First organism that transmits added DNA `letters` created

Washington: In a breakthrough, scientists have for the first time engineered a bacterium whose genetic material includes three pairs of DNA `letters` or bases instead of the two found in nature.

The bacterium`s cells can replicate the unnatural DNA bases more or less normally, as long as the molecular building blocks are supplied, researchers said. "Life on Earth in all its diversity is encoded by only two pairs of DNA bases, A-T and C-G, and what we`ve made is an organism that stably contains those two plus a third, unnatural pair of bases," said Floyd E Romesberg from The Scripps Research Institute (TSRI), who led the research. "This shows that other solutions to storing information are possible and takes us closer to an expanded-DNA biology that will have many exciting applications - from new medicines to new kinds of nanotechnology," said Romesberg. Romesberg and his laboratory have been working since the late 1990s to find pairs of molecules that could serve as new, functional DNA bases - and, in principle, could code for proteins and organisms that have never existed before. "These unnatural base pairs have worked beautifully in vitro, but the big challenge has been to get them working in the much more complex environment of a living cell," said Denis A Malyshev, a member of the Romesberg laboratory who was lead author of the new report. In the new study, the team synthesised a stretch of circular DNA known as a plasmid and inserted it into cells of the common bacterium E coli. The plasmid DNA contained natural T-A (adenine-thymine) and C-G (cytosine-guanine) base pairs along with the best-performing unnatural base pair Romesberg`s laboratory had discovered, two molecules known as d5SICS and dNaM. The goal was to get the E coli cells to replicate this semi-synthetic DNA as normally as possible. The greatest hurdle may be reassuring to those who fear the uncontrolled release of a new life form: the molecular building blocks for d5SICS and dNaM are not naturally in cells, researchers said. Thus, to get the E coli to replicate the DNA containing these unnatural bases, the researchers had to supply the molecular building blocks artificially, by adding them to the fluid solution outside the cell. Then, to get the building blocks, known as nucleoside triphosphates, into the cells, they had to find special triphosphate transporter molecules that would do the job. The researchers eventually were able to find a triphosphate transporter, made by a species of microalgae, that was good enough at importing the unnatural triphosphates. "That was a big breakthrough for us - an enabling breakthrough," said Malyshev. The team found, somewhat to their surprise, that the semi-synthetic plasmid replicated with reasonable speed and accuracy, did not greatly hamper the growth of the E coli cells, and showed no sign of losing its unnatural base pairs to DNA repair mechanisms. The study was reported in the journal Nature.