 <i>C. trachomatis</i> (CtTopA). We demonstrated that, despite the lack of sequence similarity at the CTDs between CtTopA and TopA from <i>Escherichia coli</i> (EcTopA), full-length CtTopA removed negative DNA supercoils <i>in vitro</i> and complemented the growth defect of a <i>topA</i> mutant of <i>E. coli</i>. 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 <i>Mycobacterium tuberculosis</i> 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 <i>C. trachomatis</i>. Overexpressing a <i>topA</i> mutant allele lacking the SWIB domain in <i>C. trachomatis</i> resulted in slow growth when host protein synthesis was inhibited. These data suggest that productive infection of <i>C. trachomatis</i> requires functional SWIB domain-containing CtTopA and <i>de novo</i> host protein synthesis. Because SWIB domain-containing CtTopAs are not found in prokaryotes beyond <i>Chlamydia</i> spp., our work suggests an important function of the SWIB domain on CtTopA activity during <i>C. trachomatis</i> infection.IMPORTANCE<i>Chlamydia trachomatis</i> 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.