Among the "jungle of microbes" that live inside us, scientists have marveled when they stumbled upon what may be
a whole new class
of virus-like objects.
"It's crazy,"
University of North Carolina cell biologist Mark Peifer, who was not involved in the study, said in Science magazine.
"The more we look, the more crazy things we see."
These mysterious fragments of genetic material have no detectable sequences or even
structural similarities known
to other biological agents.
That's why biologist Ivan Zheludev of Stanford University and his colleagues argue that their strange discovery may not be viruses at all, but an
entirely new group of entities
that can help close the old gap between the simplest genetic molecules and the more complex viruses.
"The 'obelisks' (as they called the unknown entities) comprise a class of diverse RNAs that
have colonized and gone unnoticed
in the human and global microbiomes," the researchers said in the study published on bioRxiv.
The obelisks' genetic sequences, which get their name from the highly symmetrical rod-like structures formed by their twisted lengths of RNA, "
are only about 1,000 nucleotides in size
. "
In fact,
"this feature
is probably one of the reasons we haven't noticed them before," they maintain.
In a study that has not yet been peer-reviewed, Zheludev and his team searched 5.4 million published genetic sequence data sets and
identified nearly 30,000 different obelisks.
They appeared in about 10 percent of the human microbiomes the team examined.
In one data set, obelisks
appeared in 50 percent
of patients' oral samples.
What's more, different types of obelisks seem to be present in different areas of our body.
"This supports the idea that obelisks
could include colonists
of such human microbiomes," the researchers explain.
They managed to isolate a type of host cell from our microbiome, the bacteria Streptococcus sanguinis, a common microbe in the human mouth.
The Obelisk of these microbes had
a loop 1,137 nucleotides long.
"Although we do not know the 'hosts' of other obelisks," write Zheludev and his colleagues, "it is reasonable to assume that
at least a fraction
may be present in bacteria."
"They all appear to include codes for a new class of protein" that the researchers
have named Oblins.
The instructions for building these proteins appear to take up at least half of the Obelisks' genetic material.
Because these proteins are so similar in all obelisks, researchers suspect that they may be involved
in the entity's replication process.
This
ability to encode proteins sets
them apart from other known RNA loops called viroids, but they also do not appear to have the genes to produce protein coats that RNA viruses (including COVID-19) live on when outside cells.
.
They are also
significantly larger than other genetic molecules
that coexist within cells, from plants to bacteria, called plasmids, which are commonly made of DNA.
However, Zheludev and his team
were unable to identify any impact of the obelisks on their bacterial hosts
, nor a means by which they could spread between cells.
"These elements might
not even be 'viral' in nature
and might look more like 'RNA plasmids,'" they concluded.
Source: Science Alert