Phase-separated ParB enforces diverse DNA compaction modes and stabilizes the parS-centered partition complex

Area of Science:

• Bacterial cell biology
• Molecular microbiology
• Genetics and genomics

Background:

• The ParABS system is crucial for bacterial chromosome segregation in most species.
• ParB proteins bind parS sites, condensing DNA into complexes for ParA-mediated partitioning.
• The precise mechanism of partition complex assembly remains incompletely understood.

Purpose of the Study:

• To elucidate the molecular mechanisms behind the dynamic assembly of the bacterial partition complex.
• To investigate the role of Bacillus subtilis ParB (Spo0J) in DNA organization and condensation.
• To understand how phase separation contributes to bacterial chromosome segregation.

Main Methods:

• Investigated Bacillus subtilis ParB (Spo0J) protein behavior.
• Utilized techniques to observe phase-separated condensate formation along DNA.
• Analyzed DNA compression and looping induced by ParB condensates.
• Examined the effect of CTP on condensate formation and DNA condensation.

Main Results:

• Bacillus subtilis ParB, via N-terminal domain multimerization, forms phase-separated condensates on single DNA molecules.
• These condensates compact DNA, compressing adjacent segments and looping distant ones.
• CTP promotes condensate formation by low ParB concentrations at parS sites, initiating a two-step DNA condensation process.
• The resulting ParB-DNA co-condensate forms a stable structure resistant to significant mechanical forces.

Conclusions:

• Phase-separated ParB condensates exhibit diverse DNA compaction strategies.
• Findings provide novel insights into the dynamic assembly and robust maintenance of bacterial partition complexes.
• The study clarifies the role of ParB in organizing chromosomal DNA for segregation.