Biofuel production with a stress-resistant and growth phase-independent promoter: mechanism revealed by in vitro transcription assays

Microbial host engineering for sustainable isobutanol production from renewable resources

Due to the limited resources and environmental problems associated with fossil fuels, there is a growing interest in utilizing renewable resources for the production of biofuels through microbial fermentation. Isobutanol is a promising biofuel that could potentially replace gasoline. However, its production efficiency is currently limited by the use of naturally isolated microorganisms. These naturally isolated microorganisms often encounter problems such as a limited range of substrates, low tolerance to solvents or inhibitors, feedback inhibition, and an imbalanced redox state. This makes it difficult to improve their production efficiency through traditional process optimization methods. Fortunately, recent advancements in genetic engineering technologies have made it possible to enhance microbial hosts for the increased production of isobutanol from renewable resources. This review provides a summary of the strategies and synthetic biology approaches that have been employed in the past few years to improve naturally isolated or non-natural microbial hosts for the enhanced production of isobutanol by utilizing different renewable resources. Furthermore, it also discusses the challenges that are faced by engineered microbial hosts and presents future perspectives to enhancing isobutanol production. KEY POINTS: • Promising potential of isobutanol to replace gasoline • Engineering of native and non-native microbial host for isobutanol production • Challenges and opportunities for enhanced isobutanol production.

Identification of functional sgRNA mutants lacking canonical secondary structure using high-throughput FACS screening

Coexpressing multiple identical single guide RNAs (sgRNAs) in CRISPR-dependent engineering triggers genetic instability and phenotype loss. To provide sgRNA derivatives for efficient DNA digestion, we design a high-throughput digestion-activity-dependent positive screening strategy and astonishingly obtain functional nonrepetitive sgRNA mutants with up to 48 out of the 61 nucleotides mutated, and these nonrepetitive mutants completely lose canonical secondary sgRNA structure in simulation. Cas9-sgRNA complexes containing these noncanonical sgRNAs maintain wild-type level of digestion activities in vivo, indicating that the Cas9 protein is compatible with or is able to adjust the secondary structure of sgRNAs. Using these noncanonical sgRNAs, we achieve multiplex genetic engineering for gene knockout and base editing in microbial cell factories. Libraries of strains with rewired metabolism are constructed, and overproducers of isobutanol or 1,3-propanediol are identified by biosensor-based fluorescence-activated cell sorting (FACS). This work sheds light on the remarkable flexibility of the secondary structure of functional sgRNA.

Engineering Transcription Factor BmoR Mutants for Constructing Multifunctional Alcohol Biosensors

Native transcription factor-based biosensors (TFBs) have the potential for the in situ detection of value-added chemicals or byproducts. However, their industrial application is limited by their ligand promiscuity, low sensitivity, and narrow detection range. Alcohols exhibit similar structures, and no reported TFB can distinguish a specific alcohol from its analogues. Here, we engineered an alcohol-regulated transcription factor, BmoR, and obtained various mutants with remarkable properties. For example, the generated signal-molecule-specific BmoRs could distinguish the constitutional isomers n-butanol and isobutanol, with insensitivity up to an ethanol concentration of 800 mM (36.9 g/L). Linear detection of 0-60 mM of a specific higher alcohol could be achieved in the presence of up to 500 mM (23.0 g/L) ethanol as background noise. Furthermore, we obtained two mutants with raised outputs and over 10⁷-fold higher sensitivity and one mutant with an increased upper detection limit (14.8 g/L n-butanol or isobutanol). Using BmoR as an example, this study systematically explored the ultimate detection limit of a TFB toward its small-molecule ligands, paving the way for in situ detection in biofuel and wine industries.

Reconstructing the transcription regulatory network to optimize resource allocation for robust biosynthesis

An ideal microbial cell factory (MCF) should deliver maximal resources to production, which conflicts with the microbe's native growth-oriented resource allocation strategy and can therefore lead to early termination of the high-yield period. Reallocating resources from growth to production has become a critical factor in constructing robust MCFs. Instead of strengthening specific biosynthetic pathways, emerging endeavors are focused on rearranging the gene regulatory network to fundamentally reprogram the resource allocation pattern. Combining this idea with transcriptional regulation within the hierarchical regulatory network, this review discusses recent engineering strategies targeting the transcription machinery, module networks, regulatory edges, and bottom network layer. This global view will help to construct a production-oriented phenotype that fully harnesses the potential of MCFs.

Converting Escherichia coli MG1655 into a chemical overproducer through inactivating defense system against exogenous DNA

Escherichia coli strain K-12 MG1655 has been proposed as an appropriate host strain for industrial production. However, the direct application of this strain suffers from the transformation inefficiency and plasmid instability. Herein, we conducted genetic modifications at a serial of loci of MG1655 genome, generating a robust and universal host strain JW128 with higher transformation efficiency and plasmid stability that can be used to efficiently produce desired chemicals after introducing the corresponding synthetic pathways. Using JW128 as the host, the titer of isobutanol reached 5.76 g/L in shake-flask fermentation, and the titer of lycopene reached 1.91 g/L in test-tube fermentation, 40-fold and 5-fold higher than that of original MG1655, respectively. These results demonstrated JW128 is a promising chassis for high-level production of value-added chemicals.

