a molecular movie of viral replication at atomic scale
Smallpox may be eradicated, but the family of viruses to which it belongs has never ceased to pose questions — or dangers. At the University of Würzburg, a team of biochemists has achieved what was once unimaginable: watching the molecular machinery of a poxvirus at work, atom by atom, as it transcribes its own genetic instructions. In doing so, they have transformed an ancient threat into a legible blueprint — one that may guide the design of antiviral defenses before the next outbreak demands them.
- Smallpox is gone from nature but not from memory — two laboratory stockpiles and a largely unvaccinated global population mean the threat has merely been suspended, not extinguished.
- Poxviruses carry their own self-contained replication machinery, making them unusually independent and unusually dangerous — and their RNA polymerase, built from fifteen protein subunits, is the engine at the heart of that system.
- Using cryo-electron microscopy and advanced computation, Würzburg researchers reconstructed six distinct states of the viral polymerase in action, effectively assembling a molecular film of viral gene expression at atomic resolution.
- The structural map now enables computer-aided drug design — a rational, targeted approach to building inhibitors that could block viral transcription before a zoonotic poxvirus like monkeypox gains the foothold smallpox once held.
Smallpox vanished from the human population in 1977, and the World Health Organization declared it eradicated by 1980. Today the virus survives only in two maximum-security laboratories, one in Russia and one in the United States. Yet the threat has not fully receded — poxviruses remain scientifically and medically significant, both for their promise in cancer therapy and for the danger they represent should a related virus, like monkeypox, adapt more fully to human transmission.
What makes poxviruses unusual is their self-sufficiency. Where many viruses hijack the biochemical tools of their host cells, poxviruses carry their own complete replication machinery. Two enzymes do the essential work: a DNA polymerase that copies the viral genome, and an RNA polymerase that transcribes those genes into messenger RNA. The vaccinia strain alone deploys an RNA polymerase requiring fifteen distinct protein subunits.
A team led by Utz Fischer at the Julius-Maximilian's University of Würzburg has now mapped this polymerase at atomic resolution — a feat previously out of reach. The researchers introduced the isolated enzyme to a DNA strand carrying the genetic signal for transcription, allowed it to begin producing messenger RNA as it would inside an infected cell, and then examined the samples under a cryo-electron microscope. Working with structural analyst Clemens Grimm, they reconstructed six distinct polymerase complexes, each representing a different phase of transcription. Arranged in sequence, these images form what Grimm described as a molecular movie of viral replication. The findings were published in Nature Structural and Molecular Biology.
The stakes behind this work are practical. No cure exists for smallpox; vaccination remains the only defense. Should the virus escape containment, or should a zoonotic relative gain the ability to spread efficiently between people, a global population with little remaining immunity would be exposed. The atomic-level map of the polymerase now allows researchers to use computational modeling to design inhibitors — drugs that could block viral gene expression at its source. The blueprint has been drawn; the work of building the defenses has begun.
Smallpox killed millions across centuries before vanishing from the human population. The last naturally occurring case appeared in Somalia in October 1977, and by 1980 the World Health Organization declared the disease eradicated—a singular triumph of public health. Today, the virus exists only in two maximum-security laboratories, one in Russia and one in the United States, preserved for research.
Yet the threat has not entirely receded, and neither has scientific interest in the virus family to which smallpox belongs. Poxviruses remain relevant for two reasons: modified versions show promise in cancer treatment, and their internal machinery for replication operates in ways that fascinate researchers. Unlike many viruses that commandeer the biochemical tools of their host cells, poxviruses carry their own complete toolkit encoded in their genome—a self-contained factory for making copies of themselves. Two enzymes do the essential work: a DNA polymerase that duplicates the viral genetic code, and an RNA polymerase that transcribes those genes into messenger RNA. The vaccinia strain of poxvirus, for instance, deploys an RNA polymerase so complex it requires fifteen separate protein subunits, each with its own specialized role.
