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The ComS diffusion system

Introduction to the system

When it faces nutriment restriction, B.subtilis has two way of reacting. One is to sporulate in order to wait for better times and come back to "life" again. The second is the competence mechanism. In this state, B.subtilis tries to catch in the medium every piece of DNA around and make homologous recombination with its genome. It then divides a lot to give a chance to its new genotype to survive the harsh conditions.

Fig1: Schematics of the MeKS system [1]	Fig2: Image of a mix between sporulated and competent state during the steady state phase. [1]

The choice between these two mechanisms is controled by a bistable system known as the MeKS system. This system is usually stochastically controled by the apparition of ComS proteins in the cell. ComS inhibits the MecA protease and allows the ComK protein to self-amplify. But as ComK inhibits the production of ComS, the system comes back to the original state within a few hours.

A comprehensive study of this phenomenon has been conducted by M. Elowitz and al. [1].

Idea behind the design

The idea behind this design is to pass ComS proteins through the nanotube. This protein is very small (40 amino-acids) and it is expected to pass quite efficiently. We also know from the M. elowitz paper [1] that very few proteins are required to trigger the switch (around 200), which makes this system a very good candidate for what we want to do.

We contacted M. Elowitz, and he kindly sensed us a strain containing a chromosomally integrated reporter that monitors the level of ComK and ComS in the cell, with the construction pComG-cfp/pComS-yfp inside. This construct could in theory directly be used as a receiver cell, but the MeKS system is known to be repressed in expodential phase. We explain later how to avoid this problem.

The design can simply be summed up by the following picture:

Fig3: Schematic of the simplified general principle of the ComS design

How this design works

Here is the explanation step by step:

Fig4: Step one, the system is repressed, and there is no ComK.

Is a nanotube is etablished, some ComS will diffuse from the first cell to the second one, and block the protease by affinity inhibtion. The ComK can start amplifying, and activate the ComG promoter.

Fig5: The ComK protein activates the ComG promoter.

The ComG promoter produce CFP that report the cell has entered a competence phase, and the ComS promoter is giving us the quantity of ComS that have passed through the nanotubes.

Fig6: Schematic of the simplified general principle of the ComS design

Then, if the nanotubes break, the ComS start to disapears and several hours later, the cell come back to its original state.

Problems linked to the growth phase

The MeKS system is a noise tolerent bi-stable system that regulate the competence of the cell. This system is working in the stationnary phase and is theorically repressed during the expodential phase. We investigated the issue using computer assisted sequence homology analysis, and we found 3 locuses in which we expect the protein CodY to bind, that is known to repress many genes of the steady state phase during the expodential phase.

In order to avoid this problem, we have created a B. Subtilis strain ∆CodY. This will allow the MeKS system to be active during the expodential growth phase, as well as thousands of stationnary phase gene. This mutation is not lethal although it reduces the growth speed significantly.

A CodY- strand is obtained thanks to Link Sonenshein [2] and this strain was crossed with the reporter strain from M. Elowitz's laboratory [2] (see the paper[3]) using DNA extraction and competence of the late expodential phase competence.

We sucessfully managed to get this strain. See the experiment page for details.

Modeling and experiments

To know more about what we have done on this system and in the experiments, we invite you to visit the corresponding modeling and experiment pages:

• Modeling
• Experiments

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