Paris/A synthetic multicellular organism

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How can we define a multicellular organism ?

There is no universally admitted definition. Wikipedia gives: "an organism (in Greek organon = instrument) is a living complex adaptive system of organs that influence each other in such a way that they function in some way as a stable whole."

For us, the organs will simply be different cells. The definition we retain is that of a multicellular entity with different types of cells fulfilling different complementary tasks.

In most organisms, some cells differentiate to realize a function useful for the organism. By useful, we mean that it will give a better fitness, or in other terms, that it will help producing more offspring. The specialized cells are usually not the same than the ones reproducing the organism, and they loose this ability. Thus, their is a notion of sacrifice of some cells to the profit of those dedicated to reproduction. The cells are able to reproduce the organism represent the germline, the ones which specialize and loose this ability represent the soma.

How to proceed ?

The question is: how can we make a cell differentiate into two distinguished lines? According to us, the answer is the Cre/loxP system. We will introduce a special genetic construction into the bacterial chromosome; the excision of a cassette framed by loxP sites will lead to the differentiation of these cells into soma. In other words, the original germ cell gives a soma cell when a particular cassette is excised consequently to Cre/LoxP-system action. The construction could be simply represented in that way:

Basic construct.JPG

This is the construction of the germline. Here, the essential gene is expressed, but the auxotrophy gene is not. Thus this strain is auxotroph for the metabolite this "auxotrophy gene" produces. If the CRE recombinase is expressed, the essential gene will be exised leading to the following construction:

Soma Construct.JPG

Here the auxotrophy gene is expressed, but the strain doesn't express the essential gene anymore. It will thus die after a given time. But in the meanwhile, it will hopefully have produced enough of the auxotrophy metabolite to feed the germline.


What essential gene ?

The essential gene must fulfill two main criteria:

  • The cells should be able to live as long as possible without it.
  • Its deletion should not impair to much the capacity of the cell to produce the auxotrophy metabolite

We choose FtsZ. For more details on our choice, see here.


What auxotrophy gene ?

The auxotrophy gene must fulfill three main criteria:

  • The soma cells must be able to overproduce and excrete it
  • Their must be a simple auxotrophy to the metabolite
  • It would be better if there is a promoter sensitive to the metabolite concentration. If this is the case, we could place the CRE recombinase under the control of this promoter. In this way, the germline will differentiate into soma only if there is a need for the auxotrophy metabolite.

We choose DAP (diaminopimulate). For more details on our choice, see here.