Genetic engineering sheds light on ancient evolutionary questions

Cyanobacteria are single-celled organisms that get their energy from light and use photosynthesis to release atmospheric carbon dioxide (CO₂) and liquid water (H₂O) converts it into respirable oxygen and carbon-based molecules such as the proteins that make up cells. Cyanobacteria were the first organisms to perform photosynthesis in the history of the Earth, flooding the early Earth with oxygen and having a major impact on the evolution of life.

Geological measurements suggest that early Earth’s atmosphere was likely rich in CO more than 3 billion years ago.₂much higher than the current level man-made climate changewhich means ancient Cyanobacteria There was a lot to “eat”.

However, over the billions of years of Earth’s history, atmospheric CO₂ To survive, these bacteria had to evolve new strategies to extract CO as concentrations dropped.₂Modern cyanobacteria are therefore quite different from their ancient ancestors, possessing a complex and fragile set of structures called COs.₂-Concentration Mechanism (CCM) to compensate for low concentrations of CO₂.

Now, a new study from Caltech sheds light on how CCMs evolved, addressing a long-standing mystery in the field of evolutionary geobiology. A new study employs a genetic approach to model the ancient ancestors of modern organisms, allowing researchers to systematically experiment with different versions of bacteria to uncover possible evolutionary pathways. can.

The study is a collaboration between Woodward Fisher, professor of geobiology at Caltech, and the lab of David Savage, associate professor of molecular biology at Caltech Berkeley and the Howard Hughes Medical Institute. was.Appearing in the diary Proceedings of the National Academy of Sciences.

“This is a new way of studying Earth’s history,” says Fisher. “Being able to recreate modern organisms in the lab allows us to test their evolutionary trajectory in rigorous laboratory experiments.”

Cyanobacteria “eat” CO₂ With the help of an enzyme called Rubisco. Simply put, Rubisco isn’t very good at that job. It acts slowly and tends to react with other molecules instead of CO.₂This is not a problem for cyanobacteria in environments with high CO concentrations₂; rubisco is inefficient and the bacteria may still have enough CO.₂ metabolize. However, atmospheric CO₂ Levels have declined significantly over billions of years, and modern cyanobacteria have evolved CCMs that concentrate CO.₂ Increases the efficiency of Rubisco in the bacteria itself.

CCM is mysterious evolutionary biologist Altering any of the 20 genes that encode different parts of the CCM renders the entire structure non-functional.

“We think of evolution as a gradual process, with each new gene adding a new function,” said Avi Flamholz, a postdoctoral researcher at Caltech and lead author of the new paper. I’m here. “For example, the ancient precursors of the modern human eye didn’t have all the functions of the eye, but they were probably able to detect light in some form. In CCM, how did they evolve?” There was no clear pathway to indicate their modern complexity.”

In a new study, the team set out to model possible ancient iterations of CCM structures. To do so, they genetically engineered E. coli to require CO.₂ for their metabolism. It is more tractable to work with this model system than the cyanobacteria themselves, as the genetic tools for working with E. coli in the laboratory have been established. The team then engineered an E. coli strain with the 20 genes that make up the CCM, systematically adding, deleting, and adjusting genes to model all possible evolutionary trajectories of the CCM structure.

In this way, Flamholz and his team discovered that there are indeed several biologically viable trajectories leading to the emergence of complex modern CCMs.

“These results global change and the evolution of the Earth’s biosphere,” Fisher says.₂ Cyanobacteria have become increasingly scarce and have been able to innovate amazing biochemical solutions. ”