Viroids and Viroid-like Circular RNAs: Do They Descend from Primordial Replicators?
Viroids are a unique class of plant pathogens that consist of small circular RNA molecules, between 220 and 450 nucleotides in size. Viroids encode no proteins and are the smallest known infectious agents. Viroids replicate via the rolling circle mechanism, producing multimeric intermediates which a...
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| Format: | Article |
| Language: | English |
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MDPI AG
2022-01-01
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| Series: | Life |
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| Online Access: | https://www.mdpi.com/2075-1729/12/1/103 |
| _version_ | 1797492733272653824 |
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| author | Benjamin D. Lee Eugene V. Koonin |
| author_facet | Benjamin D. Lee Eugene V. Koonin |
| author_sort | Benjamin D. Lee |
| collection | DOAJ |
| description | Viroids are a unique class of plant pathogens that consist of small circular RNA molecules, between 220 and 450 nucleotides in size. Viroids encode no proteins and are the smallest known infectious agents. Viroids replicate via the rolling circle mechanism, producing multimeric intermediates which are cleaved to unit length either by ribozymes formed from both polarities of the viroid genomic RNA or by coopted host RNAses. Many viroid-like small circular RNAs are satellites of plant RNA viruses. Ribozyviruses, represented by human hepatitis delta virus, are larger viroid-like circular RNAs that additionally encode the viral nucleocapsid protein. It has been proposed that viroids are direct descendants of primordial RNA replicons that were present in the hypothetical RNA world. We argue, however, that much later origin of viroids, possibly, from recently discovered mobile genetic elements known as retrozymes, is a far more parsimonious evolutionary scenario. Nevertheless, viroids and viroid-like circular RNAs are minimal replicators that are likely to be close to the theoretical lower limit of replicator size and arguably comprise the paradigm for replicator emergence. Thus, although viroid-like replicators are unlikely to be direct descendants of primordial RNA replicators, the study of the diversity and evolution of these ultimate genetic parasites can yield insights into the earliest stages of the evolution of life. |
| first_indexed | 2024-03-10T01:06:59Z |
| format | Article |
| id | doaj.art-fbf4ea6fcbe54f89964676f0c2fd6497 |
| institution | Directory Open Access Journal |
| issn | 2075-1729 |
| language | English |
| last_indexed | 2024-03-10T01:06:59Z |
| publishDate | 2022-01-01 |
| publisher | MDPI AG |
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| series | Life |
| spelling | doaj.art-fbf4ea6fcbe54f89964676f0c2fd64972023-11-23T14:25:05ZengMDPI AGLife2075-17292022-01-0112110310.3390/life12010103Viroids and Viroid-like Circular RNAs: Do They Descend from Primordial Replicators?Benjamin D. Lee0Eugene V. Koonin1National Center for Biotechnology Information, National Library of Medicine, National Institutes of Health, Bethesda, MD 20894, USANational Center for Biotechnology Information, National Library of Medicine, National Institutes of Health, Bethesda, MD 20894, USAViroids are a unique class of plant pathogens that consist of small circular RNA molecules, between 220 and 450 nucleotides in size. Viroids encode no proteins and are the smallest known infectious agents. Viroids replicate via the rolling circle mechanism, producing multimeric intermediates which are cleaved to unit length either by ribozymes formed from both polarities of the viroid genomic RNA or by coopted host RNAses. Many viroid-like small circular RNAs are satellites of plant RNA viruses. Ribozyviruses, represented by human hepatitis delta virus, are larger viroid-like circular RNAs that additionally encode the viral nucleocapsid protein. It has been proposed that viroids are direct descendants of primordial RNA replicons that were present in the hypothetical RNA world. We argue, however, that much later origin of viroids, possibly, from recently discovered mobile genetic elements known as retrozymes, is a far more parsimonious evolutionary scenario. Nevertheless, viroids and viroid-like circular RNAs are minimal replicators that are likely to be close to the theoretical lower limit of replicator size and arguably comprise the paradigm for replicator emergence. Thus, although viroid-like replicators are unlikely to be direct descendants of primordial RNA replicators, the study of the diversity and evolution of these ultimate genetic parasites can yield insights into the earliest stages of the evolution of life.https://www.mdpi.com/2075-1729/12/1/103viroidsribozyvirusesprimordial replicatorsribozymesorigin of life |
| spellingShingle | Benjamin D. Lee Eugene V. Koonin Viroids and Viroid-like Circular RNAs: Do They Descend from Primordial Replicators? Life viroids ribozyviruses primordial replicators ribozymes origin of life |
| title | Viroids and Viroid-like Circular RNAs: Do They Descend from Primordial Replicators? |
| title_full | Viroids and Viroid-like Circular RNAs: Do They Descend from Primordial Replicators? |
| title_fullStr | Viroids and Viroid-like Circular RNAs: Do They Descend from Primordial Replicators? |
| title_full_unstemmed | Viroids and Viroid-like Circular RNAs: Do They Descend from Primordial Replicators? |
| title_short | Viroids and Viroid-like Circular RNAs: Do They Descend from Primordial Replicators? |
| title_sort | viroids and viroid like circular rnas do they descend from primordial replicators |
| topic | viroids ribozyviruses primordial replicators ribozymes origin of life |
| url | https://www.mdpi.com/2075-1729/12/1/103 |
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