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キーワード:
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要旨:
The faithful segregation and inheritance of bacterial chromosomes and
low-copy number plasmids requires dedicated partitioning systems. The
most common of these, ParABS, consists of ParA, a DNA-binding ATPase and
ParB, a protein that binds to centromeric-like <i>parS</i> sequences on
the DNA cargo. The resulting nucleoprotein complexes are believed to
move up a self-generated gradient of nucleoid-associated ParA. However,
it remains unclear how this leads to the observed cargo positioning and
dynamics. In particular, the evaluation of models of plasmid positioning
has been hindered by the lack of quantitative measurements of plasmid
dynamics. Here, we use high-throughput imaging, analysis and modelling
to determine the dynamical nature of these systems. We find that F
plasmid is actively brought to specific subcellular home positions
within the cell with dynamics akin to an over-damped spring. We develop
a unified stochastic model that quantitatively explains this behaviour
and predicts that cells with the lowest plasmid concentration transition
to oscillatory dynamics. We confirm this prediction for F plasmid as
well as a distantly-related ParABS system. Our results indicate that
ParABS regularly positions plasmids across the nucleoid but operates
just below the threshold of an oscillatory instability, which according
to our model, minimises ATP consumption. Our work also clarifies how
various plasmid dynamics are achievable in a single unified stochastic
model. Overall, this work uncovers the dynamical nature of plasmid
positioning by ParABS and provides insights relevant for
chromosome-based systems.
