ENZYMES that repair DNA may check for mutations by sending electrons along sections of the strand, in much the same way that electricians test for faults in circuits. The mechanism might explain how enzymes locate problems in the genome fast enough to correct them.
Most genetic mutations are caused by free radicals scavenging electrons from DNA. This 鈥渙xidative damage鈥� introduces errors such as base-pair mismatches when the strand replicates. If these errors build up they can be extremely harmful.
All organisms have enzymes that can repair the errors. They bind to DNA and are thought to move slowly along the strand, testing each base pair to see if there is a mismatch. But Jacqueline Barton of the California Institute of Technology in Pasadena is not convinced this is what happens. 鈥淭hat would take a lot of time,鈥� she says 鈥� too long to allow the genome to be comprehensively scanned.
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Instead, Barton and her colleagues think DNA repair involves two phases: a rapid scan that locates sections of DNA with errors, and a slower pass to correct them. The researchers got the idea when they found that oxidative damage prevents a current from flowing through a strand of DNA. They started using this change in DNA鈥檚 electrical properties to test strands for mismatches, and wondered whether repair enzymes might do the same.
To test whether the repair enzyme MutY can inject electrons into DNA, the researchers set up a circuit using electrodes coated with DNA. Molecules of MutY were bound to the DNA. Sure enough, current flowed from the enzyme to the electrodes. What鈥檚 more, it stopped when the DNA strands were removed or damaged, showing that the current was passing through the DNA.
By testing the circuit with MutY molecules in which different sections had been altered, the team worked out that the electrons were being transferred by an iron-sulphur cluster within the enzyme. The cluster is found in many DNA repair enzymes, Barton says. 鈥淚t鈥檚 ubiquitous from bacteria to man.鈥�
She suspects that MutY enzymes scan a piece of DNA for errors by injecting an electron at one end of the segment. If the DNA is free of errors, the electron travels along the strand until it meets another MutY enzyme, which absorbs it. This lowers the second enzyme鈥檚 affinity for DNA, so it detaches from the strand (see Graphic). 鈥淭hat process, I would argue, constitutes a scan of that region of the genome,鈥� Barton says.
Any mismatch in the section of DNA between the two enzymes blocks the electron鈥檚 path, so the second MutY enzyme remains attached to the strand. It then works its way along the strand until it reaches the damaged area of the DNA, and fixes it. Barton believes this would enable a small number of repair enzymes to scan the entire genome, even tightly coiled regions of the double helix that are otherwise hard to reach.
Scott Rajski, who works on oxidative damage to DNA at the University of Wisconsin, Madison, says working out how repair enzymes find the sites of damage is the 鈥渞eally big question鈥� in the field. 鈥淭he major contribution [of Barton鈥檚 work] is that there is now a testable, viable hypothesis,鈥� he says. 鈥淚t鈥檚 a huge advance.鈥