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. 2016 Aug 2;44(17):8041–8051. doi: 10.1093/nar/gkw693

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© The Author(s) 2016. Published by Oxford University Press on behalf of Nucleic Acids Research.

This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by-nc/4.0/), which permits non-commercial re-use, distribution, and reproduction in any medium, provided the original work is properly cited. For commercial re-use, please contact journals.permissions@oup.com

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Figure 8. — Schematic model representing the different possibilities for the origin of tmRNA. In the beginning of the RNA world, there was a mix of different minihelices. A homodimerisation between two of these would have generated the first proto-ribosome, the Peptidyl Transferase Center (PTC). With aleatory amino acids or aminoacylated minihelices, the PTC created the first random peptide syntheses. A heterodimerization between two minihelices (plus sign) then surely generated a proto-tmRNA. In this step, the proto-tmRNA would have been used as a proto-mRNA. At the same time, the PTC must have evolved into a proto-ribosome by acquiring new RNA that improved its activity. The proto-ribosome would then have continued to evolve via the new synthesized proteins, finally ending up as the ribosomal complex seen today. In contrast to this ribosomal development, the proto-tmRNA took varying evolutionary paths. Through self-splicing, it produced a proto-tRNA that evolved into modern tRNA. It also led to modern tmRNA, which now serves as a rescue system in all bacteria. In addition, proto-tmRNA provided the first non-random genetic medium, which evolved into the RNA genome then into modern mRNA. However, we cannot exclude the theory that a proto-tRNA^Ala grew into modern tmRNA, through the insertion of nucleotides into its gene (question mark) (77).