Figure 7. Model for ParB-dependent DNA condensation around parS sequences.

(Step 1) ParB binding to parS does not require cytidine triphosphate (CTP) as observed from C-trap experiments. ParS-bound apo-ParB does not spread from parS. (Step 2) CTP binding to ParB induces a conformational change to a sliding clamp which then escapes from parS to neighbouring non-specific DNA. Potential interactions between the ParB proteins around parS are represented by interlaced blue circles. Some ParB proteins are able to slide/diffuse long distances. (Step 3) ParB spreading and diffusion promotes the interaction with other CTP-ParB dimers through the C-terminal domain (CTD) of ParB (Fisher et al., 2017), resulting in DNA condensation by forming large DNA loops. Alternatively, other protein-protein interaction such those mediated by the N-terminal domain (NTD) (shown in figure) or the central DNA-binding domain (CDBD) of ParB could result in DNA condensation. CTP hydrolysis might be a means to recover ParB dimers from the DNA (black arrow). Protein roadblocks constrain diffusion of ParB proteins (red arrow).

Figure 7—figure supplement 1. ParB diffusion is required for DNA condensation by ParB.

(A) Schematic representation of DNA substrate employed in these magnetic tweezers (MT) experiments. It contains a set of 5× EcoRI sites located at 3835 bp from the DIG labelled end, and 7× parS. The positions of the EcoRI and parS sites in the DNA cartoon are represented to scale. (B) Condensation assay using the EcoRI 7× parS DNA substrate under different experimental conditions. ParB partially condenses the DNA molecule when EcoRI^E111G is present. (C) Quantification of the extension in base pairs of the non-condensed region under different experimental conditions. In the presence of EcoRI^E111G, the length of the non-condensed region agrees well with the length of the region flanked by the DIG end and the EcoRI sites.