Our understanding of their structure is limited to the N-terminal domain (NTD) which binds ParA ( Bouet and Funnell, 1999 Davey and Funnell, 1997 Davis et al., 1992 Radnedge et al., 1998) and the central DNA binding domain (CDBD) which binds parS and possibly also nsDNA ( Leonard et al., 2004 Schumacher and Funnell, 2005). Genomically-encoded ParB proteins comprise three distinct domains ( Figure 1A and Figure 1-figure supplement 1A,B and C). In particular, 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. Despite these recent experiments converging on DNA bridging models to explain the ParB spreading phenomenon, the mechanism underpinning this behaviour remains unresolved. Recently, 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 ( Sanchez et al., 2015). Modelling suggests that a combination of 1D spreading and 3D bridging interactions can explain the condensation activity and recapitulate the polar effect of roadblocks on ParB spreading ( Broedersz et al., 2014). In cells, they are presumably anchored at parS sites by sequence-specific interactions but must also contain many interactions with non-specific DNA (nsDNA), as well as self-association interactions that bridge ParB protomers to form DNA loops. These ‘networks’ were inferred to be dynamic and poorly-ordered, consisting of several DNA loops between distally bound ParB molecules.
However, ParB foci appear to contain fewer proteins than are necessary to form a filament, and single molecule analyses using direct imaging ( Graham et al., 2014) and magnetic tweezers ( Taylor et al., 2015) have shown that binding of DNA by ParB is accompanied by condensation. Earlier models envisioned a lateral 1D spreading around parS to form a filament ( Murray et al., 2006 Rodionov et al., 1999), principally because spreading can be inhibited in a polar manner by ‘roadblocks’ placed to the side of parS sequences. The mechanistic basis for this behaviour is not well understood and a matter of active debate. In addition to sequence-specific interactions with the parS sequence, the protein also spreads extensively around the site for about 18 kbp ( Breier and Grossman, 2007 Murray et al., 2006 Lynch and Wang, 1995). These nucleoprotein complexes act as a positional marker of the origin and earmark this region for segregation in a manner somewhat analogous to eukaryotic centromeres and their binding partners. subtilis, this machinery is physically targeted to the origin proximal region of the chromosome by eight palindromic DNA sequences called parS (consensus sequence 5′-TGTTNCACGTGAAACA-3′) to which the ParB (Spo0J) protein binds ( Breier and Grossman, 2007 Lin and Grossman, 1998). We continue to take redirection and cloaking very seriously, but also acknowledge the importance of URL shorteners, particularly in social promotion.Bacterial chromosomes are actively segregated and condensed by the ParABS system and condensin ( Wang et al., 2014 Song and Loparo, 2015). We place a high value on the trust and integrity of our network and each of these services was tested to ensure proper attribution. The shortened Bitly link is visually appealing, directs your visitors to your desired location and maintains your affiliate tracking.
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