Crafting genetic diversity: unlocking the potential of protein evolution

Genetic diversity is the foundation of evolutionary resilience, adaptive potential, and the flourishing vitality of living organisms, serving as the cornerstone for robust ecosystems and the continuous evolution of life on Earth. The landscape of directed evolution, a powerful biotechnological tool inspired by natural evolutionary processes, has undergone a transformative shift propelled by innovative strategies for generating genetic diversity. This shift is fuelled by several factors, encompassing the utilization of advanced toolkits like CRISPR-Cas and base editors, the enhanced comprehension of biological mechanisms, cost-effective custom oligo pool synthesis, and the seamless integration of artificial intelligence and automation. This comprehensive review looks into the myriad of methodologies employed for constructing gene libraries, both in vitro and in vivo, categorized into three major classes: random mutagenesis, focused mutagenesis, and DNA recombination. The objectives of this review are threefold: firstly, to present a panoramic overview of recent advances in genetic diversity creation; secondly, to inspire novel ideas for further innovation in genetic diversity generation; and thirdly, to provide a valuable resource for individuals entering the field of directed evolution

Design and application of genetically-encoded malonyl-CoA biosensors for metabolic engineering of microbial cell factories

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

Adaptive laboratory evolution of cupriavidus necator H16 for carbon co-utilization with glycerol

Cupriavidus necator H16 is a non-pathogenic Gram-negative betaproteobacterium that can utilize a broad range of renewable heterotrophic resources to produce chemicals ranging from polyhydroxybutyrate (biopolymer) to alcohols, alkanes, and alkenes. However, C. necator H16 utilizes carbon sources to different efficiency, for example its growth in glycerol is 11.4 times slower than a favorable substrate like gluconate. This work used adaptive laboratory evolution to enhance the glycerol assimilation in C. necator H16 and identified a variant (v6C6) that can co-utilize gluconate and glycerol. The v6C6 variant has a specific growth rate in glycerol 9.5 times faster than the wild-type strain and grows faster in mixed gluconate–glycerol carbon sources compared to gluconate alone. It also accumulated more PHB when cultivated in glycerol medium compared to gluconate medium while the inverse is true for the wild-type strain. Through genome sequencing and expression studies, glycerol kinase was identified as the key enzyme for its improved glycerol utilization. The superior performance of v6C6 in assimilating pure glycerol was extended to crude glycerol (sweetwater) from an industrial fat splitting process. These results highlight the robustness of adaptive laboratory evolution for strain engineering and the versatility and potential of C. necator H16 for industrial waste glycerol valorization

An engineered constitutive promoter set with broad activity range for Cupriavidus necator H16

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., PphaC1, PrrsC, Pj5, and Pg25) 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 PBAD 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 PBAD 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

Fast-track adaptive laboratory evolution of Cupriavidus necator H16 with divalent metal cations

Microbial strain improvement through adaptive laboratory evolution (ALE) has been a key strategy in biotechnology for enhancing desired phenotypic traits. In this Biotech Method paper, we present an accelerated ALE (aALE) workflow and its successful implementation in evolving Cupriavidus necator H16 for enhanced tolerance toward elevated glycerol concentrations. The method involves the deliberate induction of genetic diversity through controlled exposure to divalent metal cations, enabling the rapid identification of improved variants. Through this approach, we observed the emergence of robust variants capable of growing in high glycerol concentration environments, demonstrating the efficacy of our aALE workflow. When cultivated in 10% v/v glycerol, the adapted variant Mn-C2-B11, selected through aALE, achieved a final OD600 value of 56.0 and a dry cell weight of 15.2 g L−1, compared to the wild type (WT) strain's final OD600 of 39.1 and dry cell weight of 8.4 g L−1. At an even higher glycerol concentration of 15% v/v, Mn-C2-B11 reached a final OD600 of 48.9 and a dry cell weight of 12.7 g L−1, in contrast to the WT strain's final OD600 of 9.0 and dry cell weight of 3.1 g L−1. Higher glycerol consumption by Mn-C2-B11 was also confirmed by high-performance liquid chromatography (HPLC) analysis. This adapted variant consumed 34.5 times more glycerol compared to the WT strain at 10% v/v glycerol. Our method offers several advantages over other reported ALE approaches, including its independence from genetically modified strains, specialized genetic tools, and potentially carcinogenic DNA-modifying agents. By utilizing divalent metal cations as mutagens, we offer a safer, more efficient, and cost-effective alternative for expansion of genetic diversity. With its ability to foster rapid microbial evolution, aALE serves as a valuable addition to the ALE toolbox, holding significant promise for the advancement of microbial strain engineering and bioprocess optimization

