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Modified Nucleosides Always Were: an Evolutionary Model, Page 1 of 2
< Previous page Next page > /docserver/preview/fulltext/10.1128/9781555818296/9781555811334_Chap29-1.gif /docserver/preview/fulltext/10.1128/9781555818296/9781555811334_Chap29-2.gifAbstract:
This chapter evaluates the distribution of modified nucleosides in certain RNA species of the three domains of life (archaea, bacteria, and eukaryotes). Fourteen modifications are present in all three domains and of these several are found at comparable positions in tRNAs . First, most of the conserved modified nucleosides are in or near the anticodon loop. There is strong phylogenetic evidence for the early appearance of modified nucleosides. The incorporation of modified nucleosides into nucleic acid polymers is quite plausible, and the use of a particular nucleoside may depend only on its concentration in the primordial soup. Again, considering the chemistry of nucleosides, it would seem that the moderately modified nucleosides would be present in the highest concentrations in the primordial soup along with the standard nucleosides. In parallel, the phylogenetic evidence suggests that some simple modified nucleosides existed at the point when the major organismal groups diverged. In the case of tRNAs, there is no evidence for present-day RNA guiding, although such a system could be imagined early in the evolution of replicating cells. The current evidence of the enzymes involved in RNA-guided modification leaves the door open to speculation that RNA has a greater role in this process than simply guiding modification.