Ancient Bacteria Keep on Repairing DNA

Scientists have thought that ancient bacteria survive for millions of years by shutting down their metabolism. If the bacteria truly shut themselves down, however, their DNA would fall victim to decay. A research team led by Eske Willerslev of the University of Copenhagen, in Denmark, now shows instead that bacteria probably survive indefinitely by staying active enough to continuously repair damaged DNA (Proc. Natl. Acad. Sci. USA 2007, 104, 14401). The researchers extracted bacterial DNA from samples of ancient permafrost from Antarctica, Siberia, and Canada's Yukon Territory and amplified it using the polymerase chain reaction (PCR). To make sure they analyzed DNA from live cells only, the scientists targeted a 4,000-base-pair bacterial ribosomal DNA fragment because PCR products from dead organisms rarely are longer than 500 base pairs. They were able to amplify bacterial DNA—but no plant or animal DNA—from six samples that were up to 600,000 years old. The team couldn't amplify DNA from two older samples. As a test for active metabolism, the researchers measured CO₂ production from respiration. The samples younger than 600,000 years produced CO₂, but the older samples did not.