Emulsion-based directed evolution of enzymes and proteins in yeast.

Emulsion-based selections are a unique type of directed evolution method that overcome common bottlenecks associated with purely in vivo selections. For example, emulsions including cell-free translation machinery can be useful for expression of toxic genes. However, not all cell types can efficiently produce protein in vitro, for example, the eukaryotic microbe Saccharomyces cerevisiae. compartmentalized self replication (CSR) and compartmentalized partnered replication (CPR) are two emulsion-based selection schemes that leverage the advantages of both in vivo and in vitro selections by compartmentalizing cells in water-in-oil droplets. Previous implementations of these methods utilized bacterial hosts, which has limited the technology to the directed evolution of proteins that can be heterologously expressed in prokaryotic systems. To expand the repertoire of targets that can be evolved, we have adapted emulsion-based PCR selections to be compatible with a eukaryotic host.

Direct Enzymatic Synthesis of a Deep-Blue Fluorescent Noncanonical Amino Acid from Azulene and Serine.

We report a simple, one-step enzymatic synthesis of the blue fluorescent noncanonical amino acid ß-(1-azulenyl)-l-alanine (AzAla). By using an engineered tryptophan synthase ß-subunit (TrpB), stereochemically pure AzAla can be synthesized at scale starting from commercially available azulene and l-serine. Mutation of a universally conserved catalytic glutamate in the active site to glycine has only a modest effect on native activity with indole but abolishes activity on azulene, suggesting that this glutamate activates azulene for nucleophilic attack by stabilization of the aromatic ion.