Although it hasn't produced the hype of the arsenic bacteria story, researchers have made what In the Pipeline's Derek Lowe calls "a real advance in the field of engineered 'unnatural life.'" The researchers, who recently published their work in Angewandte Chemie International, forced bacteria to use thymine-5-chlorouracil — which has a chlorine atom where thymine's methyl group is — as a substitute for the thymine in their DNA. "From a med-chem perspective, that's a good switch. The two groups are about the same size, but they're different enough that the resulting compounds can have varying properties. And thymine is a good candidate for a swap, since it's not used in RNA, thus limiting the number of systems that have to change to accommodate the new base," Lowe says. This was done using evolutionary selection pressure, with the researchers forcing the bacteria to select for mutations that would allow it to use the chlorouracil. The resulting bacteria could still grow on thymine, although with a lag in the culture, but could also make the transition back to chlorouracil immediately, "which shows that their ability to do so was now coded in their genomes," Lowe says. The next step is to create a bacteria that ignores thymine altogether and has to live on chlorouracil. "If that can be realized, the resulting organisms will be the first representatives of a new biology - no cellular life form has ever been discovered that completely switches out one of the DNA bases," Lowe adds. "These sorts of experiments open the door to organisms with expanded genetic codes, new and unnatural proteins and enzymes, and who knows what else besides."