A DNA-Based Magnetic Sensor

Northern wheatearssmall migratory songbirds that weigh about the same as an AA batteryfly 30,000 km every year from Alaska via Asia to Kenya and back again. Their ability to do so depends crucially on having an internal magnetic compass with which to sense the Earth’s magnetic field. It appears that all avian migrants have such a light-dependent compass. Although it is clear that the primary magnetoreceptors are located in the birds’ retinas, the chemical identity of the sensors and the biophysical sensory mechanism remain something of a mystery. Now, Jacqueline Barton and colleagues at the California Institute of Technology, collaborating with Dongping Zhong at Ohio State University, have suggested, intriguingly, that a DNA repair enzyme could be responsible. The enzyme in question is photolyase, a photoactive protein containing flavin adenine dinucleotide (FAD) as its principal chromophore. Photolyases repair DNA lesions, for example by splitting the cyclobutane pyrimidine dimers (CPDs) that can be formed by adjacent thymine bases after absorption of ultraviolet light (Figure 1). The repair mechanism involves blue-light photoexcitation of FADH−, the fully reduced state of the cofactor, followed by transfer of an electron from the flavin to the damaged DNA to form a pair of radicals, with one unpaired electron on the CPD and the other on the flavin (FADH•). The cyclobutane bonds that link the two thymines then break sequentially, and the electron is passed back to the flavin. The whole process is complete within a nanosecond. The new study by Zwang et al. uses a clever electrochemical method to monitor the repair of CPD lesions in monolayers of 29-base-pair DNA duplexes bound to Escherichia coli photolyase. Remarkably, the rate at which the repair proceeds depends on the intensity and direction of the external magnetic field provided by a nearby Nd−Fe−B magnet. Even magnetic fields weaker than the Earth’s (∼40 μT in Barton’s lab) appear to have a measurable effect on the efficiency of repair.