The next step is to extract the complete synthetic bacterial genome from the yeast and transplant it into bacterial cells. Extracting the genome from yeast and transporting it is the trickiest part of the process. The mycoplasma genome is relatively small, but it’s a huge molecule. The shear force of water moving around the bare DNA molecules can pull it apart. So the researchers immobilize the DNA in a pellet of gel and take it to another lab, where the transplant recipient cells have been prepared. The recipient cells, of the species Mycoplasma capricolum, are a close relative of the cells whose genome is the basis of the synthetic genome. Through trial and error, the researchers have found that there is a particular part of the cells’ cycle of growth and division at which they are most likely to take up the foreign DNA.
Getting the recipient cells to take up the synthetic genome is in part a matter of chance. A researcher mixes the recipient cells with a chemical solution to make their surfaces fluid and sticky, then adds the cells to the DNA solution. Once mixed, the sticky cells begin fusing with one another. In order to maintain a spherical shape as their surface area is increasing, the cells take on volume from the solution around them. By chance, as they fuse, some of these megacells take in copies of the M. mycoides genome.
Left for about three hours, the cells with more than one genome will divide, creating a mixture of cell types. About one in 100,000 cells has the transplanted genome, which contains an antibiotic-resistance gene. When the cell solution is streaked on plates containing the antibiotic tetracycline, only those with the transplanted genome survive. Though they were initially of a different species, the M. mycoides genome takes over to create what the researchers call a synthetic cell.
The researchers will now use these techniques to gradually shrink the genome. They’re currently using software to design new genomes with various genes removed, then using their technique to synthesize and transplant them. “We can test a staggering amount of possibilities in an experiment,” says Glass. This allows them to determine in a matter of weeks rather than years what happens when, say, 10 genes in a particular pathway are expressed at varying levels or eliminated.
Developing these techniques took about $30 to $40 million in funding, mostly from the company Synthetic Genomics. The main cost of these experiments comes from the price of synthesizing DNA, which may go down as more researchers see the promise of these types of experiments. “Other groups aren’t doing this because of the cost and because the methods have been difficult, but I’d like to think we’re making the methods simple,” says Gibson. Boston University professor James Collins agrees. “As the costs come down you’ll see a number of labs begin to synthesize on this scale. If this technique is viewed as being useful, we’ll get there on the costs.”