by Merry & Elio
More often than not, basic cellular phenomena have been deciphered first in bacteria, and then later elucidated in eukaryotic cells. A glaring exception is mitosis which was first described for eukaryotic cells in the 1880's. The corresponding story for bacterial chromosomes has remained uncharacteristically elusive. Truth is, we don't know much about it, although there has been some piecemeal progress. A review was published in 2006. A recent paper approaches the question by looking at how plasmids are partitioned to the daughter cells during division of their host bacterium.
R1 is a large plasmid (100 kb) that carries genes for resistance to drugs and heavy metals. Its 2 to 4 copies per cell are reliably partitioned between the progeny cells during cell division. 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).
The ParM protein is a homolog of actin that, given ATP, polymerizes into actin-like filaments. During cell division, bundles of ParM filaments can be seen extending the length of the cell with their ends attached to the segregating plasmid DNA. Binding to parC requires the cooperation of the ParR protein.
For their in vitro studies, the investigators mixed purified ParM and ParR proteins with parC DNA attached to 350 nm beads. When they looked at an isolated parC bead, they saw filaments forming continuously and growing out from the bead in all directions, resulting in an aster-like structure. Individual filaments grew to an average length of 1.5 microns before experiencing spontaneous catastrophic disassembly. (This property of the filaments is referred to as dynamic instability and plays a vital role according to the authors.) If, instead, the free end of the filaments contact a second parC bead, a bundle of filaments forms connecting the pair of beads. (See Movie #S-4, where parC DNA is red, the ParM filaments green.)
Does elongation takes place by addition of ParM at the ends or all along the filament? Growth is at the ends, as can be seen in the figure and in Movie #S-12 where photobleaching was used to make two reference marks on each spindle. As the spindle elongates, the distance between the bleached marks does not change, whereas the distance between each mark and its nearby parC bead increases at a constant rate.
The authors hypothesize that the ParM filaments that grow out in all directions from a ParR/parC complex efficiently search the entire bacterial cell looking for a fellow complex. If they do not meet one, they disassemble and other filaments continue the search. When they do contact a second ParR/parC complex, a stable spindle forms and elongates, separating the two plasmid chromosomes. Sounds like what a mitotic spindle does in eukaryotic cells.
An animated picture of this process is seen in Movie #S-17. (Note that this movie is an 11 MB file.)
This is in vitro – a long way from the cell – but it is heartening, nevertheless. A final thought. What is it that makes chromosome segregation in bacteria so hard to pin down? Is it just size that makes a mitotic apparatus imperceptible in bacteria? Or, as was widely considered for a while, is the cell membrane involved? Any thoughts?