Plasmid Segregation: Is a Total Understanding Within Reach?
Wiki Entry by Robin Kirkpatrick, AP 225, Fall 2011
Plasmids are small, often circular, double stranded DNA. During chromosomal segregation, plasmids partition themselves via help from cytoskeleton polymers which form a structure analogous to the eukaryotic mitotic This paper presents an in vitro model of Plasmid segregation, and discusses the relation to the in vivo model. This paper discusses the experimental evidence measured from one bacterial model system yhat has can been used to develop a simple physical model.
A commonly studied plasmid partitioning system is the one involved in the plasmid R1 which requires only three components: ParM and ParR, which the plasmid R1 encodes, and a DNA sequence, parC. Experimental evidence showed that ParM forms filaments. As ParM bridges plasmids together, this suggested that a mechanism for plasmid segregation may be via polymerization of ParM, since growth would result in pushing the two plasmids apart. In vitro studies showed that, while ParM filament nucleate, they are unstable and grow and shrink bidirectionally. Furthermore, cooperative binding of ParR to ParC promotes assembly of ParM. A model was thus formed which involved rapidly fluctuating ParM filaments searching for the ParM-parC complex, and once two plasmids come close together, ParM becomes stabilized on both ends. ParM then grows resulting in pushing plasmids to different sides of the cell, and eventually depolymerizes as a result of dynamic instability allows which then plasmids to diffuse on opposite sides of the cell. This in vitro model is shown below in Figure 1.
Experimental evidence has also shown that, after elongation, ParM filament depolymerize both in vivo and in vitro, regardless of the current phase of the cell cycle, suggesting that regulation is independent of cell cycle dependent factors. Further experimental evidence has also confirmed that in vivo results are consistent with in vitro results. Therefore, the model proposed in Figure 1 appears to be consistent with both the in vivo and in vitro systems.
The author notes that there still remains a variety of questions regarding segregation. How do more complex systems such, as those found is eukaryotes, regulate segregation? How does the system facilitate the joining of two plasmids when it requires correct orientation and proximity? Can the biophysics of the plasmid and and ParM and ParR be used to explain plasmid partition at a statistical level?