Resurrected gene allows time travel to an Earth before oxygen

The world’s commonest protein is a whiz at chemistry

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By Bob Holmes in Mesa, Arizona

A resurrected gene, brought back from the dead in the lab, is allowing molecular biologists to travel billions of years into the past to study one of the most significant transitions in Earth’s history.

About 2.5 billion years ago, oxygen began to build up in Earth’s previously anoxic atmosphere as a result of photosynthesis by cyanobacteria and other microbes. This Great Oxygenation Event must have caused an ecological upheaval, because oxygen is such a reactive molecule.

To understand more about this key point in evolution, evolutionary biologist Betül Kacar at Harvard University decided to reconstruct the ancient form of rubisco, the key enzyme in photosynthesis that converts carbon dioxide into the precursors of sugars. Rubisco has been called the most abundant protein on Earth, and its history dates back to the dawn of photosynthesis more than 3 billion years ago.

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Kacar and her team compared rubisco gene sequences from modern organisms to infer what the sequence must have been in their common ancestor. By doing that repeatedly, she says, “we can walk back down the branches of the evolutionary tree”.

Rubisco changed much more quickly around the time of the Great Oxygenation than it did either before or after it, Kacar said last week at the Astrobiology Science Conference in Mesa, Arizona.

Read more: Photosynthesis: Shaping the planet

This rapid change must have been driven by the need to adapt to the presence of oxygen, she suggests. The modern rubisco molecule has to be selective because it encounters both oxygen and carbon dioxide, but even so it sometimes goes after the wrong gas. Rubisco from before the Great Oxygenation might have been more lax because it encountered oxygen so infrequently.

Kacar’s team has now synthesised the gene sequences to make the ancient rubisco and is using CRISPR gene-editing technology to insert them into cyanobacteria. The modified bacteria, they hope, will then produce a form of rubisco molecule not seen on Earth for billions of years. “Earth’s past is alive, in a way,” says Kacar.

The team can then compare the functions of the ancient proteins and their modern relatives, to see whether the enzyme did indeed become more selective during the Great Oxygenation Event. The technique adds a new dimension to studies of the past that cannot be gleaned from the geological record, says Kacar.

“What makes Betül’s work really exciting is that she’s actually using these sequences to reconstruct the protein in the laboratory,” says Roger Summons, a geobiologist at the Massachusetts Institute of Technology.

The biggest insights, Summons adds, may come from features of the ancient protein that come as a surprise. “It’s about what you might learn that you can’t even anticipate at this point,” he says.