Title: Harnessing Saccharomyces cerevisiae Genetics for Cell Engineering
Description: Cell engineering holds the promise of creating designer microorganisms that can address some of society's most pressing needs, ranging from the production of biofuels and drugs to the detection of disease states or environmental contaminants. Realizing these goals will require the extensive reengineering of cells, which will be a formidable task due both to our incomplete understanding of the cell at the systems level and to the technical difficulty of manipulating the genome on a large scale. In Chapter 1, we begin by discussing the potential of directed evolution approaches to overcome the challenges of cell engineering. We then cover the methodologies that are emerging to adapt the mutagenesis and selection steps of directed evolution for in vivo, multi-component systems. Yeast hybrid assays provide versatile systems for coupling a function of interest to a high-throughput growth selection for directed evolution. In Chapter 2, we develop an experimental framework to characterize and optimize the performance of yeast two- and three-hybrid growth selections.

Using the LEU2 reporter gene as a model selectable marker, we show that quantitative characterization of these assay systems allows us to identify key junctures for optimization. In Chapter 3, we apply the same systematic characterization to the yeast three-hybrid counter selection, beginning with our previously reported URA3 reporter. We further develop a screening approach to identify effective new yeast three-hybrid counter selection reporters. Installing customized multi-gene pathways in the cell is arguably the first step of any cell engineering endeavor. Chapter 4 describes the design, construction, and initial validation of Reiterative Recombination, a robust in vivo DNA assembly method relying on homing endonuclease-stimulated homologous recombination. Reiterative Recombination elongates constructs of interest in a stepwise manner by employing pairs of alternating, orthogonal endonucleases and selectable markers. We anticipate that Reiterative Recombination will be a valuable tool for a variety of cell engineering endeavors because it is both highly efficient and technically straightforward.

As an initial application, we illustrate Reiterative Recombination's utility in the area of metabolic engineering in Chapter 5. Specifically, we demonstrate that we can build functional biosynthetic pathways and generate large libraries of pathways in vivo. The facility of pathway construction by Reiterative Recombination should expedite strain optimization for metabolic engineering.