Radiodurans' rings and radioresistance

The bacterium Deinococcus radiodurans has the remarkable ability to resist doses of ionising radiation many times higher than those that kill other organisms. In the January 10 Science Smadar Levin-Zaidman and colleagues at the Weizmann Institute of Science, Israel, report that the D. radiodurans genome has an unusual ring-like structure that may account for its radioresistance by restricting the diffusion of radiation-generated free DNA ends (Science 2003, 299:254-256). Scanning electron microscopy revealed that D. radioduranscells have a tetrad morphology with each quarter containing equal amounts of DNA (each contains a single copy of the bacterial genome). This compartmentalization suggests that DNA repair after radiation does not involve homologous recombination. The bacterial nucleoids adopt a toroidal morphology that presumably dictates a rigid structure, facilitating template-independent, error-free, end-joining of DNA breaks.