The Bacterial Divide

Roughly once an hour, the rod-shaped bacterium E. coli multiplies by producing a copy of its DNA and then splitting into two daughter bacteria, each carrying a complete set of genetic information. It is vital that the division occur very close to the bacterium's midpoint to ensure the viability of the daughter cells, but it has long been a mystery as to how a bacterium locates its middle in preparation for division. Researchers from Simon Fraser University (British Columbia) and Dalhousie University (Nova Scotia) believe they have solved the riddle by studying the interactions of three proteins that flow from end to end inside the bacterium (Martin Howard, 31-71-527-5515, mjhoward@lorentz.leidenuniv.nl; Andrew Rutenberg, 902-494-2952, adr@fizz.phys.dal.ca).

Biologists have known for several years that the proteins MinC, MinD, and MinE are important in cell division; the absence or incorrect distribution of any one of the three can corrupt cell division, or inhibit the process altogether. Experiments have shown that these Min proteins oscillate from end to end of the bacterium every minute or so. The effect of the oscillation is that MinC and MinD have the their highest concentration at the bacterial ends. Because MinC inhibits division, the bacterium will divide at the center, where MinC is minimized.

The nagging question concerns how these protein oscillations are driven. Jostling molecules in gases and liquids tend to spread concentrated substances around in a diffusion process; it's the reason a fragrance can drift across a room even in still air. Diffusion is also the principle transport mechanism inside bacteria, but acting alone it should evenly distribute compounds throughout the cell. As the researchers' new model shows, however, it's when protein diffusion is combined with the binding and release of proteins from the cell membrane that oscillating patterns in E. coli occur. The effect is closely related to the Turing model reaction-diffusion equations often championed as the mechanism behind complex patterns in nature, such as tiger stripes and ladybug spots (Update 558). In the case of E. coli, oscillation of the Min protein self-organizing behavior causes the division site to be at the cell midpoint. (M. Howard, A.D. Rutenberg, and S. de Vet, Physical Review Letters, 31 Dec 2001.)