Zymomonas mobilis as an emerging biotechnological chassis for the production of industrially relevant compounds

Zymomonas mobilis is a well-recognized ethanologenic bacterium with outstanding characteristics which make it a promising platform for the biotechnological production of relevant building blocks and fine chemicals compounds. In the last years, research has been focused on the physiological, genetic, and metabolic engineering strategies aiming at expanding Z. mobilis ability to metabolize lignocellulosic substrates toward biofuel production. With the expansion of the Z. mobilis molecular and computational modeling toolbox, the potential of this bacterium as a cell factory has been thoroughly explored. The number of genomic, transcriptomic, proteomic, and fluxomic data that is becoming available for this bacterium has increased. For this reason, in the forthcoming years, systems biology is expected to continue driving the improvement of Z. mobilis for current and emergent biotechnological applications. While the existing molecular toolbox allowed the creation of stable Z. mobilis strains with improved traits for pinpointed biotechnological applications, the development of new and more flexible tools is crucial to boost the engineering capabilities of this bacterium. Novel genetic toolkits based on the CRISPR-Cas9 system and recombineering have been recently used for the metabolic engineering of Z. mobilis. However, they are mostly at the proof-of-concept stage and need to be further improved.

Improving mobilization of foreign DNA into Zymomonas mobilis ZM4 by removal of multiple restriction systems

Zymomonas mobilis has emerged as a promising candidate for production of high-value bioproducts from plant biomass. However, a major limitation in equipping Z. mobilis with novel pathways to achieve this goal is restriction of heterologous DNA. Here, we characterized the contribution of several defense systems of Z. mobilis strain ZM4 to impeding heterologous gene transfer from an Escherichia coli donor. Bioinformatic analysis revealed that Z. mobilis ZM4 encodes a previously described mrr-like type IV restriction modification (RM) system, a type I-F CRISPR system, a chromosomal type I RM system (hsdMS_c), and a previously uncharacterized type I RM system, located on an endogenous plasmid (hsdRMS_p). The DNA recognition motif of HsdRMS_p was identified by comparing the methylated DNA sequence pattern of mutants lacking one or both of the hsdMS_c and hsdRMS_p systems to that of the parent strain. The conjugation efficiency of synthetic plasmids containing single or combinations of the HsdMS_c and HsdRMS_p recognition sites indicated that both systems are active and decrease uptake of foreign DNA. In contrast, deletions of mrr and cas3 led to no detectable improvement in conjugation efficiency for the exogenous DNA tested. Thus, the suite of markerless restriction-negative strains that we constructed and the knowledge of this new restriction system and its DNA recognition motif provide the necessary platform to flexibly engineer the next generation of Z. mobilis strains for synthesis of valuable products.

IMPORTANCEZymomonas mobilis is equipped with a number of traits that make it a desirable platform organism for metabolic engineering to produce valuable bioproducts. Engineering strains equipped with synthetic pathways for biosynthesis of new molecules requires integration of foreign genes. In this study, we developed an all-purpose strain, devoid of known host restriction systems and free of any antibiotic resistance markers, which dramatically improves the uptake efficiency of heterologous DNA into Z. mobilis ZM4. We also confirmed the role of a previously known restriction system as well as identifying a previously unknown type I RM system on an endogenous plasmid. Elimination of the barriers to DNA uptake as shown here will allow facile genetic engineering of Z. mobilis.

CRISpy-Pop: A Web Tool for Designing CRISPR/Cas9-Driven Genetic Modifications in Diverse Populations

CRISPR/Cas9 is a powerful tool for editing genomes, but design decisions are generally made with respect to a single reference genome. With population genomic data becoming available for an increasing number of model organisms, researchers are interested in manipulating multiple strains and lines. CRISpy-pop is a web application that generates and filters guide RNA sequences for CRISPR/Cas9 genome editing for diverse yeast and bacterial strains. The current implementation designs and predicts the activity of guide RNAs against more than 1000 Saccharomyces cerevisiae genomes, including 167 strains frequently used in bioenergy research. Zymomonas mobilis, an increasingly popular bacterial bioenergy research model, is also supported. CRISpy-pop is available as a web application (https://CRISpy-pop.glbrc.org/) with an intuitive graphical user interface. CRISpy-pop also cross-references the human genome to allow users to avoid the selection of guide RNAs with potential biosafety concerns. Additionally, CRISpy-pop predicts the strain coverage of each guide RNA within the supported strain sets, which aids in functional population genetic studies. Finally, we validate how CRISpy-pop can accurately predict the activity of guide RNAs across strains using population genomic data.

Zymomonas mobilis metabolism: Novel tools and targets for its rational engineering

Zymomonas mobilis is an α-proteobacterium that interests the biofuel industry due to its perfect ethanol fermentation yields. From its first description as a bacterial isolate in fermented alcoholic beverages to date, Z. mobilis has been rigorously studied in directions basic and applied. The Z. mobilis powerful Entner-Doudoroff glycolytic pathway has been the center of rigorous biochemical studies and, aside from ethanol, it has attracted interest in terms of high-added-value chemical manufacturing. Energetic balances and the effects of respiration have been explored in fundamental directions as also in applications pursuing strain enhancement and the utilization of alternative carbon sources. Metabolic modeling has addressed the optimization of the biochemical circuitry at various conditions of growth and/or substrate utilization; it has been also critical in predicting desirable end-product yields via flux redirection. Lastly, stress tolerance has received particular attention, since it directly determines biocatalytical performance at challenging bioreactor conditions. At a genetic level, advances in the genetic engineering of the organism have brought forth beneficial manipulations in the Z. mobilis gene pool, e.g., knock-outs, knock-ins and gene stacking, aiming to broaden the metabolic repertoire and increase robustness. Recent omic and expressional studies shed light on the genomic content of the most applied strains and reveal landscapes of activity manifested at ambient or reactor-based conditions. Studies such as those reviewed in this work, contribute to the understanding of the biology of Z. mobilis, enable insightful strain development, and pave the way for the transformation of Z. mobilis into a consummate organism for biomass conversion.

Advances and prospects in metabolic engineering of Zymomonas mobilis

Biorefinery of biomass-based biofuels and biochemicals by microorganisms is a competitive alternative of traditional petroleum refineries. Zymomonas mobilis is a natural ethanologen with many desirable characteristics, which makes it an ideal industrial microbial biocatalyst for commercial production of desirable bioproducts through metabolic engineering. In this review, we summarize the metabolic engineering progress achieved in Z. mobilis to expand its substrate and product ranges as well as to enhance its robustness against stressful conditions such as inhibitory compounds within the lignocellulosic hydrolysates and slurries. We also discuss a few metabolic engineering strategies that can be applied in Z. mobilis to further develop it as a robust workhorse for economic lignocellulosic bioproducts. In addition, we briefly review the progress of metabolic engineering in Z. mobilis related to the classical synthetic biology cycle of "Design-Build-Test-Learn", as well as the progress and potential to develop Z. mobilis as a model chassis for biorefinery practices in the synthetic biology era.