February 16, 2024
3 minute read
Strange 'obelisks' found in human intestines may be virus-like objects
Rod-shaped pieces of RNA called “obelisks” were first discovered in gut and oral bacteria.
For the first time, scientists noticed fragments of RNA colonizing the body. bacteria in our intestines. These “obelisks,” as the researchers have dubbed them, may represent an entirely new class of infectious agents.
“This is absolutely amazing,” says Kathleen Hefferon, a microbiologist at Cornell University who was not involved in the study. The study is available on the website bioRxiv preprint server and has not yet been published in a peer-reviewed scientific journal.
To discover the obelisks, lead author Ivan Zheludev of Stanford University and his colleagues analyzed data from an RNA database containing thousands of sequences isolated from human feces. RNA is similar to DNA but is usually a single chain of “letters” or base pairs rather than double-stranded, and usually carries messages from the DNA that instruct the body to make proteins. The researchers identified thousands of different loops of single-stranded RNA that did not code for proteins. They even found one group of obelisks inside a common oral bacterium. Streptococcus in the blood. They also analyzed information about the oral and gut microbiomes of 472 people from five previous studies and found obelisks in almost 10 percent of the participants.
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Obelisks are somewhat reminiscent of viroids – tiny round fragments of single-stranded RNA. Like virusesViroids require a host to replicate and can infect and cause disease in eukaryotesorganisms with cells that have nuclei. In particular, they are widely described in flowering plants, as well as in some fungi and animals. But unlike viruses, they lack an outer protein shell. This makes them one of the simplest self-replicating collections of genetic material on the planet.
Obelisks vary in shape and structure. Obelisks are rolled into a rod shape rather than remaining a flat circle, and they RNA sequences do not match any known viroid sequence. They are also the first viroid-like elements found in bacterial cells rather than in more complex organisms.
“It will be interesting to see how to classify these things,” says biochemist Zascha Weinberg of the University of Leipzig in Germany, referring to the obelisks. Because viroids and their relatives do not eat, regenerate, or copulate, they blur the line between living and nonliving things.
Given the ubiquity of viroids in the plant world, looking for something similar in bacteria makes sense, says Hefferon, who studies disease-causing viroids in commercial crops such as potatoes. Still, it's exciting to finally find them, Hefferon adds.
Future research, Hefferon hopes, will find virus-like elements in even more places. If these simple fragments of RNA are present in organisms as diverse as potatoes and Streptococcus bacteria, there is no reason why they should not penetrate other living things. “This could very well be happening in mammalian cells,” she says. But scientists will need to continue studying the obelisks to determine whether they are capable of infecting or causing disease in such organisms.
A more detailed study of the obelisks may even help scientists trace origin of life on earth. Some researchers believe that viroids and their relatives represent the oldest and most primitive forms of life, or at least their predecessors. And if they are able to infect organisms throughout the evolutionary tree, they may have played a role in shaping many of the species we know today. “There's a lot we don't know,” Hefferon says. “It feels like we’re finding fossils from a completely different world.”
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