Plasmid Segregation: Is a Total Understanding within Reach?
Original Entry: Nick Chisholm, AP 225, Fall 2009
Authors: Daniel J. Needleman
Publication: Current Biology 18, R212-R214 (2008)
Soft Matter Keywords
This review article discusses recent progress towards a quantitative, molecular understanding of DNA segregation during cell division. The author states that recent in vitro and in vivo studies of the active partitioning of bacterial plasmids have shown that this type of understanding will be possible.
In particular, it talks about the partitioning of plasmids in bacteria; these plasmids are non-essential circular pieces of DNA. These are segregated by cytoskeletal polymers that form into dynamical structures, and thus can self-organize these plasmids.
Soft Matter Discussion
Since this article comments on experiments in the field, without giving much detail on the theory behind the results, I will discuss the interesting results of the experiments.
The dynamics studied both in vitro and in vivo gave strikingly similar results, and these dynamics are shown in Figure 1. The authors are trying to determine the cause of segregation of plasmids (the green loops shown in Figure 1). It was found three components are necessary for segregation: the proteins parM and parR, which the plasmid encodes, and a centromere-like sequence of DNA called parC (contained within the plasmid). Basically, the parR binds to the parC cooperatively; the parM, meanwhile, is floating around and continuously nucleating (forming nuclei) and disassembling.
Eventually, the parM will meet two closely spaced plasmids (with attached parR), bridge them, stabilize and grow, moving the two plasmids apart. This growth, and subsequent separation of the plasmids, is caused by the polymerization of the parM protein. It turns out that the polymerization continues until the plasmids reach the cell walls (and are oriented along the long axis of the cell, i.e. until the polymer reaches the maximum possible length while remaining in the cell). Clearly, since the plasmids are paired up, and on opposite sides of the cell, the cell can pinch inwards along the short axis of the cell and divide (with equal number of plasmids in each subsequent cell).
Obviously, it's pretty cool to see polymerization in action in a biological setting, performing an important role! It's also pretty neat that the interaction is so conceptually simple.
This review is rather important, at least for inexperienced students such as myself, since it emphasizes the importance of in vitro studies. Secretly, I've always felt that in vitro studies were rather meaningless; after all, for a good portion of experiments, these do not simulate realistic conditions. However, this particular string of work has shown that in vitro experiments can be of importance (i.e. they lead to the same results as in vivo), and we already know that in vitro experiments allow one to probe the properties of more realistic systems more easily. For me, personally, reading this article made me feel more at ease about the whole idea behind in vitro experiments.
 Daniel J. Needleman, "Plasmid Segregation: Is a Total Understanding within Reach?" Current Biology 18, R212-R214 (2008).