Tag: sunlight

Filming a vitamin B12 photoreceptor in action

Filming a vitamin B12 photoreceptor in action

Using X-ray free-electron lasers and synchrotron light at facilities in Switzerland, Japan, France and the UK, a worldwide collaboration of scientists have discovered how a vitamin B12-based photoreceptor works. Understanding how photoreceptors function aids future technological applications, such as optogenetics, that involve controlling cellular processes with light. The findings are published in Nature.

Vitamin B12 is an organometallic cofactor found in many enzymes that control essential processes in various organisms, including humans. It came as a surprise a decade ago that vitamin B12 derivatives had been repurposed for light sensing by a large family of previously unknown photoreceptors in bacteria that fulfil various functions. 

The prototypical B12 photoreceptor CarH, for example, regulates the expression of genes involved in protecting bacteria against excess sunlight. It achieves this by binding to DNA in the dark, acting as a molecular doorstop. Upon illumination, its tetrameric architecture breaks apart, enabling transcription by unbinding from DNA. 

The way in which this and other B12 photoreceptors function at a molecular level has remained a mystery ever since. However, an international consortium led by scientists at the Institut de Biologie Structurale in Grenoble, France has now combined experimental techniques using X-ray free-electron lasers at the Paul Scherrer Institute PSI in Switzerland (SwissFEL) and Japan (SACLA), as well as the synchrotrons in France (ESRF) and the UK (Diamond Light Source), with quantum-chemical calculations to uncover the inner workings of CarH.

Read more on the PSI website

Image: John Beale is responsible for macromolecular crystallography at the Cristallina experimental station of SwissFEL

Credit: © Paul Scherrer Institute PSI / Markus Fische

Repairing genetic damage with sunlight

Repairing genetic damage with sunlight

DNA damage to the genetic material DNA drives cancer, ageing, and cell death. Therefore, DNA repair is crucial for all organisms, and a deeper understanding of this basic function helps us better comprehend how life around us survives and thrives. An international team of researchers has now revealed how the enzyme photolyase efficiently channels the energy of sunlight into DNA repair chemistry.

All life under the sun must cope with harmful UV rays. UV damage can take many forms, but DNA, the molecule that carries the genetic information of all living organisms, is especially vulnerable. For instance, UV can drive chemical cross-linking reactions of DNA, potentially introducing errors into the genetic code. This cross-linking can lead to cell death or – in the worst cases – mutagenesis and cancer. Such damage is not uncommon; under bright sunlight, a human skin cell can undergo 50-100 cross linking reactions per second.

“To survive, life has evolved powerful DNA repair mechanisms. One especially elegant solution is provided by the enzyme photolyase,” explains DESY scientist Thomas J. Lane, who is also a researcher in the Cluster of Excellence “CUI: Advanced Imaging of Matter” at Universität Hamburg. The enzyme uses sunlight to repair damage caused by sunlight. Photolyase is able to recognize the location where UV irradiation has cross-linked DNA and grabs onto those bits of damaged DNA. Then, it can capture a blue photon from the sun, and use it to perform repair chemistry, turning the DNA back into its original, healthy form.

To better understand how photolyase works, the scientists were particularly interested first in the form of the enzyme immediately after absorbing a photon, but before repairing the DNA. Second, they wanted to find out the exact sequence of bond-breaking chemical reactions necessary to turn damaged DNA into healthy DNA. As a third step, the team sought to better understand how photolyase can specifically recognize which DNA is damaged.

Conducting time-resolved crystallography at the SwissFEL X-ray free-electron laser of PSI the scientists were able to capture the excited state of the photolyase chromophore, letting them understand how the enzyme efficiently channels the energy of sunlight into DNA repair chemistry. “This research was only made possible by the recent development of X-ray free-electron laser sources. Their intense femtosecond-duration pulses let us record flash X-ray photographs that freeze all atomic motion so that we can follow the reaction step by step at the speed of molecules,” says first author Nina-Eleni Christou from DESY.

Read more on PSI website

Image: PSI researcher Camila Bacellar is pleased about the success in precisely analysing the DNA repair enzyme photolyase at the Alvra beamline of the Swiss X-ray free-electron laser SwissFEL.

Credit: Paul Scherrer Institute/Markus Fischer