Researchers are helping to unveil the “blueprint” for photosynthesis

Researchers from Michigan State University and colleagues from the University of California, Berkeley, the University of South Bohemia and the Lawrence Berkeley National Laboratory helped reveal the most detailed image yet important biological “antennas”.

Nature evolved these structures to harness the sun’s energy through photosynthesis, but these sunlight receptors are not found in plants. They are found in microbes known as cyanobacteria, the evolutionary descendants of the first organisms on earth that can take in sunlight, water and carbon dioxide and turn them into sugars and oxygen.

Published in the magazine on August 31st Nature, The results immediately shed new light on microbial photosynthesis, specifically how light energy is captured and channeled to where it’s needed to drive the conversion of carbon dioxide into sugars. In the future, the knowledge could also help researchers eliminate harmful bacteria in the environment, design artificial photosynthetic systems for renewable energy, and incorporate microbes in sustainable production that starts with raw materials carbon dioxide and sunlight.

“There is a lot of interest in using cyanobacteria as solar-powered factories that capture sunlight and turn it into a type of energy that can be used to make important products,” said Cheryl Kerfeld, Hannah Distinguished Professor of Structural Bioengineering at the College of Natural Science. “With a plan like the one we provided in this study, you can start thinking about tuning and optimizing the light-harvesting component of photosynthesis. »

“Once you see how something works, you have a better idea of ​​how to change and manipulate it. That’s a huge benefit,” said Markus Sutter, senior research associate at the Kerfeld Lab, which is based at MSU and the Berkeley Lab in California.

The antennae structures of cyanobacteria, called phycobilisomes, are complex assemblages of pigments and proteins that assemble into relatively massive complexes.

For decades, researchers have been trying to visualize the different building blocks of phycobilisomes to try to understand how they are assembled. Phycobilisomes are fragile and require this step-by-step approach. In the past, researchers have not been able to get the high-resolution images of intact antennas needed to understand how they capture and transmit light energy.

Thanks to an international team of experts and advances in a technique known as cryo-electron microscopy, the structure of a cyanobacterial light-harvesting antenna is now available with near-atomic resolution. The team included researchers from MSU, the Berkeley Lab, the University of California, Berkeley, and the University of South Bohemia in the Czech Republic.

“We were fortunate to be a team of people with complementary skills, people who worked well together,” said Kerfeld, who is also a member of the MSU-DOE Plant Research Laboratory, which is supported by the US Department of Energy. “The group had the right chemistry. »

“A long journey full of beautiful surprises”

“This work is a breakthrough in the field of photosynthesis,” said Paul Sauer, a postdoctoral researcher in Professor Eva Nogales’ Laboratory for Cryogenic Electron Microscopy at the Berkeley Lab and UC Berkeley.

“Until now, the complete light-collecting antenna structure of a cyanobacterium was missing,” says Sauer. “Our discovery helps us understand how evolution found ways to convert carbon dioxide and light into oxygen and sugars in bacteria long before plants existed on our planet. »

In addition to Kerfeld, Sauer is the corresponding author of the new article. The team documented several notable results, including the discovery of a new phycobilisome protein and the observation of two new ways in which the phycobilisome directs its light-capturing rods that had not previously been elucidated.

“It’s 12 pages of discoveries,” said María Agustina Domínguez-Martín of the Nature report. As a postdoctoral fellow at the Kerfeld Lab, Domínguez-Martín initiated the study at MSU and conducted it at the Berkeley Lab. She is currently a Marie Skówdoska-Curie Postdoctoral Fellow at the University of Cordoba in Spain. “It has been a long journey full of beautiful surprises. »

For example, one surprise was how a relatively small protein can act as a surge suppressor for the massive antenna. Prior to this work, researchers knew that if the phycobilisome had absorbed too much sunlight, the phycobilisome might contain molecules called orange carotenoid proteins, or OCPs. OCPs release excess energy as heat and protect a cyanobacterium’s photosynthetic system from combustion.

Up until now it has been a matter of debate how many OCPs the phycobilisome can bind and where these binding sites are located. The new research answers these fundamental questions and offers potentially practical insights.

Of course, this type of surge protection system — which is called photoprotection and has analogies in the plant world — tends to be wasteful. Cyanobacteria are slow to turn off their photoprotection after they’ve done their job. Now, with a complete picture of how surge protectors work, researchers can find ways to design “smart” and cheaper photoprotectors, Kerfeld said.

And while they help make the planet habitable for humans and countless other organisms that need oxygen to survive, cyanobacteria have a dark side. Cyanobacteria blooms in lakes, ponds, and reservoirs can produce toxins that are deadly to native ecosystems as well as humans and their pets. Having a map of how bacteria not only get energy from the sun but also protect themselves from too much of it could lead to new ideas for fighting harmful buds.

In addition to the new answers and possible applications this work offers, the researchers are also excited about the new questions it raises and the research it could inspire.

“If you look at it like Lego, you can keep stacking, right? Proteins and pigments are like blocks that make up the phycobilisome, but that’s then part of the photosystem, which is located in the cell membrane, which is part of the whole cell. ‘ said Suter. “We’re kind of climbing the ladder. We found something new on our squadron, but we can’t say we’ve fixed the system. »

“We answered some questions, but we opened the doors for others and that makes it a breakthrough for me,” said Domínguez-Martín. “I’m excited to see how the field develops from here. »

This work was supported by the US Department of Energy’s Office of Science, the National Institutes of Health, the Czech Science Foundation, and the European Union’s Horizon 2020 research and innovation program.

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