Spontaneous breaks of replicating DNA in bacteria occur at a rate much lower than suspected, according to a study to be published online in Nature Genetics. This is the first direct measurement of DNA breakage in normal living cells, and suggests that the potential for such damage to cause larger chromosomal rearrangements is much greater than previously realized.
The cellular machinery that replicates DNA sometimes stalls. While these stalled 'replication forks' can be resolved by cleaving the DNA, which allows replication to start again, the frequency of these spontaneous breaks has been unknown. Jeanine Pennington and Susan Rosenberg devised a way to visualize individual bacteria (E. coli) with such a break by inserting a gene encoding a green fluorescent protein that would be triggered by the cell's response to DNA damage.
The authors observed that approximately one per cent of the cells in their experiment had at least one spontaneous break per generation. Previous reports based on indirect measurements had suggested a rate 20- to 100-fold higher. This low rate of spontaneous breakage suggests that each break must be a much more potent cause of the dangerous genetic instability that can result from the attempt to repair them. The authors suggest that cells, including those of higher organisms, have likely developed robust ways to avoid the creation of such DNA breaks.
Susan Rosenberg (Baylor College of Medicine, Houston, TX, USA)
Abstract available online.
(C) Nature Genetics press release.
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