Deciphering molecular mechanisms in the evolution of new functions

Sammanfattning: The evolution of new genes and functions is considered to be a major contributor to biological diversity in organisms. Through de novo origination, “duplication and divergence”, and horizontal gene transfer, organisms can acquire new genetic material that can evolve to perform novel functions. In this thesis, we investigate how functional trade-offs, “gene duplication and amplification”, and neutral divergence contribute to the emergence of a new function from a preexisting gene. In Paper i, we investigated the ability of Salmonella enterica to compensate for the loss of peptide release factor 1 (RFI) and the potential of peptide release factor 2 (RF2) to gain a new function to replace RFI. The amplification of RF2 and accumulated mutations within RF2 were the main evolutionary routes by which the fitness cost was restored. However, further characterization of the evolved RF2 showed a toxic effect to the cell due to the termination on tryptophan codon (UGG). This evolutionary trade-off - which we named “collateral toxicity” - might present a serious barrier for evolving an efficient RF2 to replace RF1.In Paper ii, we determined whether we could evolve a generalist enzyme with two functions (HisA + TrpF) from the specialist enzyme HisA, which can only synthesize histidine. In a previous study, we showed that HisA evolved a TrpF activity through strong trade-off trajectories. Here, we developed a selection scheme in which we constantly selected for keeping the original function (HisA), while intermittently selecting for the new function (TrpF). Our results showed that all evolved lineages shared the same “stepping stone” mutations in the hisA gene, which enabled them to grow well in the absence of both histidine and tryptophan. Additional accumulated mutations in the hisA gene gave the strains an increased ability to grow without both amino acids, indicating that the HisA enzyme evolved to be an efficient generalist.  In Paper iii, we explored how differences between diverged orthologs influence evolvability. We generated artificial orthologs using a random mutagenesis approach. First, we screened for orthologs with a lower HisA activity and then selected for orthologs with a higher HisA activity; these steps were repeated in alternating rounds. We then tested the ability of each ortholog to evolve  TrpF activity. As expected, the orthologs showed varying abilities to evolve the new function. In particular, orthologs with higher HisA activity levels showed both a higher potential to evolve the new function and a higher TrpF activity when they acquired the new function.