A CTP-dependent gating mechanism enables ParB spreading on DNA.

Abstract

Proper chromosome segregation is essential in all living organisms. The ParA-ParB-<i>parS</i> system is widely employed for chromosome segregation in bacteria. Previously, we showed that <i>Caulobacter crescentus</i> ParB requires cytidine triphosphate to escape the nucleation site <i>parS</i> and spread by sliding to the neighboring DNA (Jalal et al., 2020). Here, we provide the structural basis for this transition from nucleation to spreading by solving co-crystal structures of a C-terminal domain truncated <i>C. crescentus</i> ParB with <i>parS</i> and with a CTP analog. Nucleating ParB is an open clamp, in which <i>parS</i> is captured at the DNA-binding domain (the DNA-gate). Upon binding CTP, the N-terminal domain (NTD) self-dimerizes to close the NTD-gate of the clamp. The DNA-gate also closes, thus driving <i>parS</i> into a compartment between the DNA-gate and the C-terminal domain. CTP hydrolysis and/or the release of hydrolytic products are likely associated with reopening of the gates to release DNA and recycle ParB. Overall, we suggest a CTP-operated gating mechanism that regulates ParB nucleation, spreading, and recycling.