UCSF

Although the mechanism of DNA segregation in Eukaryotes has been known for many decades, Prokaryotic DNA segregation has remained a mystery. I have studied a form of plasmid segregation in bacteria in order to get a better understanding of this process. The R1 Par operon is a self-contained plasmid partitioning system composed of three parts: parC, ParR and ParM. parC is a stretch of DNA consisting of 10 sequential repeats, each of which binds ParR. The ParR/parC complex in turn binds the actin homolog ParM. Previous studies have shown that ParM forms filaments nearly identical to those of eukaryotic actin filaments and that ParM filament bundles appear to position plasmids at each end of a rod-shaped cell. We demonstrate that ParM filaments are dynamically unstable and can elongate bidirectionally in vitro. ParM filaments also nucleate at a speed much faster than that of eukaryotic actins. Addition of ParR and parC coated beads induced long bundles of filaments with a bead at each end, indicating that the ParR/parC complex can suppress ParM dynamic instability. ParM filaments are also dynamically unstable in vivo and that the majority of filaments undergo assembly and rapid disassembly in less than a minute. These observations led to a model in which ParM filaments continually search the cytoplasm and eventually capture a ParR bound parC region on a plasmid. Insertional polymerization at the ParM/ParR interface will then push the plasmids to opposite ends of the cell and hold them in place until cell division, ensuring that each daughter cell receives a copy.