Tuesday, May 15, 2007

Bacterial Mitosis

A really nice article about Bacterial Mitosis can be found at Small Things Considered. It evaluates a recent paper on how plasmids are partitioned to the daughter cells during division of their host bacterium. I always assumed that plasmids segregated randomly. For this reason, I found it puzzling that low-copy number plasmids existed. Say a dividing bacterial cell has 2 plasmids. If the plasmids are randomly distributed within the cytoplasm, then the occurrence of plasmids in the daughter cells will follow a Poisson Distribution. That is, the pair of daughter cells will have three possible states: 1:1, 0:2 and 2:0 number of plasmids. Obviously that isn't good if the plasmids "aim" to maximize their reproduction. Two of the three cases do not entail any reproduction at all.

But actually bacterial cells appear to undergo a primitive form of mitosis.

"The researchers isolated a three-component system from these cells that acts somewhat like a mitotic spindle when reconstituted in vitro. Those three components are two proteins encoded by the plasmid par operon (ParM and ParR) and a specific centromere-like DNA sequence within the operon (parC)."

This system allows the dividing cell to partition plasmids equally between daughters. How cool is that?


  1. Very cool. I like Small Things Considered, too. Do I understand from the paper that the mechanism is encoded by the plasmid itself? So every plasmid has to have its own? Imagine if each of our chromosomes had a different mitotic mechanism!

  2. That appears to be the case, the R1 plasmid encodes the three proteins required for the segregation machinery. Apparently other plasmids have similar (or even the same) machinery. The original paper mentions that "some extrachromosomal DNA elements have evolved their own polymer-based DNA segregation machinery" with cites for some more examples.

    Alternatively, plasmids might just reproduce intracellularly with high enough frequency to ensure their random segregation in each daughter cell (i.e. each daughter would get several/many copies). I'm sensing there is quite an opportunity for an evolutionary perspective on plasmid transmission (if it isn't already out there).

  3. Many lab staple plasmids have had par genes removed (deliberately or by repeated selection) to allow higher copy numbers. This is why rapidly dividing cultures can quickly lose their plasmids in the absence of strong selection. In other words, always add your antibiotics kids!! Low copy number plasmids don't suffer so much from this effect as they usually have intact par loci, but obviously the extremely high copy number of non-par plasmids mediates against loss too.