Relief of ParB autoinhibition by DNA catalysis and recycling of ParB by CTP hydrolysis promote bacterial centromere assembly.

Three-component ParABS systems are widely distributed factors for plasmid partitioning and chromosome segregation in bacteria. ParB acts as adaptor protein between the 16–base pair centromeric DNA sequences and the DNA segregation proteins ParA and Smc (structural maintenance of chromosomes). Upon cytidine triphosphate (CTP) and DNA binding, ParB dimers form DNA clamps that spread onto -flanking DNA by sliding, thus assembling the so-called partition complex. We show here that CTP hydrolysis is essential for efficient chromosome segregation by ParABS but largely dispensable for Smc recruitment. Our results suggest that CTP hydrolysis contributes to partition complex assembly via two mechanisms. It promotes ParB unloading from DNA to limit the extent of ParB spreading, and it recycles off-target ParB clamps to allow for retargeting, together superconcentrating ParB near . We also propose a model for clamp closure involving a steric clash when binding ParB protomers to opposing half sites.