While investigating the many microbes that have colonized the human body, a team of researchers stumbled upon something strange: a previously unrecognized class of virus-like objects, which they have dubbed “Obelisks”. The newly described entities are thought to inhabit human mouth and gut bacteria and have circular RNA genomes, sequences of which have been found all over the world.
The researchers came across almost 30,000 different Obelisks, with examples from all seven continents, while trawling through over 5 million datasets of published genetic sequences.
“It’s insane,” Mark Peifer, a cell and developmental biologist at the University of North Carolina at Chapel Hill who was not involved with the research, told Science Magazine. “The more we look, the more crazy things we see.”
The work, which is yet to be peer-reviewed, found Obelisks in 7 percent of the analyzed stool samples, and 50 percent of oral samples. Their sequences were also found to be distinct depending on where they were found in the body.
They bear no sequence or structural similarity to known biological agents, and so “comprise a class of diverse RNAs that have colonised, and gone unnoticed in, human, and global microbiomes,” the team write in their preprint.
Obelisks are named after the rod-like structures predicted to encompass their entire genomes, which resemble the monument of the same name.
The virus-like entities’ genomes are just 1,000 bases long, which likely sets them apart from RNA viruses as they tend to have much longer sequences.
Obelisks also include genes encoding a novel protein superfamily called “Oblins”, which the researchers suspect may be involved in RNA replication. This distinguishes them from simpler RNA loops called viroids that do not code for proteins.
So, Obelisks are like viruses but they’re not viruses, and they’re also not viroids. They might not even behave that much like viruses: “these elements might not even be ‘viral’ in nature,” the researchers explain, as it is currently “not possible to assign transmission mode, host impact, or replication mode.” Instead, they might more closely resemble RNA plasmids, the team conclude.
Because of these unknowns, Obelisks' effect on human health also remains up in the air.
Still, the findings help to flesh out what we know about viral evolution, something that is still shrouded in a lot of mystery. “This is one of the most exciting parts of being in this field right now,” Simon Roux of the DOE Joint Genome Institute at Lawrence Berkeley National Laboratory, also not involved in the research, told Science. “We can see the picture of the long-term evolution of viruses on Earth start to slowly emerge.”
The study, which has not been peer-reviewed, is available at bioRxiv.