The structural basis for dynamic DNA binding and bridging interactions which condense the bacterial centromere.

Gemma Lm Fisher,Timothy Craggs,James A Taylor,Frank Sobott,Annika Butterer,Victoria A Higman,César L Pastrana,Mark S Dillingham,Matthew P Crump,Alan Koh,Heath Murray,Fernando Moreno-Herrero

doi:10.7554/elife.28086

Abstract

The ParB protein forms DNA bridging interactions around parS to condense DNA and earmark the bacterial chromosome for segregation. The molecular mechanism underlying the formation of these ParB networks is unclear. We show here that while the central DNA binding domain is essential for anchoring at parS, this interaction is not required for DNA condensation. Structural analysis of the C-terminal domain reveals a dimer with a lysine-rich surface that binds DNA non-specifically and is essential for DNA condensation in vitro. Mutation of either the dimerisation or the DNA binding interface eliminates ParB-GFP foci formation in vivo. Moreover, the free C-terminal domain can rapidly decondense ParB networks independently of its ability to bind DNA. Our work reveals a dual role for the C-terminal domain of ParB as both a DNA binding and bridging interface, and highlights the dynamic nature of ParB networks in Bacillus subtilis.

Highlights

Bacterial chromosomes are actively segregated and condensed by the ParABS system and condensin [1]
We show here that the central DNA binding domain is essential for anchoring at parS, and that this interaction is not required for DNA condensation
Structural analysis of the C-terminal domain reveals a dimer with a lysine-rich surface that binds DNA non- and is essential for DNA condensation in vitro

Summary

Introduction

Bacterial chromosomes are actively segregated and condensed by the ParABS system and condensin [1]. ParB foci appear to contain fewer proteins than are necessary to form a filament, and single molecule analyses using direct imaging [6] and magnetic tweezers [7] have shown that binding of DNA by ParB is accompanied by condensation These “networks” were inferred to be dynamic and poorly-ordered, consisting of several DNA loops between distally bound ParB molecules. Single-molecule imaging of the F-plasmid SopB led to a broadly similar model, defining ParB networks as fluid structures that localise around parS using a “nucleation and caging” mechanism [9] Despite these recent experiments converging on DNA bridging models to explain the ParB spreading phenomenon, the mechanism underpinning this behaviour remains unresolved. The relationship between these dynamic nucleoprotein complexes and the molecular architecture of the ParB protein is unclear and is the subject of the work presented here

Methods

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Discussion

Conclusion