Abstract
Viroids are non-coding circular RNA molecules with rod-like or branched structures. They are often ribozymes, characterized by catalytic RNA. They can perform many basic functions of life and may have played a role in evolution since the beginning of life on Earth. They can cleave, join, replicate, and undergo Darwinian evolution. Furthermore, ribozymes are the essential elements for protein synthesis of cellular organisms as parts of ribosomes. Thus, they must have preceded DNA and proteins during evolution. Here, we discuss the current evidence for viroids or viroid-like RNAs as a likely origin of life on Earth. As such, they may also be considered as models for life on other planets or moons in the solar system as well as on exoplanets.
Highlights
IntroductionViruses or virus-like entities arose first during evolution, with cells being a later evolutionary achievement, the origin of life may be something similar to the simplest virus-like entities we know
Viroids or ribozymes are composed of naked closed circular RNA with zero genes, i.e., they do not code for triplets for amino acids as we know them on Earth
RNA--like molecule that supported the transfer of the amino acid to a ribozyme, which fixed a peptide bond to another amino acid
Summary
Viruses or virus-like entities arose first during evolution, with cells being a later evolutionary achievement, the origin of life may be something similar to the simplest virus-like entities we know These are the viroids or ribozymes, which are composed of RNA and typically do not encode any proteins. The viroids consist of a self-complementary rod-like or branched structured covalently closed circular RNA They comprise two families, but only one of them harbors an enzymatically active hammerhead motif, the other one relies on host factors. Today’s viroids act as plant pathogens and cause diseases presumably by leading to the formation of viroid-derived small RNAs, which play a role in pathogenicity mediated by small interfering siRNAs, which cause silencing of important host genes involving RISC, the RNA-Induced Silencing Complex It mediates transcriptional or post-transcriptional dysregulation of host protein expression, other yet unknown mechanisms cannot be excluded [24,25].
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