An obelisk is a microscopic genetic element that consists of a type of infectious agent composed of

RNA Ribonucleic acid (RNA) is a polymeric molecule that is essential for most biological functions, either by performing the function itself (non-coding RNA) or by forming a template for the production of proteins (messenger RNA). RNA and deoxyrib ...

. Described as a "

viroid Viroids are small single-stranded, circular RNAs that are infectious pathogens. Unlike viruses, they have no protein coating. All known viroids are inhabitants of angiosperms (flowering plants), and most cause diseases, whose respective eco ...

-like element", they consist of RNA in a circular rod shape without any

protein Proteins are large biomolecules and macromolecules that comprise one or more long chains of amino acid residue (biochemistry), residues. Proteins perform a vast array of functions within organisms, including Enzyme catalysis, catalysing metab ...

shell coating. Obelisks were identified in 2024 through computational analysis of vast genetic datasets. Their RNA sequences are entirely novel, and their placement within the

tree of life The tree of life is a fundamental archetype in many of the world's mythology, mythological, religion, religious, and philosophy, philosophical traditions. It is closely related to the concept of the sacred tree.Giovino, Mariana (2007). ''The ...

remains uncertain as they do not appear to have a shared ancestry with any other life form,

virus A virus is a submicroscopic infectious agent that replicates only inside the living Cell (biology), cells of an organism. Viruses infect all life forms, from animals and plants to microorganisms, including bacteria and archaea. Viruses are ...

, or viroid. Obelisks are currently classified as an enigmatic taxon, forming a distinct phylogenetic group.

Discovery

Obelisks were first described in a January 2024

preprint In academic publishing, a preprint is a version of a scholarly or scientific paper that precedes formal peer review and publication in a peer-reviewed scholarly or scientific journal. The preprint may be available, often as a non-typeset versi ...

, by

Stanford University Leland Stanford Junior University, commonly referred to as Stanford University, is a Private university, private research university in Stanford, California, United States. It was founded in 1885 by railroad magnate Leland Stanford (the eighth ...

(USA), Valencia University (Spain) and

University of Toronto The University of Toronto (UToronto or U of T) is a public university, public research university whose main campus is located on the grounds that surround Queen's Park (Toronto), Queen's Park in Toronto, Ontario, Canada. It was founded by ...

(Canada) scientists which sifted through genetic data. Currently, only a few methods are available for the identification of these elements from NGS data. The authors of the paper say that "Obelisks form their own distinct phylogenetic group", as their

RNA sequence A nucleic acid sequence is a succession of bases within the nucleotides forming alleles within a DNA (using GACT) or RNA (GACU) molecule. This succession is denoted by a series of a set of five different letters that indicate the order of the nu ...

s, discovered by computer-aided

metatranscriptomics Metatranscriptomics is the set of techniques used to study gene expression of microbes within natural environments, i.e., the metatranscriptome. While metagenomics focuses on studying the genomic content and on identifying which microbes are prese ...

, are not homologous with the genomic sequence of any other life form. With their relationship to other organisms being unknown, they are an example of the ''

incertae sedis or is a term used for a taxonomy (biology), taxonomic group where its broader relationships are unknown or undefined. Alternatively, such groups are frequently referred to as "enigmatic taxa". In the system of open nomenclature, uncertainty ...

'', or "enigmatic taxa". The authors of the January preprint named these sequences "obelisks" due to a predicted rod-like

secondary structure Protein secondary structure is the local spatial conformation of the polypeptide backbone excluding the side chains. The two most common Protein structure#Secondary structure, secondary structural elements are alpha helix, alpha helices and beta ...

: "At 1164 nt in length, the rod-like secondary structure was striking .. Viroids were known to exist in plants and cause pathology, and there had been no evidence that they were in animals or bacteria. This marks the first time a viroid or viroid-like object has been found in bacteria or animals.

Distribution and pathology

Obelisks have been found in human stool samples, and inside specimens of ''

Streptococcus sanguinis ''Streptococcus sanguinis'', formerly known as ''Streptococcus sanguis'', is a Gram-positive facultative anaerobic coccus species of bacteria and a member of the Viridans Streptococcus group. ''S. sanguinis'' is a normal inhabitant of the healt ...

'', a species of bacteria, taken from human mouths. Some human subjects hosted obelisks for more than 300 days. The initial study showed the presence of obelisks in about 7 percent of the stool samples, and about 50 percent of

saliva Saliva (commonly referred as spit or drool) is an extracellular fluid produced and secreted by salivary glands in the mouth. In humans, saliva is around 99% water, plus electrolytes, mucus, white blood cells, epithelial cells (from which ...

samples, surveying individuals globally. The effect of obelisks on human health, if any, is yet to be determined, as are issues such as their life cycles, and what factors their replication depend on.

Genetics and biochemistry

Features of obelisks include

circular RNA In molecular biology, circular ribonucleic acid (or circRNA) is a type of single-stranded RNA which, unlike linear RNA, forms a covalently closed continuous loop. In circular RNA, the 3' and 5' ends normally present in an RNA molecule have been ...

genome assemblies with around 1000

base pair A base pair (bp) is a fundamental unit of double-stranded nucleic acids consisting of two nucleobases bound to each other by hydrogen bonds. They form the building blocks of the DNA double helix and contribute to the folded structure of both DNA ...

s, and rod-like secondary structures that encompass the entire genome. In contrast to viroids, their RNA is translated into

s, tentatively called "oblins" in the preprint. The two proteins listed there have been named Oblin-1 and Oblin-2. First structural predictions say that Oblin-1 can bind metal ions and thus could be involved in

cellular signalling In biology, cell signaling (cell signalling in British English) is the Biological process, process by which a Cell (biology), cell interacts with itself, other cells, and the environment. Cell signaling is a fundamental property of all Cell (biol ...

. Oblin-2 features a binding site which is typical of

protein complex A protein complex or multiprotein complex is a group of two or more associated polypeptide chains. Protein complexes are distinct from multidomain enzymes, in which multiple active site, catalytic domains are found in a single polypeptide chain. ...

es, and might therefore bind to

enzyme An enzyme () is a protein that acts as a biological catalyst by accelerating chemical reactions. The molecules upon which enzymes may act are called substrate (chemistry), substrates, and the enzyme converts the substrates into different mol ...

s of its host cell.

References

{{Life, non-cellular life, and comparable structures RNA Incertae sedis Subviral agents