Scientists learn about the wide variety of viroids and viroid-like entities

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To recognize and understand viroids and viroid-like covalently closed circular RNAs, a team of researchers from the National Library of Medicine (NLM) and partnering university research institutes have created a computer pipeline. Compared to linear RNA, this kind of single-stranded RNA forms a continuous, covalently closed circle. The journal Cell reported the findings.

Viroids are the smallest and most basic known infectious agents. They are circular RNAs with just 250–400 nucleotides. They had been thought to solely infect plants, up until now. The lack of knowledge about the diversity of viroid and viroid-like RNAs encouraged researchers to learn more about these sub-viral agents and their potential prevalence in various environments and hosts.

Researchers discovered 11,378 viroid-like cccRNAs spanning 4,409 species-level clusters by examining a collection of 5,131 metatranscriptomes and 1,344 plant transcriptomes. Comparing this discovery to the previously reported viroid-like elements, there was a five-fold increase.

Researchers found that this unique family of pathogens is widespread in all sorts of habitats and hosts, similar to the more well-known RNA viruses, and is not restricted in its spread to a few plants as was initially assumed. Additionally, completely unknown viral types were discovered, and distant relatives of the human Hepatitis Delta Virus (Hepatitis D) were found, providing insight into the origin of this important human illness.

Eugene V. Koonin, Ph.D., a co-author of the paper and a senior investigator in the Computational Biology Branch of NLM’s Intramural Research Program, stated that this work opens up new paths for researchers worldwide.  He said they are working on some follow-up investigations.


Benjamin D. Lee et al. (2023). Mining metatranscriptomes reveals a vast world of viroid-like circular RNAs, CellDOI: 10.1016/j.cell.2022.12.039

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