“The first time this happened, it gave rise to all forms of complex life.”

Primary endosymbiosis is the process by which a prokaryotic cell is phagocytosed by a eukaryotic cell and develops Beyond symbiosis to become an organelle. To date, only three examples of this process are known. But the magazine Sciences echo the room.

An international team of scientists has discovered the first nitrogen-fixing organelle inside a eukaryotic cell.

“It's very rare for organelles to arise from this kind of thing.”explains the first author of one of the last two articles on this topic and a postdoctoral researcher at the University of California, Santa Cruz, Tyler Coyle.

“The first time we thought this happened, it led to the emergence of all complex life forms. Everything more complex than a bacterial cell owes its existence to this event,” he noted, referring to the origin of mitochondria.

“About a billion years ago, this happened again with chloroplasts,” he continues. This gave us plants“.

The next known case of endosymbiosis involves a chloroplast-like microbe. The fourth and final is the first example of a nitrogen-fixing organelle, which researchers call a nitroplast.

His discovery was a combination of decades of work and some luck. In 1998, Jonathan ZehrA distinguished professor of marine sciences at the University of California, Santa Cruz, has found a short DNA sequence of what appears to be an unknown nitrogen-fixing cyanobacteria in seawater in the Pacific Ocean. Zahr and his colleagues spent years studying the mysterious object, which they named UCYN-A.

Optical micrograph shows the marine haptophyte alga Braarudosphaera bigelowii with a black arrow indicating the nitroplast organelle. Tyler Cole Society for Research and TechnologySebastian Carrasco via Europa Press

Meanwhile, Kyoko Hagino, a paleontologist at Kochi University in Japan, was actively trying to grow seaweed. It turned out to be the host organism of UCYN-A. It took more than 300 sampling trips and more than a decade, but Hagino finally succeeded in growing the algae on the farm. This allowed other researchers to begin studying UCYN-A and their host seaweed together in the laboratory.

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For many years, scientists considered UCYN-A to be an endosymbiont closely related to algae. But the two latest papers suggest that UCYN-A evolved with its host in a previous symbiosis, and now meets the criteria for being an organelle.

In an article published in cell In March 2024, Zahr and colleagues V Massachusetts Institute of Technology, Barcelona Institute of Marine Sciences, and University of Rhode Island showed that the size relationship between UCYN-A and its host algae is similar in different species of the marine algae Braarudosphaera bigelowii.

The researchers used a model to demonstrate that host cell growth and UCYN-A are controlled by the exchange of nutrients. Their metabolic processes are related. This synchronization of growth rates led to Researchers call UCYN-A “organelle-like.”.

“This is exactly what happens with organelles,” Zahr said. “If we look at mitochondria and chloroplasts, It's the same thing: they grow with the cell“.

But scientists did not confidently call UCYN-A an organelle until they confirmed other evidence. The cover story of Science, published on Friday, April 12, features Qualley, Kendra Turk Kubo, Wing-Kwan Esther Mak of the University of California, Santa Cruz, and collaborators from the University of California, San Francisco, and Lawrence National Laboratory in Berkeley, N.Y., Taiwan. Ocean University and Kochi University in Japan show that UCYN-A imports proteins from host cells.

“This is one of the properties of something that goes from an endosymbiont to an organelle,” Zahr said. “They start getting rid of bits of DNA, their genome gets smaller and smaller, and they start relying on the parent cell to carry those gene products (or the protein itself) into the cell.”

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Cole worked on the study proteins. He compared the proteins found within the isolated UCYN-A to those proteins he Found throughout the algal host cell. He discovered that the host cell makes proteins and labels them with a specific amino acid sequence, which tells the cell to send them to the nitroplast.

The nitroblast then imports and uses the proteins. Cole identified the function of some proteins filling in gaps in specific pathways within UCYN-A. “It's like a magic puzzle that fits together and really works,” Zahr said.

In the same article, UC San Francisco researchers showed this UCYN-A replicates in association with the algal cell It is inherited like other organelles.

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