Protein-based biorefining driven by nitrogen-responsive transcriptional machinery

BACKGROUND

Protein-based bioconversion has been demonstrated as a sustainable approach to produce higher alcohols and ammonia fertilizers. However, owing to the switchover from transcription mediated by the bacterial RNA polymerase σ⁷⁰ to that mediated by alternative σ factors, the biofuel production driven by σ⁷⁰-dependent promoters declines rapidly once cells enter the stationary phase or encounter stresses. To enhance biofuel production, in this study the growth phase-independent and nitrogen-responsive transcriptional machinery mediated by the σ⁵⁴ is exploited to drive robust protein-to-fuel conversion.

RESULTS

We demonstrated that disrupting the Escherichia coli ammonia assimilation pathways driven by glutamate dehydrogenase and glutamine synthetase could sustain the activity of σ⁵⁴-mediated transcription under ammonia-accumulating conditions. In addition, two σ⁵⁴-dependent promoters, argTp and glnAp2, were identified as suitable candidates for driving pathway expression. Using these promoters, biofuel production from proteins was shown to persist to the stationary phase, with the net production in the stationary phase being 1.7-fold higher than that derived from the optimal reported σ⁷⁰-dependent promoter P _LlacO₁. Biofuel production reaching levels 1.3- to 3.4-fold higher than those of the σ⁷⁰-dependent promoters was also achieved by argTp and glnAp2 under stressed conditions. Moreover, the σ⁵⁴-dependent promoters realized more rapid and stable production than that of σ⁷⁰-dependent promoters during fed-batch fermentation, producing up to 4.78 g L ^- 1 of total biofuels.

CONCLUSIONS

These results suggested that the nitrogen-responsive transcriptional machinery offers the potential to decouple production from growth, highlighting this system as a novel candidate to realize growth phase-independent and stress-resistant biofuel production.

Context-dependency of synthetic minimal promoters in driving gene expression: a case study

Synthetic promoters are considered ideal candidates in driving robust gene expression. Most of the available synthetic promoters are minimal promoters, for which the upstream sequence of the 5' end of the core region is usually excluded. Although the upstream sequence has been shown to mediate transcription of natural promoters, its impact on synthetic promoters has not been widely studied. Here, a library of chromosomal DNA fragments is randomly fused with the 5' end of the J23119 synthetic promoter, and the transcriptional performance of the promoter is evaluated through β-galactosidase assay, fluorescence intensity and chemical biosynthesis. Results show that changes in the upstream sequence can induce significant variation in the promoter strength of up to 5.8-fold. The effect is independent of the length of the insertions and the number of potential transcription factor binding sites. Several DNA fragments that are able to enhance the transcription of both the natural and the synthetic promoters are identified. This study indicates that the synthetic minimal promoters are susceptible to the surrounding sequence context. Therefore, the upstream sequence should be treated as an indispensable component in the design and application of synthetic promoters, or as an independent genetic part for the fine-tuning of gene expression.

Engineering transcription factor BmoR for screening butanol overproducers

The wild-type transcription factors are sensitive to their corresponding signal molecules. Using wild-type transcription factors as biosensors to screen industrial overproducers are generally impractical because of their narrow detection ranges. This study took transcription factor BmoR as an example and aimed to expand the detection range of BmoR for screening alcohols overproducers. Firstly, a BmoR mutation library was established, and the mutations distributed randomly in all predicted functional domains of BmoR. Structure of BmoR-isobutanol complex were modelled, and isobutanol binding sites were confirmed by site-directed mutagenesis. Subsequently, the effects of the mutations on the detection range or output were confirmed in the BmoR mutants. Four combinatorial mutants containing one increased-detection-range mutation and one enhanced-output mutation were constructed. Compared with wild-type BmoR, F276A/E627N BmoR and D333N/E627N BmoR have wider detection ranges (0-100 mM) and relatively high outputs to the isobutanol added quantitatively or produced intracellularly, demonstrating they have potential for screening isobutanol overproduction strains. This work presented an example of engineering the wild-type transcription factors with physiological significance for industrial utilization.

A novel protein purification strategy mediated by the combination of CipA and Ssp DnaB intein

Protein purification is an indispensable step in diverse fields of biological research or production process. Conventional purification methods including the affinity purification or the usage of self-aggregating tags suffered from many drawbacks such as the complicated steps, high cost and low efficiency. Moreover, the fusion tag usually had negative effects on the activity of the target protein. To address the above issues, here we propose a novel protein purification method which needs simple operation steps, and this method is mediated by the combination of CipA protein and a mini-intein (Synechocystis sp. PCC6803 DnaB, Ssp DnaB), depending on the assembly function of CipA and the self-cleavage function of Ssp DnaB. To realize the purification, CipA-DnaB-eGFP protein was expressed and assembled into protein crystalline inclusions (PCIs) in E. coli. Then, only cell lysis, cleavage and centrifugation steps were required to purify eGFP. Purified eGFP was in the supernatant with a purity of over 90%. The cleavage efficiency and the yield of eGFP reached 51.96% and 13.99 ± 0.88 mg/L fermentation broth, respectively. Furthermore, to broaden the application of this approach, three other proteins which were maltose binding protein (MBP), ketoisovalerate decarboxylase (Kivd) and alcohol dehydrogenase (AdhP) were purified with high cleavage efficiency. The purified Kivd and AdhP remained high specific activities. This work demonstrated an effective and convenient protein purification method.