Photosynthetic bacteria helped shape Planet Earth. Among them are cyanobacteria that produced the oxygen in our atmosphere and made complex life possible, captivating scientists for decades. Now, researchers at the Institute of Science and Technology Austria (ISTA) report a surprising new discovery—a system thought to separate DNA has developed to sculpt the shape of the cell in cyanobacteria instead. The results, published in Science , shed light on how protein systems evolve and how multicellularity emerged in this type of ecologically essential 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.”
Still today, these organisms remain vital by contributing significantly to global biomass production and playing key roles in carbon and nitrogen cycles. They thrive in some of Earth’s most extreme environments—from hot springs to the Arctic—and even on roofs and walls on urban buildings. Among them is Anabaena sp. PCC 7120 (or simply Anabaena ), a multicellular cyanobacterium that has been the subject of research for more than 30 years.
Working in the group of Professor Martin Loose in collaboration with the Schur group at ISTA , as well as the Institut Pasteur de Montevideo (Uruguay), Kiel University (Germany), and the University of Zürich (Switzerland), Springstein and his colleagues now show that Anabaena , and likely many other multicellular cyanobacteria, have undergone a major evolutionary shift, transforming an ancient DNA segregation system into a new cytoskeleton that controls cell shape.
DNA in bacteria: A brief primer
Like all bacteria, Anabaena reproduce by cell division, which requires precise replication and distribution of its genetic material. This genetic material—the DNA—is tightly packed into chromosomes, much like a wire around a spool. Often present in multiple copies, chromosomes must be reliably inherited during cell division for daughter cells to remain viable.
Bacterial DNA exists in two main forms: chromosomes, which carry genes crucial for survival, and plasmids that contain additional, often non-essential genes. Plasmids are especially mobile, as they can easily be transferred from one bacterium to another, allowing bacteria to rapidly acquire new traits and evolve swiftly.
A DNA segregation system—until it was not
Since 2014, Springstein has been captivated by Anabaena , exploring their evolutionary and molecular mysteries. When the COVID-19 pandemic brought research to a halt and laboratories closed, he turned to reviewing literature on the topic while writing a review and found something surprising that proved worth following up.
“I made a serendipitous observation,” he recalls.
He noted that Anabaena and some other select multicellular cyanobacteria possess a so-called ParMR system that is encoded on their chromosomes. This system is traditionally associated with plasmid segregation and was previously only found on plasmids—the bacteria’s mobile gene storage site. This observation made him hypothesize that this system might actively segregate chromosomes—and not plasmids—during cell division to ensure the proper maintenance of its DNA.
Springstein then later joined ISTA and the Loose group as an IST-Bridge Fellow to test this idea. However, his experiments told a different story. One component, ParR, for instance, could not bind to the DNA anymore; instead, it associated with lipid membranes, particularly the inner cell membrane. Rather than forming filament bundles in the cytoplasm to segregate chromosomes, Anabaena’s ParM forms filament networks just underneath the inner cell membrane to assemble into an array of protein polymers like a cell cortex.
In other words, instead of generating spindle-like cytoplasmic structures as expected for a chromosome segregation system, it appeared to function through membrane-associated organization.
Cells lose their shape
To unravel this mystery further, the researchers rebuilt the system outside living cells using purified components. In these in vitro reconstitution experiments, they observed that the filaments showed dynamic instability—they grew before suddenly collapsing during disassembly, a behavior well known from microtubules in eukaryotic cells.
To understand the structural basis of this behavior, the Loose group teamed up with the group of ISTA Professor Florian Schur and his PhD student Manjunath Javoor. Using cryo-electron microscopy—a technique that captures molecular structures at near-atomic resolution—the researchers examined the architecture of these filaments. Their discovery: Unlike the plasmid-encoded ParMR system in other bacteria, which forms polar filaments, Anabaena filaments are bipolar, meaning they can grow and shrink from both ends.
The functional consequences became quite clear when the system was removed from living cells.
“Cells lacking the system lost their normal rectangular-like cell shape and instead became round and swollen,” Springstein explains.
Similar defects are often seen in mutations of cell-shape maintenance genes in other bacteria, strongly indicating that this system plays a role in controlling cell morphology rather DNA segregation.
Reflecting on its newly uncovered function and their distinct location in the cell, the researchers renamed the system “CorMR.”
Four steps to a new function
Multicellular cyanobacteria evolved from single-celled ancestors through a gradual increase in cellular complexity. Bioinformatic analyses by collaborator Daniela Megrian from the Institut Pasteur in Montevideo, Uruguay, shed light on how the CorMR system evolved—an adaptation that did not arise all at once but rather through a series of changes.
The transformation likely unfolded in four key steps: the system moved from a plasmid to the chromosome; its components changed in size and structure; new membrane-binding capabilities emerged, and the system came under the control of an additional protein system. Together, these changes turned an ancient DNA-segregation machinery into one that controls cell shape.
Science
Experimental study
Cells
Repurposing of a DNA segregation machinery into a cytoskeletal system controlling cell shape
16-Apr-2026
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