Collateral Toxicity Limits the Evolution of Bacterial Release Factor 2 toward Total Omnipotence

When new genes evolve through modification of existing genes, there are often tradeoffs between the new and original functions, making gene duplication and amplification necessary to buffer deleterious effects on the original function. We have used experimental evolution of a bacterial strain lacking peptide release factor 1 (RF1) in order to study how peptide release factor 2 (RF2) evolves to compensate the loss of RF1. As expected, amplification of the RF2-encoding gene prfB to high copy number was a rapid initial response, followed by the appearance of mutations in RF2 and other components of the translation machinery. Characterization of the evolved RF2 variants by their effects on bacterial growth rate, reporter gene expression, and in vitro translation termination reveals a complex picture of reduced discrimination between the cognate and near-cognate stop codons and highlights a functional tradeoff that we term "collateral toxicity." We suggest that this type of tradeoff may be a more serious obstacle in new gene evolution than the more commonly discussed evolutionary tradeoffs between "old" and "new" functions of a gene, as it cannot be overcome by gene copy number changes. Further, we suggest a model for how RF2 autoregulation responds to alterations in the demand not only for RF2 activity but also for RF1 activity.