Recognition and Condensation of the Bacterial Centromere by ParB

Abstract

In Bacillus subtilis chromosome segregation is a multipartite task shared by the ParABS system and the Structural Maintenance of Chromosomes (SMC) condensin complex. At the most basic level of this hierarchal assembly, ParB binds at and around parS sequences, locally condensing DNA and acting as a positional marker of the replication origin. This complex is a cargo for ParA-mediated segregation and a loading site for condensin. However, the mechanisms of ParB assembly at parS and the ensuing segregation and condensation of the chromosome are poorly understood.In previous work we suggested a model for ParB association and condensation around parS involving distinct specific and non-specific DNA binding loci. Here we test this model by assessing the properties of mutant ParB proteins using biochemical and biophysical assays. Mutation of the central helix-turn-helix region of the protein completely eliminates specific binding to parS sequences, but non-specific DNA binding and condensation are largely unaffected. Moreover, we provide direct evidence for a second DNA binding locus in the C-terminal domain of ParB and show this region is also important for oligomerisation leading to DNA condensation. A refined model for ParB-DNA complexes at the bacterial origin is discussed.