Photosynthetic bacteria played a major role in forming the Earth as we know it. Among them, cyanobacteria stand out for producing the oxygen that fills our atmosphere and allowing the emergence of complex life. Now, scientists at the Austrian Institute of Science and Technology (ISTA) have discovered a surprising twist in the workings of these organisms. Biological systems once thought to separate DNA have instead evolved to control the shape of cyanobacterial cells. The survey results are scienceprovide new insights into how protein systems change over time and how multicellular life developed in these ecologically important bacteria.
“Cyanobacteria are essentially the pioneers of oxygenic photosynthesis,” says Benjamin Springstein, a postdoctoral researcher in the Ruth group at the Austrian Institute of Science and Technology (ISTA).
“They are responsible for the Great Oxygenation Event about 2.5 billion years ago, when oxygen accumulated in the atmosphere and aerobic life became possible. It’s safe to say that without them, none of us would be here today.”
Today, cyanobacteria remain essential to life on Earth. They contribute significantly to global biomass and play a central role in the carbon and nitrogen cycles. These organisms are highly adaptable, living in extreme conditions from hot springs to the Arctic, and even on surfaces such as city roofs and walls. One species in particular, Anabaena sp. PCC 7120 (or simply Anabaena), has been studied for more than 30 years and serves as a model for understanding multicellular cyanobacteria.
Evolution transforms DNA systems into cell-forming structures
Springstein worked with Professor Martin Roos’ group, along with collaborators from ISTA, the Institut Pasteur de Montevideo (Uruguay), the University of Kiel (Germany), and the University of Zurich (Switzerland). Together, they discovered that Anabaena, and perhaps other multicellular cyanobacteria, are undergoing major evolutionary changes. The ancient system once used to separate DNA during cell division has been repurposed into cytoskeleton-like structures that help determine cell shape.
Explain bacterial DNA
Like all bacteria, Anabaena reproduces by dividing into new cells. This process relies on the accurate copying and distribution of DNA so that each new cell receives the genetic information it needs to survive. DNA is tightly packed into chromosomes like thread on a spool, and often exists in multiple copies that must be passed down during division.
Bacterial DNA comes in two main forms. Chromosomes contain essential genes needed for survival, but plasmids contain additional genes that are often nonessential. Plasmids can move between bacteria, allowing traits to spread quickly and allowing rapid adaptation.
DNA separation systems with new roles
Springstein has been studying Anabaena since 2014, exploring its biology and evolution. When lab work paused during the COVID-19 pandemic, he spent time reviewing the scientific literature and noticed something unexpected.
“I made a serendipitous observation,” he recalls.
He discovered that Anabaena and several related cyanobacteria contain a system known as ParMR encoded within their chromosomes. Traditionally, this system was associated with plasmid isolation and was found only on plasmids, which are mobile genetic elements. This unusual arrangement led him to suspect that the system had adapted to separate chromosomes instead.
After joining ISTA as an IST-Bridge Fellow, Springstein set out to test the idea. His experiments revealed something completely different. One of the components, ParR, no longer binds to DNA. Instead, it attaches to lipid membranes, especially the inner membranes of cells. On the other hand, ParM does not form structures for moving DNA within the cytoplasm. Instead, it forms a filament network just beneath the inner membrane, forming a layer of protein polymers that resembles the cell cortex.
Rather than functioning like typical DNA separation systems, which form spindle-like structures inside cells, this system appears to operate at the membrane level and organize cellular structures.
Filaments that behave like cytoskeleton
To better understand how this system works, the researchers used purified components to recreate it outside of living cells. In these in vitro reconstitution experiments, the filaments were observed to exhibit dynamic instability. They grow and then rapidly disintegrate, a behavior similar to microtubules in more complex cells.
To investigate further, the team collaborated with ISTA professor Florian Schur and doctoral student Manjunath Javoor. The researchers looked at how these filaments are assembled using cryo-electron microscopy, which allows scientists to see the molecular structure in detail. They discovered that, unlike the polar filaments formed by similar systems in other bacteria, Anabaena’s filaments are bipolar and can extend and retract from both ends.
Cell shape changes due to loss of system
The true functionality of this system was revealed when it was extracted from living cells.
“Cells lacking this system lost their normal rectangular cell shape and instead became round and bulging,” Springstein explains.
This type of change is usually seen when genes involved in maintaining cell shape are disrupted in other bacteria. This strongly suggests that the primary role of this system is to control cellular structure rather than managing DNA distribution.
Given its new function and location within cells, the researchers renamed the system “CorMR.”
How evolution reused ancient systems
Multicellular cyanobacteria evolved gradually from unicellular ancestors, increasing in complexity over time. Bioinformatics analysis by co-investigator Daniela Megurian of the Pasteur Institute in Montevideo, Uruguay, helped clarify how the CorMR system came to be.
Change didn’t happen all at once. Rather, it appears to have occurred through a series of changes. First, the system was moved from a plasmid to a chromosome. Then the size and structure of its components changed. and developed the ability to bind to cell membranes. Eventually, it came under the control of additional protein systems.
Together, these steps transformed an ancient DNA separation mechanism into a system that forms cells themselves, providing a striking example of how evolution can give an old biological tool an entirely new purpose.
