Engineering a functional 1-deoxy-D-xylulose 5-phosphate (DXP) pathway in Saccharomyces cerevisiae.

Isoprenoids are used in many commercial applications and much work has gone into engineering microbial hosts for their production. Isoprenoids are produced either from acetyl-CoA via the mevalonate pathway or from pyruvate and glyceraldehyde 3-phosphate via the 1-deoxy-D-xylulose 5-phosphate (DXP) pathway. Saccharomyces cerevisiae exclusively utilizes the mevalonate pathway to synthesize native isoprenoids and in fact the alternative DXP pathway has never been found or successfully reconstructed in the eukaryotic cytosol. There are, however, several advantages to isoprenoid synthesis via the DXP pathway, such as a higher theoretical yield, and it has long been a goal to transplant the pathway into yeast. In this work, we investigate and address barriers to DXP pathway functionality in S. cerevisiae using a combination of synthetic biology, biochemistry and metabolomics. We report, for the first time, functional expression of the DXP pathway in S. cerevisiae. Under low aeration conditions, an engineered strain relying solely on the DXP pathway for isoprenoid biosynthesis achieved an endpoint biomass 80% of that of the same strain using the mevalonate pathway.

Rewriting yeast central carbon metabolism for industrial isoprenoid production.

A bio-based economy has the potential to provide sustainable substitutes for petroleum-based products and new chemical building blocks for advanced materials. We previously engineered Saccharomyces cerevisiae for industrial production of the isoprenoid artemisinic acid for use in antimalarial treatments. Adapting these strains for biosynthesis of other isoprenoids such as ß-farnesene (C15H24), a plant sesquiterpene with versatile industrial applications, is straightforward. However, S. cerevisiae uses a chemically inefficient pathway for isoprenoid biosynthesis, resulting in yield and productivity limitations incompatible with commodity-scale production. Here we use four non-native metabolic reactions to rewire central carbon metabolism in S. cerevisiae, enabling biosynthesis of cytosolic acetyl coenzyme A (acetyl-CoA, the two-carbon isoprenoid precursor) with a reduced ATP requirement, reduced loss of carbon to CO2-emitting reactions, and improved pathway redox balance. We show that strains with rewired central metabolism can devote an identical quantity of sugar to farnesene production as control strains, yet produce 25% more farnesene with that sugar while requiring 75% less oxygen. These changes lower feedstock costs and dramatically increase productivity in industrial fermentations which are by necessity oxygen-constrained. Despite altering key regulatory nodes, engineered strains grow robustly under taxing industrial conditions, maintaining stable yield for two weeks in broth that reaches >15% farnesene by volume. This illustrates that rewiring yeast central metabolism is a viable strategy for cost-effective, large-scale production of acetyl-CoA-derived molecules.

Production of benzylisoquinoline alkaloids in Saccharomyces cerevisiae.

The benzylisoquinoline alkaloids (BIAs) are a diverse class of metabolites that exhibit a broad range of pharmacological activities and are synthesized through plant biosynthetic pathways comprised of complex enzyme activities and regulatory strategies. We have engineered yeast to produce the key intermediate reticuline and downstream BIA metabolites from a commercially available substrate. An enzyme tuning strategy was implemented that identified activity differences between variants from different plants and determined optimal expression levels. By synthesizing both stereoisomer forms of reticuline and integrating enzyme activities from three plant sources and humans, we demonstrated the synthesis of metabolites in the sanguinarine/berberine and morphinan branches. We also demonstrated that a human P450 enzyme exhibits a novel activity in the conversion of (R)-reticuline to the morphinan alkaloid salutaridine. Our engineered microbial hosts offer access to a rich group of BIA molecules and associated activities that will be further expanded through synthetic chemistry and biology approaches.

The regulatory roles of the galactose permease and kinase in the induction response of the GAL network in Saccharomyces cerevisiae.

The GAL genetic switch of Saccharomyces cerevisiae exhibits an ultrasensitive response to the inducer galactose as well as the "all-or-none" behavior characteristic of many eukaryotic regulatory networks. We have constructed a strain that allows intermediate levels of gene expression from a tunable GAL1 promoter at both the population and the single cell level by altering the regulation of the galactose permease Gal2p. Similar modifications to other feedback loops regulating the Gal80p repressor and the Gal3p signaling protein did not result in similarly tuned responses, indicating that the level of inducer transport is unique in its ability to control the switch response of the network. In addition, removal of the Gal1p galactokinase from the network resulted in a regimed response due to the dual role of this enzyme in galactose catabolism and transport. These two activities have competing effects on the response of the network to galactose such that the transport effects of Gal1p are dominant at low galactose concentrations, whereas its catabolic effects are dominant at high galactose concentrations. In addition, flow cytometry analysis revealed the unexpected phenomenon of multiple populations in the gal1delta strains, which were not present in the isogenic GAL1 background. This result indicates that Gal1p may play a previously undescribed role in the stability of the GAL network response.

Poly-L-lysine templated silicas: using polypeptide secondary structure to control oxide pore architectures.

Utilizing polypeptide secondary structure as a means for controlling oxide pore architectures is explored. Poly-L-lysine is used as a model polypeptide as its folding behavior is well understood and compatible with the sol-gel chemistry of silica. Here, we show that silicas synthesized with poly-L-lysine in a alpha-helix conformation possess cylindrical pores that are approximately 1.5 nm in size, whereas silicas synthesized with poly-L-lysine in a beta-sheet conformation possess larger pores, the size of which are a function of the poly-L-lysine concentration, or in other words the size of the aggregate. In both cases, highly porous materials are obtained. In-situ circular dichroism measurements of the synthesis mixtures show that the poly-L-lysine secondary structure is not perturbed during synthesis. Infrared spectroscopy of the as-synthesized materials is consistent with the poly-L-lysine retaining its secondary structure. Grand canonical Monte Carlo simulations were also performed to validate the interpretation of the experimental adsorption results. The experimental isotherms are consistent with simulated isotherms of cylindrical pores 1.3-1.7 nm in size, in good agreement with expected values. Our results suggest a new avenue for synthesizing porous oxides with highly tuneable pore sizes and shapes under mild conditions.