An efficient transformation method for the bioplastic-producing "Knallgas" bacterium Ralstonia eutropha H16

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 was 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 OD600 0.6. These cells were made competent by a 15-min incubation in 50 mM CaCl2 , followed by two cell washes and final resuspension in 0.2 M sucrose prior to electroporation using 2.3 kV. This protocol achieved a transformation efficiency of (3.86 ± 0.29) x 10(5) cfu/μg DNA, a 10(3) -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

Accelerated directed evolution of dye-decolorizing peroxidase using a bacterial extracellular protein secretion system (BENNY)

Background Dye-decolorizing peroxidases (DyPs) are haem-containing peroxidases that show great promises in industrial biocatalysis and lignocellulosic degradation. Through the use of Escherichia coli osmotically-inducible protein Y (OsmY) as a bacterial extracellular protein secretion system (BENNY), we successfully developed a streamlined directed evolution workflow to accelerate the protein engineering of DyP4 from Pleurotus ostreatus strain PC15. Result After 3 rounds of random mutagenesis with error-prone polymerase chain reaction (epPCR) and 1 round of saturation mutagenesis, we obtained 4D4 variant (I56V, K109R, N227S and N312S) that displays multiple desirable phenotypes, including higher protein yield and secretion, higher specific activity (2.7-fold improvement in kcat/Km) and higher H2O2 tolerance (sevenfold improvement based on IC50). Conclusion To our best knowledge, this is the first report of applying OsmY to simplify the directed evolution workflow and to direct the extracellular secretion of a haem protein such as DyP4

Random and combinatorial mutagenesis for improved total production of secretory target protein in Escherichia coli

Signal peptides and secretory carrier proteins are commonly used to secrete heterologous recombinant protein in Gram-negative bacteria. The Escherichia coli osmotically-inducible protein Y (OsmY) is a carrier protein that secretes a target protein extracellularly, and we have previously applied it in the Bacterial Extracellular Protein Secretion System (BENNY) to accelerate directed evolution. In this study, we reported the first application of random and combinatorial mutagenesis on a carrier protein to enhance total secretory target protein production. After one round of random mutagenesis followed by combining the mutations found, OsmY(M3) (L6P, V43A, S154R, V191E) was identified as the best carrier protein. OsmY(M3) produced 3.1 ± 0.3 fold and 2.9 ± 0.8 fold more secretory Tfu0937 β-glucosidase than its wildtype counterpart in E. coli strains BL21(DE3) and C41(DE3), respectively. OsmY(M3) also produced more secretory Tfu0937 at different cultivation temperatures (37 °C, 30 °C and 25 °C) compared to the wildtype. Subcellular fractionation of the expressed protein confirmed the essential role of OsmY in protein secretion. Up to 80.8 ± 12.2% of total soluble protein was secreted after 15 h of cultivation. When fused to a red fluorescent protein or a lipase from Bacillus subtillis, OsmY(M3) also produced more secretory protein compared to the wildtype. In this study, OsmY(M3) variant improved the extracellular production of three proteins originating from diverse organisms and with diverse properties, clearly demonstrating its wide-ranging applications. The use of random and combinatorial mutagenesis on the carrier protein demonstrated in this work can also be further extended to evolve other signal peptides or carrier proteins for secretory protein production in E. coli

Production of trimeric SARS‐CoV‐2 spike protein by CHO cells for serological COVID‐19 testing

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