Condensins load onto DNA to organize chromosomes. Smc-ScpAB clearly loads onto the parS sites bound by Spo0J, but other loading site(s) must operate independently of parS. In this study, we asked where and how Smc-ScpAB normally selects its loading site. Our results suggest that rDNA is also a loading site. A pull-down assay revealed that Smc-ScpAB preferentially loads onto rDNA in the wild-type cell and even in a Δspo0J mutant but not in a Δsmc mutant. Moreover, we showed that deletion mutants of rDNAs cause a defect in nucleoid separation, and at least two rDNAs near oriC are essential for separation. Full-length rDNA, including promoters, is required for loading and nucleoid separation. A synthetic defect by deletions of both rDNA and spo0J resulted in more aberrant nucleoid separation. We propose that a single-stranded segment of DNA that is exposed at highly transcribed rRNA operons would become a target for Smc-ScpAB loading.
Keywords: R-loop; Smc; bacteria; chromosome; oriC resolution; pull-down; single-stranded DNA; topological binding; transcription.
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