Abstract
The genetic code evolved around the reading of the tRNA anticodon on the primitive ribosome, and tRNA-34 wobble and tRNA-37 modifications coevolved with the code. We posit that EF-Tu, the closing mechanism of the 30S ribosomal subunit, methylation of wobble U34 at the 5-carbon and suppression of wobbling at the tRNA-36 position were partly redundant and overlapping functions that coevolved to establish the code. The genetic code devolved in evolution of mitochondria to reduce the size of the tRNAome (all of the tRNAs of an organism or organelle). “Superwobbling” or four-way wobbling describes a major mechanism for shrinking the mitochondrial tRNAome. In superwobbling, unmodified wobble tRNA-U34 can recognize all four codon wobble bases (A, G, C and U), allowing a single unmodified tRNA-U34 to read a 4-codon box. During code evolution, to suppress superwobbling in 2-codon sectors, U34 modification by methylation at the 5-carbon position appears essential. As expected, at the base of code evolution, tRNA-37 modifications mostly related to the identity of the adjacent tRNA-36 base. TRNA-37 modifications help maintain the translation frame during elongation.
Highlights
This review was written to support an interpretation of a confluence of recent and older data
As expected, at the base of genetic code evolution, tRNA-37 modifications primarily depend on the adjacent tRNA-36 base, which corresponds to genetic code rows 1–4
We propose that cm5 Ubased modifications suppress superwobbling, which is observed for 4-codon sectors in mitochondrial tRNAs [1,2,5]
Summary
This review was written to support an interpretation of a confluence of recent and older data. At the base of genetic code evolution, it appears that tRNA-U34 may often or always have been modified, in part, to suppress superwobbling and to allow evolution of 2-codon sectors [3,4,12,13]. We posit that 5-carbon U34 methylation-based wobble modifications were essential for the initial evolution of the genetic code. The genetic code can be mostly traced, along with relevant tRNA modification data through evolution of life on Earth [19]. We find that a simple narrative for the evolution of life on Earth is obtained by comparing genetic codes, tRNA-34 and tRNA-37 modifications, aaRS and tRNAome data from a small number of reference organisms
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