Synthetic Biology New Letters for Lifes Alphabet

The five bases found in nucleic acids define the ‘alphabet’ used to encode life on Earth. The construction of an organism that stably propagates an unnatural DNA base pair redefines this fundamental feature of life.

Malyshev et al. now describe the development of a bacterium capable of faithfully replicating a plasmid — a small, circular DNA molecule — containing the hydrophobic d5SICS:dNaM base pair (Fig. 1), thus creating the first organism to harbour an engineered and expanded genetic alphabet. This feat was far from simple: the authors first had to find a way of getting the bacterium to take up unnatural nucleotides, and then to work within the constraints of the billion-year-old habits of polymerases, the enzymes that synthesize polymeric nucleic acids.

Romesberg thought that multiple additional challenges remained, including getting the cell’s DNA polymerases to recognize and replicate the base pairs and preventing the cell’s DNA repair mechanisms from recognizing the novel nucleotides as aberrant. But, much to the team’s surprise, the unnatural plasmids immediately began to replicate.

“Once we had the triphosphates in the cell, then we really thought that our work would begin, but quite surprisingly it worked the first time,” Romesberg said.

The E. coli continued to replicate plasmids containing synthetic DNA as long as they were fed triphosphates. The bacteria only slowly lost their synthetic base pairs after their synthetic triphosphate food was removed.