A research team at the Julius-Maximilian's University of Würzburg has now accomplished something previously impossible: they have watched this polymerase machine at work at the atomic scale. Led by Utz Fischer, the chair of the university's Department of Biochemistry I, the group isolated the viral RNA polymerase and introduced it to a strand of DNA containing a promoter—the genetic signal that tells the enzyme where to begin transcription. The polymerase recognized the signal and began producing messenger RNA, exactly as it would inside an infected cell. The researchers then examined the samples using a cryo-electron microscope, working with Bettina Böttcher from a neighboring department. Using advanced computational methods, they reconstructed the three-dimensional atomic structure of what they observed.
The result was remarkable in its detail. From a single sample examined under the microscope, the team reconstructed six distinct polymerase complexes, each representing a different phase of the transcription process. Clemens Grimm, who led the structural analysis, described the breakthrough in terms that capture its significance: the individual images could be arranged sequentially, like frames in a film, to show the early transcription phase unfold with temporal resolution. They had created, in effect, a molecular movie of viral replication. The findings appeared in Nature Structural and Molecular Biology.
The practical motivation for this work extends beyond academic curiosity. No effective cure exists for smallpox infection; prevention through vaccination is the only reliable defense. If the virus samples held in those two laboratories were somehow released—through accident, theft, or deliberate act—they would encounter a global population almost entirely without immunity. The risk may seem remote, but Fischer points to a more immediate concern: zoonotic diseases, in which viruses adapted to animals jump to humans. Monkeypox, a relative of smallpox, already causes sporadic human infections and can produce severe illness. Should such a virus adapt further to its human host and begin spreading person to person, an epidemic could emerge rapidly.
This is where the atomic-level structural knowledge becomes a tool for defense. Understanding how the polymerase operates in its various states allows researchers to use computer modeling to design inhibitors—drugs that would block viral gene expression. Rather than relying on trial-and-error experimentation, scientists can now take a rational, structure-based approach to antiviral development. Such computational studies are already underway. The map of the polymerase at atomic resolution has become a blueprint for building the drugs that might contain a threat that, though dormant, has not truly disappeared.
Notable Quotes
If the still existing virus samples were to be spread again, for example by a terrorist attack, they would hit a population that has no immunization.— Utz Fischer, Chair of Department of Biochemistry I, Julius-Maximilian's University of Würzburg
We can string the individual pictures together as in a movie and thus represent the early transcription phase with time resolution.— Clemens Grimm, structural analysis lead
The Hearth Conversation Another angle on the story
Why does it matter that we can see this enzyme at atomic resolution? Couldn't we already study how poxviruses replicate?
We could study the outcome, yes—but not the mechanism in motion. This is like the difference between knowing a car moves and watching every gear turn. At atomic scale, we see exactly how the polymerase grabs the DNA, positions it, and builds RNA. That precision is what lets us design drugs that fit into the machinery and jam it.
So this is really about preparing for a threat that might never come?
It's about preparing for threats we can't predict. Smallpox is locked away, but monkeypox is loose in animal populations right now. If it mutates the right way, we need to be ready. The atomic map is insurance.
The team reconstructed six different complexes from one sample. How is that even possible?
The polymerase goes through distinct shapes as it works—like a hand opening and closing around an object. In the microscope, you capture thousands of snapshots of the enzyme at different moments. The computer sorts them by shape, groups the similar ones, and reconstructs each stage in three dimensions. One sample contains millions of molecules, so you get many snapshots of each stage.
And they turned this into a movie?
Exactly. When you arrange the six stages in order, you see the polymerase at work over time. It's not a video in the traditional sense—it's a sequence of atomic structures—but it shows the process unfolding. That's what makes it powerful for drug design. You can see where to insert a blocker.
Why did it take until now to see this?
The technology didn't exist before. Cryo-electron microscopy has only recently become precise enough to resolve structures at atomic scale. It's a combination of better microscopes, better sample preparation, and computers fast enough to process the data. All three had to mature simultaneously.