X-ray laser images Megavirus in high-resolution 3D

Last month, scientists imaged samples of some of the largest known viruses using the Linac Coherent Light Source (LCLS) x-ray laser at Stanford University (Stanford, CA), producing the highest-resolution 3D images of these mysterious viruses to date. The work could bring researchers closer to understanding the viruses' functions and origins.

The experiments marked the first visit of Megavirus to LCLS, and a return visit for a slightly smaller giant virus, called Mimivirus, which had been studied in previous sets of LCLS experiments.

These massive viruses, which are large enough to be observed by standard light microscopes, were miscategorized as bacteria until about a decade ago because of their size and complex genetics. The experiments at LCLS may help scientists unravel the origins and evolution of Earth's earliest microscopic inhabitants.

Giant viruses such as Mimivirus and Megavirus do fit the strict definition of viruses: Microorganisms that cannot grow or reproduce apart from a living cell. But their size, genetic complexity, and other unconventional viral characteristics raise questions about whether they should be considered living or nonliving. Researchers have even found evidence that giant viruses can be infected by smaller viruses.

Mimivirus and Megavirus, which target small organisms such as amoeba, are larger than some bacteria, and each contain more than 1,000 genes—representing hundreds more than what can be found in many forms of bacteria.

Janos Hajdu, visiting professor of photon science at Stanford University and professor of molecular biophysics at Uppsala University in Sweden, has participated in several rounds of experiments at SLAC's Linac Coherent Light Source studying giant viruses, including samples of Megavirus and Mimivirus
Janos Hajdu, visiting professor of photon science at Stanford University and professor of molecular biophysics at Uppsala University in Sweden, has participated in several rounds of experiments at SLAC's Linac Coherent Light Source studying giant viruses, including samples of Megavirus and Mimivirus. (Photo by Glenn Roberts Jr.)

"It looks like they may have a similar type of genome to our chromosomes," says Janos Hajdu, a key investigator in the giant virus experiments at LCLS. He is a visiting professor of photon science at Stanford University and a professor of molecular biophysics at Uppsala University (Uppsala, Sweden).

Scientists have offered up theories "that these giant viruses have a link or common origin" to cellular nuclei, Hajdu says, so it is important to understand their chemical makeup and structure. These viruses measure hundreds of nanometers in diameter and are tens of times larger than their previously known relatives.

A key part of the latest LCLS experiments was to observe the viruses using specific x-ray energies that allow selective imaging of nucleic acid, the essential chemical component of DNA and other genetic material, and its surroundings within the virus, Hajdu notes.

"If these viruses contain chromatin-type nucleic acids," which cram DNA into cell nuclei, "that is something that supports the notion that there is a link" between viruses and cells, Hajdu says.

As Hajdu spoke, a stream of x-ray images from single virus particles captured by intense x-ray pulses from the LCLS flashed on a screen behind him. His team of collaborators gathered data from millions of such images, which they will now sort through in analyzing and reconstructing the viruses. The virus samples, not harmful to humans, were readied in a portable lab building near LCLS that had been shipped from Sweden.

Researchers had conducted preparatory experiments at FLASH, a lower-energy x-ray free-electron laser (FEL) at German Electron Synchrotron (commonly known as DESY; Hamburg, Germany). Then, the researchers could perform the high-resolution experiments at LCLS, Hajdu explains.

Past experiments at LCLS had focused on two-dimensional imaging of Mimivirus; now, the researchers would like to extend it to 3D at high resolution, Hajdu says.

Hajdu notes that while the science of FELs has improved, higher-intensity x-ray lasers are needed to achieve higher-resolution imaging.

The major goal is to be able to image the structure of a living cell at or near atomic resolution. "At that point, we would be able to see everything and where it worked naturally," Hajdu says.

If and when the structures of cellular components are ever resolved with an x-ray laser, the next challenge would be to differentiate the functions performed by similar structures.

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