Malonyl-CoA is the basic building block for synthesizing a range of important compounds including fatty acids, phenylpropanoids, flavonoids and non-ribosomal polyketides. Centering around malonyl-CoA, we summarized here the various metabolic engineering strategies employed recently to regulate and control malonyl-CoA metabolism and improve cellular productivity. Effective metabolic engineering of microorganisms requires the introduction of heterologous pathways and dynamically rerouting metabolic flux towards products of interest. Transcriptional factor-based biosensors translate an internal cellular signal to a transcriptional output and drive the expression of the designed genetic/biomolecular circuits to compensate the activity loss of the engineered biosystem. Recent development of genetically-encoded malonyl-CoA sensor has stood out as a classical example to dynamically reprogram cell metabolism for various biotechnological applications. Here, we reviewed the design principles of constructing a transcriptional factor-based malonyl-CoA sensor with superior detection limit, high sensitivity and broad dynamic range. We discussed various synthetic biology strategies to remove pathway bottleneck and how genetically-encoded metabolite sensor could be deployed to improve pathway efficiency. Particularly, we emphasized that integration of malonyl-CoA sensing capability with biocatalytic function would be critical to engineer efficient microbial cell factory. Biosensors have also advanced beyond its classical function of a sensor actuator for in situ monitoring of intracellular metabolite concentration. Applications of malonyl-CoA biosensors as a sensor-invertor for negative feedback regulation of metabolic flux, a metabolic switch for oscillatory balancing of malonyl-CoA sink pathway and source pathway and a screening tool for engineering more efficient biocatalyst are also presented in this review. We envision the genetically-encoded malonyl-CoA sensor will be an indispensable tool to optimize cell metabolism and cost-competitively manufacture malonyl-CoA-derived compounds.

We describe scalable and cost‐efficient production of full length, His‐tagged severe acute respiratory syndrome coronavirus 2 (SARS‐CoV‐2) spike glycoprotein trimer by Chinese hamster ovary (CHO) cells that can be used to detect SARS‐CoV‐2 antibodies in patient sera at high specificity and sensitivity. Transient production of spike in both human embryonic kidney (HEK) and CHO cells mediated by polyethyleneimine was increased significantly (up to 10.9‐fold) by a reduction in culture temperature to 32°C to permit extended duration cultures. Based on these data GS‐CHO pools stably producing spike trimer under the control of a strong synthetic promoter were cultured in hypothermic conditions with combinations of bioactive small molecules to increase yield of purified spike product 4.9‐fold to 53 mg/L. Purification of recombinant spike by Ni‐chelate affinity chromatography initially yielded a variety of co‐eluting protein impurities identified as host cell derived by mass spectrometry, which were separated from spike trimer using a modified imidazole gradient elution. Purified CHO spike trimer antigen was used in enzyme‐linked immunosorbent assay format to detect immunoglobulin G antibodies against SARS‐CoV‐2 in sera from patient cohorts previously tested for viral infection by polymerase chain reaction, including those who had displayed coronavirus disease 2019 (COVID‐19) symptoms. The antibody assay, validated to ISO 15189 Medical Laboratories standards, exhibited a specificity of 100% and sensitivity of 92.3%. Our data show that CHO cells are a suitable host for the production of larger quantities of recombinant SARS‐CoV‐2 trimer which can be used as antigen for mass serological testing.

Ralstonia eutropha H16 (also known as Cupriavidus necator H16) is a Gram-negative lithoautotrophic β-proteobacterium with increasing biotechnological applications, including carbon capture and utilization, biopolymer synthesis, and biofuel production. Engineering of this organism is supported by the availability of its genome sequence and suitable plasmid systems. However, the lack of a simple and robust transformation method remains a challenge as it limits both the pace and ease of engineering this organism. To overcome this limitation, a systematic study is performed to evaluate the effects of different parameters on the transformation efficiency of R. eutropha H16. The optimized electroporation protocol uses R. eutropha H16 cells grown to OD 0.6. These cells are made competent by a 15-min incubation in 50 mM CaCl , followed by two cell washes and final resuspension in 0.2 M sucrose prior to electroporation using 2.3 kV. This protocol achieves a transformation efficiency of (3.86 ± 0.29) × 10 cfu µg DNA, a 10 -fold improvement compared to a previously published value for the same plasmid. This transformation method is a valuable tool for R. eutropha H16 research and will further enable the development of other advanced molecular biology methods for this industrially relevant microorganism.

Well-characterized promoters with variable strength form the foundation of heterologous pathway optimization. It is also a key element that bolsters the success of microbial engineering and facilitates the development of biological tools like biosensors. In comparison to microbial hosts such as Escherichia coli and Saccharomyces cerevisiae, the promoter repertoire of Cupriavidus necator H16 is highly limited. This limited number of characterized promoters poses a significant challenge during the engineering of C. necator H16 for biomanufacturing and biotechnological applications. In this article, we first examined the architecture and genetic elements of the four most widely used constitutive promoters of C. necator H16 (i.e., P , P, P , and P) and established a narrow 6-fold difference in their promoter activities. Next, using these four promoters as starting points and applying a range of genetic modifications (including point mutation, length alteration, incorporation of regulatory genetic element, promoter hybridization, and configuration alteration), we created a library of 42 constitutive promoters, all of which are functional in C. necator H16. Although these promoters are also functional in E. coli, they show different promoter strength and hierarchical rank of promoter activity. Subsequently, the activity of each promoter was individually characterized, using l-arabinose-inducible P promoter as a benchmark. This study has extended the range of constitutive promoter activities to 137-fold, with some promoter variants exceeding the l-arabinose-inducible range of P promoter. Not only has the work enhanced our flexibility in engineering C. necator H16, it presented novel strategies in adjusting promoter activity in C. necator H16 and highlighted similarities and differences in transcriptional activity between this organism and E. coli.

We describe scalable and cost-efficient production of full length, His-tagged SARS-CoV-2 spike glycoprotein trimer by CHO cells that can be used to detect SARS-CoV-2 antibodies in patient sera at high specificity and sensitivity. Transient production of spike in both HEK and CHO cells mediated by PEI was increased significantly (up to 10.9-fold) by a reduction in culture temperature to 32ºC to permit extended duration cultures. Based on these data GS-CHO pools stably producing spike trimer under the control of a strong synthetic promoter were cultured in hypothermic conditions with combinations of bioactive small molecules to increase yield of purified spike product 4.9-fold to 53 mg/L. Purification of recombinant spike by Ni-chelate affinity chromatography initially yielded a variety of co-eluting protein impurities identified as host cell derived by mass spectrometry, which were separated from spike trimer using a modified imidazole gradient elution. Purified CHO spike trimer antigen was used in ELISA format to detect IgG antibodies against SARS-CoV-2 in sera from patient cohorts previously tested for viral infection by PCR, including those who had displayed COVID-19 symptoms. The antibody assay, validated to ISO 15189 Medical Laboratories standards, exhibited a specificity of 100% and sensitivity of 92.3%. Our data show that CHO cells are a suitable host for the production of larger quantities of recombinant SARS-CoV-2 trimer which can be used as antigen for mass serological testing.