The point of this brief history is that a surprisingly complex biological function can be reconstituted through self-assembly of dispersed components. Why not try to reconstitute a whole cell? If this turns out to be possible, perhaps it will help us untangle what we mean by “life” and even elucidate the major steps that led to the origin of cellular life.
Pier Luigi Luisi and his research associates in Zurich made the first attempt by encapsulating ribosomes in lipid vesicles in 1999, together with a synthetic form of RNA that told the ribosomes to incorporate the amino acid phenylalanine into a protein. A few short peptides were produced, but lipid bilayers are impermeable to amino acids, so synthesis was limited to those phenylalanines that happened to be inside the vesicles. Vincent Noireaux and Albert Libchaber at the Rockefeller University had a clever solution for the permeability problem: why not add a channel to the lipid bilayer of the vesicles? They reported in 2004 that they had succeeded in encapsulating a complete translation system isolated from E. coli, along with messenger RNA that directs ribosomal synthesis of green fluorescent protein (GFP) and of hemolysin, a protein that serves as a channel allowing externally added amino acids and ATP to enter the vesicles. The system worked for as long as four days, and at the end of the incubation period the vesicles glowed green from the accumulated GFP. Tetsuya Yomo and his research group at Osaka University have gone a step further with a similar encapsulated translation system in which the GFP gene is present in a strand of DNA. They refer to their system as a genetic cascade, because the GFP gene is transcribed into messenger RNA, which then directs synthesis of the protein.
These encapsulated translation systems exhibit a fundamental property of life: they use genetic information to synthesize a protein, but only a few specific proteins are produced, and everything else is left behind. To be truly alive, the protocells would need a DNA strand with genes for more than 200 different proteins and RNA species, including genes for a polymerase enzyme so that the DNA can be replicated. Enzymes that catalyze lipid synthesis must also be present, because the membrane boundary needs to grow. Transport proteins must be incorporated into the lipid bilayer; otherwise the vesicles have no access to external sources of nutrients and energy. A whole set of regulatory processes should also be in place, so that all this growth is coördinated. Finally, when the vesicles grow to approximately twice their original size, they need to divide into daughter cells that share the original genetic information.
It follows that even the simplest life today is astonishingly complex and could not have sprung into existence on the early Earth with a full complement of hundreds of genes. There must have been something simpler–a kind of scaffold life that was left behind in the evolutionary debris several billion years ago. Given all this, how likely is it that the ultimate promise of synthetic biology will be fulfilled–that an artificial version of a primitive living cell can be assembled? The best bet is probably a ribozyme that catalyzes its own complete synthesis from ATP, UTP, GTP, and CTP–the four nucleotide monomers of RNA–using genetic information encoded in its structure. If someone succeeds, we will have in hand the essential property that is lacking so far in artificial cell models: reproduction of the catalyst itself. Given such a ribozyme, we already know how to incorporate it into a system of lipid vesicles that can grow along with the ribozyme and allow nutrient nucleotides to enter the cell to support growth. The encapsulated ribozymes will have the capacity to evolve, as Bartel and Szostak demonstrated 15 years ago. In short, the system will be alive.
And then what happens? There will be headlines, of course; textbooks will be rewritten; and early in the morning someone will probably be awakened by a phone call from Stockholm. But after all the hullabaloo dies down; someone else will ask, “Well, so what?” That same question could have been asked when the double-helix structure of DNA was published in 1953. The magnitude of the discovery was not apparent until years later. I think that the first system of molecules capable of reproducing itself will also seem to be an academic exercise at first. But to put it in proper perspective, recall that food, antibiotics, oil, wood, methane, and hydrogen are produced by living cells resulting from more than three billion years of evolution. I think the next revolution in technology will begin when the synthetic functions of life can be performed by simplified versions of cells that are designed from blueprints rather than through evolution.
David Deamer is a research professor of biomolecular engineering at the University of California, Santa Cruz. He is currently writing a book on the origin of life, to be published by the University of California Press.