A Non-neutral Origin for Error Minimization in the Origin of the Genetic Code

The error minimization of the genetic code would have been determined by natural selection and not by a neutral evolution

I discuss the mechanisms by which the error minimization observed in the genetic code would have been produced; that is, the ability of the genetic code to buffer, for example, the deleterious effects of translation errors. Here, I analyse whether the error minimization was produced by the intervention of natural selection or whether it is an emergent, that is, neutral property; in other words, whether it is a by-product of another mechanism that was structuring the genetic code. In particular, I criticize Massey's simulations (2008) - favouring the neutral hypothesis - which, containing elements of natural selection, would render his conclusions at least partly tautological. Furthermore, I criticize some of Koonin's (2017) interpretations regarding Massey's simulations. Finally, I criticize the opinion of Janzen et al. (2022) according to which their self-aminoacylating ribozyme system would have been capable of generating an error minimization of the genetic code as its emergent property. That is to say, I criticize, more generally, a neutral origin of error minimization. Indeed, any mechanism for structuring the genetic code would be capable of generating, in theory, such an emergent property. The problem is that to demonstrate this, it would be necessary to show that the level of optimization achieved by the genetic code would be that expected under the neutral hypothesis, the one that Janzen et al. (2022) instead they did not make. Therefore, their view is only a hypothesis and is very far from being corroborated by their results. Instead, in the literature there is a strong evidence that the level of optimization achieved by the genetic code is so high that it would imply, per se, an intervention of natural selection in the origin of error minimization of the genetic code. On the other hand, this level of optimization would be very far from what might have been produced by a neutral process.

Little Evidence the Standard Genetic Code Is Optimized for Resource Conservation

Selection for resource conservation can shape the coding sequences of organisms living in nutrient-limited environments. Recently, it was proposed that selection for resource conservation, specifically for nitrogen and carbon content, has also shaped the structure of the standard genetic code, such that the missense mutations the code allows tend to cause small increases in the number of nitrogen and carbon atoms in amino acids. Moreover, it was proposed that this optimization is not confounded by known optimizations of the standard genetic code, such as for polar requirement or hydropathy. We challenge these claims. We show the proposed optimization for nitrogen conservation is highly sensitive to choice of null model and the proposed optimization for carbon conservation is confounded by the known conservative nature of the standard genetic code with respect to the molecular volume of amino acids. There is therefore little evidence the standard genetic code is optimized for resource conservation. We discuss our findings in the context of null models of the standard genetic code.

The Combinatorial Fusion Cascade to Generate the Standard Genetic Code

Combinatorial fusion cascade was proposed as a transition stage between prebiotic chemistry and early forms of life. The combinatorial fusion cascade consists of three stages: eight initial complimentary pairs of amino acids, four protocodes, and the standard genetic code. The initial complimentary pairs and the protocodes are divided into dominant and recessive entities. The transitions between these stages obey the same combinatorial fusion rules for all amino acids. The combinatorial fusion cascade mathematically describes the codon assignments in the standard genetic code. It explains the availability of amino acids with the even and odd numbers of codons, the appearance of stop codons, inclusion of novel canonical amino acids, exceptional high numbers of codons for amino acids arginine, leucine, and serine, and the temporal order of amino acid inclusion into the genetic code. The temporal order of amino acids within the cascade is congruent with the consensus temporal order previously derived from the similarities between the available hypotheses. The control over the combinatorial fusion cascades would open the road for a novel technology to develop artificial microorganisms.

The Mutational Robustness of the Genetic Code and Codon Usage in Environmental Context: A Non-Extremophilic Preference?

The genetic code was evolved, to some extent, to minimize the effects of mutations. The effects of mutations depend on the amino acid repertoire, the structure of the genetic code and frequencies of amino acids in proteomes. The amino acid compositions of proteins and corresponding codon usages are still under selection, which allows us to ask what kind of environment the standard genetic code is adapted to. Using simple computational models and comprehensive datasets comprising genomic and environmental data from all three domains of Life, we estimate the expected severity of non-synonymous genomic mutations in proteins, measured by the change in amino acid physicochemical properties. We show that the fidelity in these physicochemical properties is expected to deteriorate with extremophilic codon usages, especially in thermophiles. These findings suggest that the genetic code performs better under non-extremophilic conditions, which not only explains the low substitution rates encountered in halophiles and thermophiles but the revealed relationship between the genetic code and habitat allows us to ponder on earlier phases in the history of Life.

Adaptive Properties of the Genetically Encoded Amino Acid Alphabet Are Inherited from Its Subsets

Life uses a common set of 20 coded amino acids (CAAs) to construct proteins. This set was likely canonicalized during early evolution; before this, smaller amino acid sets were gradually expanded as new synthetic, proofreading and coding mechanisms became biologically available. Many possible subsets of the modern CAAs or other presently uncoded amino acids could have comprised the earlier sets. We explore the hypothesis that the CAAs were selectively fixed due to their unique adaptive chemical properties, which facilitate folding, catalysis, and solubility of proteins, and gave adaptive value to organisms able to encode them. Specifically, we studied in silico hypothetical CAA sets of 3–19 amino acids comprised of 1913 structurally diverse α-amino acids, exploring the adaptive value of their combined physicochemical properties relative to those of the modern CAA set. We find that even hypothetical sets containing modern CAA members are especially adaptive; it is difficult to find sets even among a large choice of alternatives that cover the chemical property space more amply. These results suggest that each time a CAA was discovered and embedded during evolution, it provided an adaptive value unusual among many alternatives, and each selective step may have helped bootstrap the developing set to include still more CAAs.

Genetic Code Error Minimization as a Non-Adaptive But Beneficial Trait

In a recent Letter, Di Giulio questions the use of the term 'neutral' when describing the process by which error minimization may have arisen as a side-product of genetic code expansion, resulting from the addition of similar amino acids to similar codons (Di Giulio, in J Mol Evol 86(9):593-597, 2018). However, I point out that in this scenario error minimization is non-adaptive, and so 'neutral' is an appropriate term to describe its imperviousness to direct selection. Error minimization is a form of mutational robustness, and so commonly viewed as beneficial. This in turn implies that not all beneficial traits may be adaptations generated by direct selection for that trait.