Ancient Bacteria Turned a DNA System Into a Cell Skeleton

Fluorescently labeled CorM filaments inside Anabaena. These represent a newly discovered cytoskeleton in multicellular cyanobacteria. Credit: © Loose group | ISTA

A system once tied to DNA organization in cyanobacteria has evolved into a structure that shapes the cell itself. This shift reveals how evolution can turn old biological tools into entirely new functions.

Photosynthetic bacteria played a crucial role in shaping our planet. Among them, cyanobacteria stand out for producing the oxygen that filled Earth’s atmosphere and enabled complex life to evolve. Now, scientists at the Institute of Science and Technology Austria (ISTA) have uncovered an unexpected twist in how these organisms function. A system long believed to separate DNA has instead taken on a completely different job, helping determine the shape of cyanobacterial cells. The study, published in Science, provides new clues about how protein systems evolve and how multicellular life arose in these environmentally important bacteria.

“Cyanobacteria are essentially pioneers of oxygenic photosynthesis,” says Benjamin Springstein, a postdoc in the Loose group at the Institute of Science and Technology Austria (ISTA).

“They are responsible for the Great Oxygenation Event about 2.5 billion years ago, when oxygen accumulated in the atmosphere and made aerobic life possible. Without them, it’s safe to say that none of us would be here today.”

Even now, cyanobacteria remain vital to Earth’s ecosystems. They contribute significantly to global biomass and play key roles in carbon and nitrogen cycling. These organisms are remarkably adaptable, thriving in extreme environments ranging from hot springs to the Arctic, as well as on surfaces such as roofs and walls in cities. One well-studied species, Anabaena sp. PCC 7120 (or simply Anabaena), has been a model organism for more than 30 years.

Down to the core. From left to right: When rebuilt outside of living cells, CorM forms dynamic filaments. Cryo-electron microscopy (cryo-EM) image of purified CorM filaments. Successive zoom-ins show the reconstructed 3D electron density map of the CorM filament, followed by the corresponding atomic model, illustrating the filaments’ assembly into a bipolar double-stranded filament. Credit: © Springstein et al. / ScienceEvolution Repurposes a DNA System Into a Cell-Shaping Structure

Springstein worked in the group of Professor Martin Loose alongside collaborators from ISTA, the Institut Pasteur de Montevideo (Uruguay), Kiel University (Germany), and the University of Zürich (Switzerland). Their research shows that Anabaena, and likely many other multicellular cyanobacteria, have undergone a major evolutionary change. An ancient system once used to separate DNA has been transformed into a cytoskeleton-like structure that helps control cell shape.

High-resolution image of CorM filaments in Anabaena. Green corresponds to CorM filaments while purple shows cyanobacterial photosynthetic pigments. Credit: © Springstein et al. / ScienceDNA in Bacteria: A Brief Primer

Like all bacteria, Anabaena reproduce through cell division. This process requires accurate copying and distribution of DNA so that each new cell receives the genetic material it needs. DNA is tightly packed into chromosomes, similar to thread wound around a spool, and is often present in multiple copies that must be reliably passed on during division.

Bacterial DNA exists in two main forms. Chromosomes carry genes essential for survival, while plasmids contain additional genes that are often not required. Plasmids are highly mobile and can move between bacteria, allowing traits to spread quickly and enabling rapid adaptation.

First author Benjamin Springstein. The ISTA postdoc uses high-resolution microscopes to examine cyanobacteria known as Anabaena. Credit: © ISTAA DNA Segregation System Finds a New Role

Springstein has been studying Anabaena since 2014, focusing on its evolutionary and molecular features. During the COVID-19 pandemic, when laboratory work paused, he reviewed scientific literature and noticed something unusual.

“I made a serendipitous observation,” he recalls.

He found that Anabaena and some related cyanobacteria contain a system called ParMR encoded on their chromosomes. This system is typically linked to plasmid segregation and had previously only been observed on plasmids, which act as mobile gene storage units. This unusual placement led him to suspect that the system might have adapted to separate chromosomes instead.

After joining ISTA as an IST-Bridge Fellow, Springstein tested this idea experimentally. The results revealed a different function. One component, ParR, no longer binds to DNA. Instead, it attaches to lipid membranes, particularly the inner cell membrane. Meanwhile, ParM does not form structures in the cytoplasm to move DNA. Instead, it builds filament networks just beneath the inner membrane, creating an array of protein polymers that resembles a cell cortex.

Rather than forming spindle-like structures inside the cell, as expected for DNA segregation, the system appears to organize itself at the membrane and contribute to cell structure.

Collaboration between the Loose and the Schur groups at the Insitute of Science and Technology Austria (ISTA). In the back, from left to right: Roman Hajdu, Martin Loose, Florian Schur. In the front, from left to right: Manjunath Javoor, Benjamin Springstein, Bettina Zens. Credit: © ISTAFilament Dynamics Reveal Cytoskeleton-Like Behavior

To better understand this system, the researchers reconstructed it outside living cells using purified components. In these in vitro reconstitution experiments, they observed that the filaments exhibit dynamic instability. They grow and then rapidly collapse, a pattern also seen in microtubules in more complex cells.

To examine their structure in detail, the team collaborated with ISTA Professor Florian Schur and his PhD student Manjunath Javoor. Using cryo-electron microscopy, which allows scientists to visualize molecular structures at very high resolution, they analyzed the architecture of the filaments. They found that, unlike similar systems in other bacteria that form polar filaments, the filaments in Anabaena are bipolar, meaning they can grow and shrink from both ends.

Loss of the System Changes Cell Shape

The system’s true role became clear when it was removed from living cells.

“Cells lacking the system lost their normal rectangular-like cell shape and instead became round and swollen,” Springstein explains.

Such changes are commonly observed when genes responsible for maintaining cell shape are disrupted in other bacteria. This strongly suggests that the system’s primary function is to control cell morphology rather than manage DNA distribution.

Given its newly identified role and its position near the cell membrane, the researchers renamed the system “CorMR.”

Stepwise Evolution of a New Cellular Function

Multicellular cyanobacteria evolved from single-celled ancestors through gradual increases in complexity. Bioinformatic analysis by collaborator Daniela Megrian from the Institut Pasteur in Montevideo, Uruguay, helped clarify how the CorMR system developed.

The transition likely occurred in several stages rather than all at once. First, the system moved from a plasmid to the chromosome. Next, its components changed in size and structure. Then, it gained the ability to bind to cell membranes. Finally, it came under the control of an additional protein system.

Together, these steps transformed an ancient DNA segregation system into one that shapes the cell itself, highlighting how evolution can repurpose existing biological machinery to create entirely new functions.

Reference: “Repurposing of a DNA segregation machinery into a cytoskeletal system controlling cell shape” by Benjamin L. Springstein, Manjunath G. Javoor, Daniela Megrian, Roman Hajdu, Dustin M. Hanke, Bettina Zens, Gregor L. Weiss, Florian K. M. Schur and Martin Loose, 16 April 2026, Science.
DOI: 10.1126/science.aea6343

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