Inducible directed evolution of complex phenotypes in bacteria

Directed evolution is a powerful method for engineering biology in the absence of detailed sequence-function relationships. To enable directed evolution of complex phenotypes encoded by multigene pathways, we require large library sizes for DNA sequences >5-10 kb in length, elimination of genomic hitchhiker mutations, and decoupling of diversification and screening steps. To meet these challenges, we developed Inducible Directed Evolution (IDE), which uses a temperate bacteriophage to package large plasmids and transfer them to naive cells after intracellular mutagenesis. To demonstrate IDE, we evolved a 5-gene pathway from Bacillus licheniformis that accelerates tagatose catabolism in Escherichia coli, resulting in clones with 65% shorter lag times during growth on tagatose after only two rounds of evolution. Next, we evolved a 15.4 kb, 10-gene pathway from Bifidobacterium breve UC2003 that aids E. coli's utilization of melezitose. After three rounds of IDE, we isolated evolved pathways that both reduced lag time by more than 2-fold and enabled 150% higher final optical density. Taken together, this work enhances the capacity and utility of a whole pathway directed evolution approach in E. coli.