Obelisk RNA represents a recently identified category of genetic elements. These remarkably small, circular RNA molecules are found in various organisms, distinguishing them from known viruses, viroids, or plasmids. Their discovery marks a notable expansion in our understanding of biological diversity.
Characteristics of Obelisk RNAs
Obelisk RNAs have distinctive structural features. Their genome is a single, circular RNA strand, typically around 1,000 nucleotides long. This forms a rod-like secondary structure.
Unlike viroids, which do not encode proteins, obelisk RNAs contain a novel gene coding for a protein, tentatively named “oblin.” This protein has no detectable sequence or structural similarities to any known proteins. In contrast to viruses, obelisks lack genes for a protein shell or capsid. Plasmids, also circular genetic elements, are typically DNA-based and larger.
The presence of an encoded protein, combined with their small, circular RNA structure, places obelisks in a unique biological space. This combination of features suggests a distinct mode of existence and potential interaction with their hosts.
Discovery and Distribution
Obelisk RNAs were identified through advanced computational analysis of vast genetic datasets, using metagenomic sequencing. Researchers sifted through millions of published genetic sequences, leading to the discovery of almost 30,000 different obelisk types. This allowed detection of these elements directly from environmental and biological samples.
Obelisk RNAs are widespread across various environments and host organisms. They have been found in about 7% of human stool samples and 50% of saliva samples, indicating a significant presence in the human gut and oral microbiomes. Beyond humans, obelisks have been detected in global datasets, including oceanic environments, where they can exceed the abundance of RNA viruses in the prokaryotic fraction. This broad distribution suggests that obelisks are common and have colonized diverse biological niches.
Potential Biological Roles
The specific functions of the oblin protein are still speculative and under investigation. Initial structural predictions suggest that Oblin-1, one of the two identified oblin proteins, may bind metal ions, potentially indicating a role in cellular signaling pathways. Oblin-2 features a binding site characteristic of protein complexes, suggesting it might interact with enzymes or other proteins within its host cell.
Obelisks may interact with host cells or their associated microbes in various ways. These interactions might involve influencing host gene expression, modulating microbial metabolism, or acting as a new type of genetic parasite. For instance, in Streptococcus sanguinis, a common bacterium in the human oral microbiome, a specific obelisk called Obelisk-S.s is highly abundant within the cell’s RNA, despite being absent from the bacterium’s genomic DNA. This high abundance suggests a persistent presence and potential influence on the host bacterium.
The mechanisms by which obelisks replicate and spread are also being explored. Some obelisks code for specific variants of the type-III hammerhead self-cleaving ribozyme, which could point to a viroid-like replication system. While their precise impact on human health or disease remains to be determined, their consistent presence in human microbiomes points to a potential interaction.
Implications of the Discovery
The discovery of obelisk RNA significantly expands our understanding of the “dark matter” of the biological world, challenging existing definitions of life and genetic elements. Their unique characteristics, which do not align perfectly with viruses, viroids, or plasmids, suggest a new phylogenetic group. This finding highlights the vast amount of undiscovered genetic diversity that still exists within biological systems.
This discovery opens numerous avenues for future research in virology, microbiology, and the broader study of biological diversity. Researchers will likely focus on elucidating the exact biological roles of obelisk RNAs, particularly the functions of their encoded oblin proteins. Understanding their replication cycles and how they interact with their hosts will be another significant area of investigation.
The potential role of obelisks in human health or disease is also a compelling area for future study, given their prevalence in human microbiomes. Furthermore, exploring their evolutionary origins could provide insights into the early history of life on Earth. The unique properties of obelisks might also be harnessed as novel biological tools in various applications.