How a bacterium finds its center in order to localize the division machinery is a long-standing question. The center is millions of molecules away from the poles. Thus, what type of signaling system enables the position of the division apparatus? Crucial is a pol-to-pol oscillation of a molecule called MinD, which blocks together with another molecule (MinC) the initiation of cell division. Cell division becomes restricted to the centre where, on average, the MinC/D concentration is lowest. The figure below shows snapshots of the MinD oscillation. The numbers are seconds. Each full cycle requires about 50 seconds [1,2].
What type of molecular interaction allows this pole-to-pole oscillation? Although more or less all molecules involved in this center-finding process were known for long, it remained an open problem of how these components work together. In a review Shapiro and Losick wrote in 2000 : "we are left with two topological mysteries: how does a bacterial cell knows where its middle is and what is the medial mark that triggers polymerization" .
To facilitate an understanding of this highly complex process, first an analogy should be given (knowing that all analogies are a bit dangerous). Imagine a strip of very rapidly growing grass on which a cow is grazing. After eating up all grass in the immediate surroundings, the cow will start to move into a region in which more grass is available. In which direction to go first is more or less random but it will continue in this direction since less grass is left behind. After reaching the end of the strip, the cow will move rapidly to the other side of the strip where still fresh grass is available. After reaching the other end, the process will start anew, assuming that the grass recovered meanwhile. Since the cow passes the center twice as frequent as the poles, there will remain less grass in the center.
In the bacterium, a protein called MinD would cover uniformly the membrane (the grass). According to our model, a local signal (MinE, the cow) needs MinD to bind to the membrane but removes thereby MinD from the membrane. Thus, the MinE signal will move into a region where more MinD is available. Thus, the minE signal sweeps over the field like a windshield wiper of a car. On time average, the MinD concentration is lowest in the center, allowing the initiation of the division apparatus (FtsZ-ring) (for more details clicke here): The analogy in not perfect in two aspects. The Min molecules but not the grass is recycled. Further, the cow is anyway a localized object. To achieve a similar localization of the MinE distribution , a pattern forming reaction is required. The following simulation shows a typical computer simulation using our model (Meinhardt, H, de Boer, P.A.J., 2001) [PDF]
On the subsequent pages, the elements of this model are described in more detail
For further details and references see Meinhardt, H, de Boer, P.A.J. (2001). Pattern formation in Escherichia coli: A model for the pole-to-pole oscillations of Min proteins and the localization of the division site. PNAS 98, 14202-14207 [ PDF ]. Around the same time, two alternative but somewhat related models were published [7,8].