Phages take breaks while ejecting DNA.

The bacteria that prey on animals and plants have their own parasites: a family of viruses, called phages, that infect the microbes by injecting them with long strings of DNA. A new study suggests that the DNA-injection process is more complicated than researchers had previously presumed, happening in fits and starts instead of in a smooth and steady stream.

Almost all phages have a tail structure, which latches on to bacterial prey, and a head, which stores all the virus' DNA. The length of this DNA string is about 50 times the diameter of a phage's head. As such, the molecule is held under immense pressure, about 10 times that inside a bottle of champagne.

Scientists had proposed that this pressure pushes all a phage's DNA out of its head as soon as its syringelike tail touches a bacterium's membrane.

To test this hypothesis, Lucienne Letellier of Paris-Sud University in Orsay, France, and her colleagues took time-lapse photomicrographs of DNA emerging from phages into a solution containing bacterial proteins. The team set up the study so that the exiting DNA would attract molecular tags that would then fluoresce. This enabled the researchers to measure the lengths of emerged DNA strings at timed intervals.

In the March 8 Current Biology, Letellier's group reports finding that the phages released their DNA in several steps. Further examination of photographs from the experiment showed that phages made their ejections at specific locations along the DNA that have preexisting "nicks," or breaks, in one strand of the double helix. The scientists speculate that each section between nicks encodes instructions for assembling a different piece of a new virus.--C.B.