Scientists Get First Glimpse At How Plants, Most Animals Repair UV-damaged DNA

The Ohio State University study revealed how the enzyme photolyase uses energy from visible light to repair UV damage.

Thisenzyme is missing in all mammals, including humans, although all plantsand all other animals have it. Greater understanding of how photolyaseworks could one day lead to drugs that help repair UV damage in humanDNA.

In the online edition of the Proceedings of the NationalAcademy of Sciences, Dongping Zhong and his colleagues reportexperimental evidence of what scientists have long suspected -- thatvisible light excites the photolyase molecule and boosts the energy ofelectrons in its atoms. This in turn enables the enzyme to inject anelectron into the DNA molecule at the UV damage site temporarily toperform repairs.

They also report something that was unexpected:Water plays a key role in the process, by regulating how long thedonated electron stays inside the damage site before returning to thephotolyase molecule.

Scientists believe that all placentalmammals lost the ability to make this enzyme some 170 million yearsago, said Zhong, an assistant professor of physics and adjunctassistant professor of chemistry and biochemistry at Ohio State.

That'swhy humans, mice, and all other mammals are particularly vulnerable tocancer-causing UV rays from the sun. But the rest of the animal kingdom– insects, fish, birds, amphibians, marsupials, and even bacteria,viruses and yeast – retained a greater ability to repair such damage.

Sincethe 1940s, scientists have been trying to understand how the DNA inplants and some animals can be damaged by UV light, and then –seemingly – repaired by visible light. In the 1960s, they identifiedthe enzyme that was responsible for the repair, and named itphotolyase, but they didn't know exactly how the enzyme worked.

Bythe 1980s, scientists proposed a mechanism for photolyase – that itdonated an electron to damaged DNA – but nobody could prove it. Thereaction happened too fast to be seen with normal laboratory tools.

Scientistsalso knew that the enzyme formed a tiny water-filled pocket to host thedamage site within a cell nucleus, said Zhong. But until his latestseries of experiments, nobody knew how water affected the reaction.

TheOhio State researchers mixed photolyase with UV-damaged DNA, and hitthe mixture with a kind of blue strobe light to simulate the energythat it would receive from visible light.

Because the lightpulses lasted less than a trillionth of a second, the researchers wereable to make a very fast series of measurements to follow how thechemical reaction evolved over time.

They assembled their measurements together like a series of stop-motion photographs to reveal the individual steps of UV repair.

Zhongexplained the damage and repair processes this way: When a UV photonstrikes a portion of DNA, the atoms in the DNA molecule become excited.Sometimes an accidental bond forms between them. The bond is called aphoto-lesion, and can lead to a kind of molecular injury called adimer. Dimers prevent DNA from replicating properly, and cause geneticmutations that lead to diseases such as cancer.

In cell nucleithat contain photolyase, the enzyme forms a water-filled pocket withthe right shape and size to accommodate the dimer for the repair.Normally, the enzyme wouldn't be able to reach the dimer, which ishidden inside the coiled DNA molecule, Zhong said. But electricalinteractions between the DNA molecule and the enzyme cause the portionof the DNA that contains the dimer to flip outwards from the coil andinto the pocket.

Then, when a photon of visible light hits thepocket, the enzyme becomes excited, and expels one of its own electronsinto the dimer, which forces a rearrangement of the atoms in the DNA.

“Fromour work, we see that in less than a billionth of a second, the damagedDNA bases can recover their original form and the dimer will be gone,as if the UV damage never occurred,” Zhong said. But even within thatshort time, there's a danger of the donated electron jumping back tothe photolyase enzyme before the repair is done.

Zhong's work has revealed how the water in the pocket performs a critical function at this point.

Whenthe photolyase enzyme becomes excited, it jostles the water molecules,and that motion within the pocket delays the electron's exit from thedimer until just after the repair is done.

As far as scientistscan tell, photolyase's only function is to repair DNA, and it's verygood at it. The enzyme harnesses energy from the visible portion ofsunlight to repair UV damage in plants and animals with 90 percentefficiency.

“Unfortunately, during evolution, mammals lost thisenzyme,” said Zhong. “So we humans have more of a chance of gettingskin cancer than an insect or a frog.”

Scientists would like todevelop drugs that use photolyase's mechanism to repair UV damage inhuman skin, but they've had trouble with the first steps – replicatingthe photolyase reaction in the laboratory, and fully understanding it.Zhong hopes his latest study will change that.

“Maybe now that weknow how light, enzyme, and water work together to control the timing,we can modulate this function and mimic what nature does,” he said. “Wewant to understand why this timing is so perfect.”

Zhongconducted this study with graduate students Ya-Ting Kao and ChaitanyaSaxena and research associate Lijuan Wang, all of Ohio State, and AzizSancar of the University of North Carolina at Chapel Hill School ofMedicine.