Magnetic-field effect on the photoactivation reaction of Escherichia coli DNA photolyase

Abstract

One of the two principal hypotheses put forward to explain the primary magnetoreception event underlying the magnetic compass sense of migratory birds is based on a magnetically sensitive chemical reaction. It has been proposed that a spin-correlated radical pair is produced photochemically in a cryptochrome and that the rates and yields of the subsequent chemical reactions depend on the orientation of the protein in the Earth's magnetic field. The suitability of cryptochrome for this purpose has been argued, in part, by analogy with DNA photolyase, although no effects of applied magnetic fields have yet been reported for any member of the cryptochrome/photolyase family. Here, we demonstrate a magnetic-field effect on the photochemical yield of a flavin-tryptophan radical pair in Escherichia coli photolyase. This result provides a proof of principle that photolyases, and most likely by extension also cryptochromes, have the fundamental properties needed to form the basis of a magnetic compass.avian compass ͉ cryptochrome ͉ radical pair states ͉ transient absorption spectroscopy ͉ flavoprotein

Magnetically sensitive light-induced reactions in cryptochrome are consistent with its proposed role as a magnetoreceptor

“…The difference between the 2-5 μs and 6-9 μs data below 400 nm in Fig. 2B is consistent with the FAD •− → FADH • protonation reaction (23).…”

“…At each wavelength the mean AE standard deviation is plotted. The minor differences between the data shown in (C) and (D) and the data reported by Henbest et al (23) are attributed to the different experimental conditions of the latter: 278 K, 20% (v∕v) glycerol, 5 mM potassium ferricyanide.…”

“…3), substantially larger than the 3-4% effects previously reported for EcPL in a solution with a lower glycerol content (23). A possible explanation for this difference can be found in the earlier observation (30) that the deprotonation rate (k f ) of the terminal TrpH •þ radical in EcPL decreases with increasing concentration of glycerol, an effect attributed to release of the proton to the solvent.…”

“…Our results show that formation of a secondary species (RP2) from the magnetically sensitive radical pair RP1, [FAD •− TrpH •þ ], avoids the need for an abnormally rapid protein rearrangement or exceptionally slow spin-decoherence. Protonation of the FAD •− radical (as may occur in AtCry) or deprotonation of the TrpH •þ radical [as occurs in EcPL (23,30)] allows the magnetic-field effect on RP1 to be "stored" in the form of a changed quantum yield of the much longer lived state RP2, from which the signaling state can subsequently be generated. The fact that there is no need for the reactions of RP2 to be magnetically sensitive means that its lifetime can greatly exceed its spin-decoherence time without ill effect.…”

“…The expression and purification of EcPL (as a mutant that does not bind the methenyltetrahydrofolate cofactor) are described elsewhere (23,47). To ensure the FAD cofactor was in its fully oxidized state, the protein was pretreated with potassium ferricyanide as described previously (23).…”