Dining on DNA

DNA may be the blueprint for life, but for many bacteria it's also decent food in a pinch. Now, researchers have identified genes that allow Escherichia coli and many other types of bacteria to eat DNA when other nutrients aren't readily available.

DNA from dead cells and organisms floats around in the environment, and some bacteria can usher it inside their cell walls. Scientists believe microbes use the DNA to diversify their genetic material, or perhaps to repair it. But there's a third possibility: In 2001, bacteriologist Steven Finkel of the University of Southern California (USC) in Los Angeles and a colleague showed that E. coli could survive even when DNA was its only source of nutrition. That was puzzling because most researchers thought E. coli was incapable of taking up extracellular DNA to incorporate into its own genome. But now Finkel and Vyacheslav Palchevskiy, also of USC, have found genes that apparently allow E. coli to consume the stuff.

The researchers screened E. colifor genes that resembled those involved in taking up DNA in other bacteria. They found eight and made mutant strains with broken versions of each of the genes. When allowed to grow in individual cultures with abundant food, all the mutants grew normally. When provided only DNA, however, the mutants grew 40 times slower than an unmutated E. coli. And when all eight were grown in the same culture and had to compete for regular food with a normal E. coli, the mutants' growth lagged, with some strains disappearing all together, the researchers report in the June issue of the Journal of Biochemistry. That suggests that the ability to eat DNA gave the normal E. coli an advantage only when other food sources were used up. The team found similar genes in 26 proteobacteria genera, indicating that DNA-eating may be a widespread talent.

As the munchies of last resort, DNA could be an important food in harsh environments, Finkel says. DNA-eating could even explain why infectious bacteria seem to thrive in the mucus in the lungs of cystic fibrosis patients, which has higher-than-normal concentrations of DNA, he says.

Microbiologist Rosemary Redfield of the University of British Columbia in Vancouver, Canada, hopes that this study convinces people that DNA-eating is really going on in nature. "It's been treated as this weird, flakey idea," she says. But for microbes that live in DNA-rich environments, says Redfield, this study "suggests that ... the ability to take up DNA may be really important."

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