The first microbe to live entirely by genetic code synthesized by humans is now proliferating in a lab at the J. Craig Venter Institute (JCVI). Venter and his colleagues have used a synthetic genome—the genetic instruction set for life—to build and operate a new Mycoplasma mycoides bacteria, according to an online report published May 20 by Science.

"For the past 15 years, the genomes of many organisms have been sequenced and deposited in databases. We call this digitizing biology," says JCVI molecular biologist Daniel Gibson. "We now show that it is possible to reverse this and synthesize cells starting from this digitized information….We refer to the cell we have created as being a synthetic cell because it is a cell controlled by a genome assembled from chemically synthesized pieces of DNA."

In other words, a chemical synthesizer stitched together various short iterations of man-made adenine, cytosine, guanine and thymine that were then assembled into a working genome that can successfully produce the proteins that enable life. Using stretches of DNA, known as cassettes, roughly 1,000 base-pairs in length, the researchers assembled a simplified version of M. mycoides genome from scratch in a succession of E. coli and yeast cells. The final synthetic genome—more than a million base-pairs long—was then inserted into an existing Mycoplasma capricolum cell. The synthetic cell then went on to behave as a M. mycoides, producing proteins from the instructions encoded by the synthetic genome and even dividing and growing.

"It is a big deal," geneticist and technology developer George Church of Harvard Medical School says of the achievement. "It's not incremental, but it's not final either," noting that other groups are already delivering useful products from partially reengineered genomes, such as biofuels from engineered E. coli.

"It's not genesis, it's not as if mice are coming from a pile of dirty rags in a corner," says biological engineer Drew Endy of Stanford University. "The correct word is poesis, human construction. We can now go from information and get a reproducing organism. It lays down the gauntlet for us to learn how to engineer genomes."

Getting to this point was not without its challenges, including requiring at least $30 million in investment into the experiments required in the past 15 years. The researchers started with the intention of synthesizing the genome of Mycoplasma genitalium, which has the smallest known natural genetic instruction set. But that organism's slow growth and other properties led them to ditch it in favor of genetically more complex cousins such as M. mycoides and M. capricolum. To simplify things, they deleted 14 genes from M. mycoides natural genome, still leaving behind hundreds.

Then the researchers could not find a way to transfer genomes from one bacteria to another, eventually enlisting the yeast as an assembly waystation, permitting easier manipulation of genetic material and overcoming natural resistance in the microbes to tinkering with their DNA. The yeast also multiply copies the synthetic genome with its own to allow spares for experiments, while adding its own genetic twists, such as eight single nucleotide polymorphisms now found in the synthetic genome. In fact, there are 19 total nucleotide sequence differences between the synthetic genome and its natural analog.

Finally, the researchers also produced synthetic genomes that didn't work along the way, "and we did not know why," Gibson says. By laboriously cross-checking the entire genome gene by gene, they finally found the fatal flaw: a single missing base in the dnaA gene, which is required for life.

Of course, the rest of the original cell remains "naturally" made, from the cytoplasm on down, but daughter cells are assembled entirely from proteins encoded by the synthetic genome. Once the synthetic M. mycoides genome was inserted into M. capricolum, it quickly booted up the natural cell's machinery and busily set to work living, making proteins and, ultimately, dividing and thriving, producing a blue colony of M. capricolum living as synthetically driven M. mycoides. "The cells with only the synthetic genome are self-replicating and capable of logarithmic growth," the researchers wrote, and grow "slightly faster" than their natural peers.

Venter and his colleagues also included four "watermarks" in the code to distinguish the synthetic microbe—dubbed Mycoplasma mycoides JCVI-syn1.0—from natural organisms, including names, sentences, an email address and a web site. "If one is able to translate the watermark sequences, they will be able to send us an email and prove that they decoded the sequences," Gibson says.

The mere fact of human-directed life in the lab raises its own concerns, including the potential for synthetic life to escape the lab and exterminate its natural cousins, or infect them with synthetic DNA through horizontal gene transfer. Various methods to control this have been suggested, including building genetic sequences that cannot exist in nature, engineering in weaknesses to man-made cells, or even suicide genes that kill the organism if it is removed from its lab environment.

Of course, man-made creations are likely to be fragile compared to their robust natural counterparts that have been engineered by billions of years of evolution, Church notes, but also calls for strict oversight to be built into the process of working with or creating such synthetic organisms. "The first safeguard turns out to be to have other people review the work you're going to do so it's not one person coming up with an idea at the bench," Endy adds. "It's a buddy system if you will."

After all, the JCVI scientists "are now ready to build different organisms," Gibson says. "We would like to use available sequencing information and create cells that can produce energy, pharmaceuticals, industrial compounds and sequester carbon dioxide."

Tackling even more complex genomes remains a daunting task so the researchers will now attempt to create the simplest genome possible that can still permit life. "We can whittle away at the synthetic genome and repeat transplantation experiments until no more genes can be disrupted and the genome is as small as possible," Gibson says, estimating that could be less than half of the more than a million base pairs required by this first synthetic genome. "This will help us to understand the function of every gene in a cell and what DNA is required to sustain life in its simplest form." As well as what DNA might be desired for a future synthetic biology.