Should We Take Alternative Genetic Codes Seriously?

Janella Baxter (University of Pittsburgh)

Abstract: Our understanding of the genetic code – the precise assignment between nucleic acid triplets and amino acids – has always been characterized by an internal tension. Shortly after “decoding” the genetic code biologists discovered that some mitochondria and chloroplast genomes have diverse nucleic acid triplet-amino acid assignments. Nevertheless, biologists widely dismiss this diversity as being unrepresentative of the majority of life. The received approach has been to presuppose that the standard genetic code is relatively universal. That is from bacterium to elephant, the assignment between nucleic acid triplet and amino acid is the same. In this talk, I wish to argue that recent developments in both the genome sequencing of wild populations and development of synthetic genetic codes add to this tension. The sequencing of microbial genomes in the wild has revealed that alternative assignments between some nucleic acid triplets and natural amino acids have not only evolved from the standard assignment, but that these assignments are much more common than had previously been thought (Ivanova et al. 2014). The ubiquity of alternative genetic codes in microbial populations puts tension on two distinct hypotheses about the origin and evolution of the genetic code. One hypothesis is that a single genetic code emerged as a consequence of competition between communal organisms (Vetsigian et al. 2006). The prevalence of diverse genetic codes in communal organisms, however, shows that multiple genetic codes can be sustained in some environments. Another hypothesis is that once the standard genetic code was established, selection pressures “froze” it in place (Sella et al. 2006). Yet, discovery of alternative codes adds further evidence that the standard genetic code is not “frozen,” but allows for some degree of evolvability. Finally, I wish to highlight some ways synthetic genetic codes might enable the testing of some hypotheses about the origin and evolution of the standard genetic code. Synthetic genetic codes have recently been treated merely as evidence of how unlikely alternative genetic codes are to evolve in nature (Koonin 2017). Yet, some of the methods employed by synthetic biologists to expand the genetic code might be treated as analogous to what has happened in wild microbial populations. Furthermore, this technology might be useful for studying how robust life can be with an expanded genetic code. In the end, biologists should take alternative genetic codes into account.