Engineered short branched-chain acyl-CoA synthesis in E. coli and acylation of chloramphenicol to branched-chain derivatives

Cell-free chemoenzymatic starch synthesis from carbon dioxide

[Figure: see text].

A Desaturase-Like Enzyme Catalyzes Oxazole Formation in Pseudomonas Indolyloxazole Alkaloids

Indolyloxazole alkaloids occur in diverse micro- and macroorganisms and exhibit a wide range of pharmacological activities. Despite their ubiquitous occurrence and simple structures, the biosynthetic pathway remained unknown. Here, we used transposon mutagenesis in the labradorin producer Pseudomonas entomophila to identify a cryptic biosynthetic locus encoding an N-acyltransferase and a non-heme diiron desaturase-like enzyme. Heterologous expression in E. coli demonstrates that both enzymes are sufficient to produce indolyloxazoles. Probing their function in stable-isotope feeding experiments, we provide evidence for an unusual desaturase mechanism that generates the oxazole by decarboxylative cyclization.

Enabling Heterologous Synthesis of Lupulones in the Yeast Saccharomyces cerevisiae

Lupulones, naturally produced by glandular trichomes of hop (Humulus lupulus), are prenylated phloroglucinol derivatives that contribute the bitter flavor of beer and demonstrate antimicrobial and anticancer activities. It is appealing to develop microbial cell factories such that lupulones may be produced via fermentation technology in lieu of extraction from limited plant resources. In this study, the yeast Saccharomyces cerevisiae transformants harboring a synthetic lupulone pathway that consisted of five genes from hop were constructed. The transformants accumulated several precursors but failed to accumulate lupulones. Overexpression of 3-hydroxy-3-methyl glutaryl co-enzyme A reductase, the key enzyme in precursor formation in the mevalonate pathway, also failed to achieve a detectable level of lupulones. To decrease the consumption of the precursors, the ergosterol biosynthesis pathway was chemically downregulated by a small molecule ketoconazole, leading to successful production of lupulones. Our study demonstrated a combination of molecular biology and chemical biology to regulate the metabolism for heterologous production of lupulones. The strategy may be valuable for future engineering microbial process for other prenylated natural products.

Synthetic Pathway for the Production of Olivetolic Acid in Escherichia coli

Type III polyketide synthases (PKS IIIs) contribute to the synthesis of many economically important natural products, most of which are currently produced by direct extraction from plants or through chemical synthesis. Olivetolic acid (OLA) is a plant secondary metabolite sourced from PKS III catalysis, which along with its prenylated derivatives has various pharmacological activities. To demonstrate the potential for microbial cell factories to circumvent limitations of plant extraction or chemical synthesis for OLA, here we utilize a synthetic approach to engineer Escherichia coli for the production of OLA. In vitro characterization of polyketide synthase and cyclase enzymes, OLA synthase and OLA cyclase, respectively, validated their requirement as enzymatic components of the OLA pathway and confirmed the ability for these eukaryotic enzymes to be functionally expressed in E. coli. This served as a platform for the combinatorial expression of these enzymes with auxiliary enzymes aimed at increasing the supply of hexanoyl-CoA and malonyl-CoA as starting and extender units, respectively. Through combining OLA synthase and OLA cyclase expression with the required modules of a β-oxidation reversal for hexanoyl-CoA generation, we demonstrate the in vivo synthesis of olivetolic acid from a single carbon source. The integration of additional auxiliary enzymes to increase hexanoyl-CoA and malonyl-CoA, along with evaluation of varying fermentation conditions enabled the synthesis of 80 mg/L OLA. This is the first report of OLA production in E. coli, adding a new example to the repertoire of valuable compounds synthesized in this industrial workhorse.

Biosynthesis of phlorisovalerophenone and 4-hydroxy-6-isobutyl-2-pyrone in Escherichia coli from glucose

Background

Type III polyketide synthases (PKSs) contribute to the synthesis of many economically important natural products, which are typically produced by direct extraction from plants or synthesized chemically. For example, humulone and lupulone (Fig. 1a) in hops (Humulus lupulus) account for the characteristic bitter taste of beer and display multiple pharmacological effects. 4-Hydroxy-6-methyl-2-pyrone is a precursor of parasorboside contributing to insect and disease resistance of plant Gerbera hybrida, and was recently demonstrated to be a potential platform chemical.Fig. 1

Sequence-based Network Completion Reveals the Integrality of Missing Reactions in Metabolic Networks

Genome-scale metabolic models are central in connecting genotypes to metabolic phenotypes. However, even for well studied organisms, such as Escherichia coli, draft networks do not contain a complete biochemical network. Missing reactions are referred to as gaps. These gaps need to be filled to enable functional analysis, and gap-filling choices influence model predictions. To investigate whether functional networks existed where all gap-filling reactions were supported by sequence similarity to annotated enzymes, four draft networks were supplemented with all reactions from the Model SEED database for which minimal sequence similarity was found in their genomes. Quadratic programming revealed that the number of reactions that could partake in a gap-filling solution was vast: 3,270 in the case of E. coli, where 72% of the metabolites in the draft network could connect a gap-filling solution. Nonetheless, no network could be completed without the inclusion of orphaned enzymes, suggesting that parts of the biochemistry integral to biomass precursor formation are uncharacterized. However, many gap-filling reactions were well determined, and the resulting networks showed improved prediction of gene essentiality compared with networks generated through canonical gap filling. In addition, gene essentiality predictions that were sensitive to poorly determined gap-filling reactions were of poor quality, suggesting that damage to the network structure resulting from the inclusion of erroneous gap-filling reactions may be predictable.