Abstract
BackgroundThe coevolution theory of the origin of the genetic code suggests that the genetic code is an imprint of the biosynthetic relationships between amino acids. However, this theory does not seem to attribute a role to the biosynthetic relationships between the earliest amino acids that evolved along the pathways of energetic metabolism. As a result, the coevolution theory is unable to clearly define the very earliest phases of genetic code origin. In order to remove this difficulty, I here suggest an extension of the coevolution theory that attributes a crucial role to the first amino acids that evolved along these biosynthetic pathways and to their biosynthetic relationships, even when defined by the non-amino acid molecules that are their precursors.ResultsIt is re-observed that the first amino acids to evolve along these biosynthetic pathways are predominantly those codified by codons of the type GNN, and this observation is found to be statistically significant. Furthermore, the close biosynthetic relationships between the sibling amino acids Ala-Ser, Ser-Gly, Asp-Glu, and Ala-Val are not random in the genetic code table and reinforce the hypothesis that the biosynthetic relationships between these six amino acids played a crucial role in defining the very earliest phases of genetic code origin.ConclusionAll this leads to the hypothesis that there existed a code, GNS, reflecting the biosynthetic relationships between these six amino acids which, as it defines the very earliest phases of genetic code origin, removes the main difficulty of the coevolution theory. Furthermore, it is here discussed how this code might have naturally led to the code codifying only for the domains of the codons of precursor amino acids, as predicted by the coevolution theory. Finally, the hypothesis here suggested also removes other problems of the coevolution theory, such as the existence for certain pairs of amino acids with an unclear biosynthetic relationship between the precursor and product amino acids and the collocation of Ala between the amino acids Val and Leu belonging to the pyruvate biosynthetic family, which the coevolution theory considered as belonging to different biosyntheses.ReviewersThis article was reviewed by Rob Knight, Paul Higgs (nominated by Laura Landweber), and Eugene Koonin.
Highlights
The coevolution theory of the origin of the genetic code suggests that the genetic code is an imprint of the biosynthetic relationships between amino acids
All this leads to the hypothesis that there existed a code, GNS, reflecting the biosynthetic relationships between these six amino acids which, as it defines the very earliest phases of genetic code origin, removes the main difficulty of the coevolution theory
The hypothesis here suggested removes other problems of the coevolution theory, such as the existence for certain pairs of amino acids with an unclear biosynthetic relationship between the precursor and product amino acids and the collocation of Ala between the amino acids Val and Leu belonging to the pyruvate biosynthetic family, which the coevolution theory considered as belonging to different biosyntheses
Summary
The extended coevolution theory In order to eliminate some criticisms on certain pairs of amino acids that are in an unclear precursor-product relationship [14,16] and, above all, to provide a more complete description of the very earliest phases of genetic code origin, I have been forced to suggest the following theory. This observation that the first amino acids to evolve along the biosynthetic pathways are the same ones that are mostly codified by codons of the GNN type leads us to suppose, in compliance with the extended coevolution theory, that there existed a type of primitive genetic code (mRNA) that possessed only the codons of the type GNC (or GNG) and codified only for the amino acids Ala, Asp and Ser or Gly (or Ala, Glu and Ser or Gly) (Fig. 3) from which the GNS code codifying for Val, Ala, Asp, Glu, Ser and/or Gly (Fig. 3) might have evolved This is suggested by exploiting the results of Ikehara et al [48] who, for quite different reasons, suggested a genetic code origin that is, in some respects, similar. Eigen et al [49] had suggested a primitive code with codons of the GNY type, which is partly compatible with what is maintained here, partly because it might be derived from a GNC code (Fig. 3) [50]
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