A giant leap in sequence space reveals the intracellular complexities of evolving a new function

Selection for a promiscuous enzyme activity provides substantial opportunity for competition between endogenous and new substrates to influence the evolutionary trajectory, an aspect that has generally been overlooked in laboratory directed evolution studies. We evolved theEscherichia colinitro/quinone reductase NfsA to detoxify chloramphenicol by randomising eight active site residues simultaneously and interrogating ∼250,000,000 reconfigured NfsA variants. Analysis of every possible evolutionary intermediate of the two best chloramphenicol reductases revealed complex epistatic interactions that restrict each hypothetical trajectory. In both cases, improved chloramphenicol detoxification was only possible after one essential substitution had eliminated activity with endogenous quinone substrates. Unlike the predominantly weak trade-offs seen in previous experimental studies, this substrate incompatibility suggests endogenous metabolites have considerable potential to shape evolutionary outcomes. Unselected prodrug-converting activities were mostly unaffected, which emphasises the importance of negative selection to effect enzyme specialisation, and offers an application for the evolved genes as dual-purpose selectable/counter-selectable markers.