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An obelisk is a microscopic genetic element similar to a viroid that scientists discovered in 2024. They have been described as "viroid-like element" where a viroid is an infectious agent made of RNA. Researchers first found obelisks using computer tools that analyze massive amounts of genetic data. The RNA sequences of obelisks are unlike any previously described and do not match the DNA or RNA of any known plant, animal, bacterium, or virus. Obelisks are thus considered an enigmatic taxon with their own phylogenetic group and it is unclear how they fit into the tree of life.

Discovery

They were first described in a January 2024 preprint, whose authors say that "Obelisks form their own distinct phylogenetic group", as their RNA sequences, discovered by computer-aided metatranscriptomics, are not homologous with the genomic sequence of any other life form. Currently, only a few methods are available for the identification of these elements from NGS data. With their relationship to other organisms being unknown, they are an example of the incertae sedis, or "enigmatic taxa".

Distribution and pathology

Obelisks have been found in human stool samples, and inside specimens of Streptococcus sanguinis, a species of bacteria, taken from human mouths. Some human subjects harboured 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 samples, from individuals living on all continents.

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 genome assemblies with around 1000 base pairs, and rod-like secondary structures that encompass the entire genome. In contrast to viroids, their RNA is translated into proteins, 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. Oblin-2 features a binding site which is typical of protein complexes, and might therefore bind to enzymes of its host cell.

Name

The preprint says: "Owing to a strong predicted rod-like secondary structure, we term this group of RNAs Obelisk-alpha. At 1164 nt [nucleotides] in length, the rod-like secondary structure was striking " (Secondary with respect to the circular assembly structure.)

References

1. ^ Ivan N. Zheludev; Robert C. Edgar; Maria Jose Lopez-Galiano; Marcos de la Peña; Artem Babaian; Ami S. Bhatt; Andrew Z. Fire (21 January 2024). "Viroid-like colonists of human microbiomes". bioRxiv. doi:10.1101/2024.01.20.576352. PMC 10827157. PMID 38293115. Wikidata Q124389714.
2. ^ Sidik, Saima (29 January 2024). "'Wildly weird' RNA bits discovered infesting the microbes in our guts". Nature. doi:10.1038/d41586-024-00266-7. PMID 38291328. S2CID 267332809. Archived from the original on 30 January 2024. Retrieved 31 January 2024.
3. Pennisi, Elizabeth (26 January 2024). "'It's insane': New viruslike entities found in human gut microbes". Science. doi:10.1126/science.znxt3dk. Archived from the original on 30 January 2024. Retrieved 31 January 2024.
4. Kremer, Frederico; de Barros, Daniele (2024). "Tormentor: An obelisk prediction and annotation pipeline". BioRxiv.org. BioRxiv. doi:10.1101/2024.05.30.596730. Retrieved 3 June 2024.

• RNA
• Incertae sedis
• Subviral agents