Toward the engineering of minimal living cells

The article focuses on the notion of a synthetic or semi-synthetic minimal cell, defined as a system that has the minimal and sufficient structural conditions for cellular life. It is emphasized that two complementary approaches are in principle possible, defined as "bottom-up" and "top-down" approaches. The first one aims at the construction of a minimal cell starting from scratch, and it is argued that a very serious bottle-neck to this pathway lies in the origination of specific macro-molecular sequences, as in nature those were constructed most likely by a particular contingent set of conditions. The top-down approaches utilize extant genes and enzymes, and the work in this case is based on the incorporation of the minimal and sufficient amount of such macromolecules into liposomes, as models for the shell of biological cells. The first phase of this ambitious project foresees the study of conditions under which complex molecular biology reactions takes place in the compartments of liposomes. Examples of these reactions are provided, for example, the production of RNA throughout Q-beta replicase in a self-reproducing vesicle system; or PC Reaction in phospholipid vesicles; or even the incorporation of ribosomes in liposomes, with the production of polypeptide chains. The use of giant vesicles is also illustrated. These systems, due to their large size, offer the advantage that by way of special micro-injection techniques, all sort of biochemical agents can be directly introduced in the compartment; and that the reaction can be followed by optical microscopy. In the final part of the article, the outlook of increasing the complexity of these liposome systems so as to arrive at first semi-synthetic cells is discussed.