Metabolic engineering of Bacillus subtilis for l-valine overproduction. 2018

Adam W Westbrook, and Xiang Ren, and Murray Moo-Young, and C Perry Chou
Department of Chemical Engineering, University of Waterloo, Waterloo, Ontario, Canada.

Bacillus subtilis has been commonly applied to industrial enzyme production due to its genetic tractability, "generally recognized as safe (GRAS)" status, and robust growth characteristics. In spite of its ideal attributes as a biomanufacturing platform, B. subtilis has seen limited use in the production of other value-added biochemicals. Here, we report the derivation of engineered strains of B. subtilis for l-valine overproduction using our recently developed CRISPR (clustered regularly interspaced palindromic repeats)-Cas9 (CRISPR-associated [protein] 9) toolkit. We first manipulate the native l-valine biosynthetic pathway by relieving transcriptional and allosteric regulation, resulting in a >14-fold increase in the l-valine titer, compared to the wild-type strain. We subsequently identify and eliminate factors limiting l-valine overproduction, specifically increasing pyruvate availability and blocking the competing l-leucine and l-isoleucine biosynthetic pathways. By inactivating (a) pdhA, encoding the E1α subunit of the pyruvate dehydrogenase complex, to increase the intracellular pyruvate pool, and (b) leuA and ilvA, respectively encoding 2-isopropylmalate synthase and l-threonine dehydratase, to abolish the competing pathways, the l-valine titer reached 4.61 g/L in shake flask cultures. Our engineered l-valine-overproducing strains of B. subtilis are devoid of plasmids and do not sporulate due to the inactivation of sigF, encoding the sporulation-specific transcription factor σ F , making them attractive for large-scale l-valine production. However, acetate dissimilation was identified as limiting l-valine overproduction in ΔpdhA B. subtilis strains, and improving acetate dissimilation or identifying alternate modes of increasing pyruvate pools to enhance l-valine-overproduction should be explored.

UI MeSH Term Description Entries
D010957 Plasmids Extrachromosomal, usually CIRCULAR DNA molecules that are self-replicating and transferable from one organism to another. They are found in a variety of bacterial, archaeal, fungal, algal, and plant species. They are used in GENETIC ENGINEERING as CLONING VECTORS. Episomes,Episome,Plasmid
D005822 Genetic Vectors DNA molecules capable of autonomous replication within a host cell and into which other DNA sequences can be inserted and thus amplified. Many are derived from PLASMIDS; BACTERIOPHAGES; or VIRUSES. They are used for transporting foreign genes into recipient cells. Genetic vectors possess a functional replicator site and contain GENETIC MARKERS to facilitate their selective recognition. Cloning Vectors,Shuttle Vectors,Vectors, Genetic,Cloning Vector,Genetic Vector,Shuttle Vector,Vector, Cloning,Vector, Genetic,Vector, Shuttle,Vectors, Cloning,Vectors, Shuttle
D000072669 Gene Editing Genetic engineering or molecular biology techniques that involve DNA REPAIR mechanisms for incorporating site-specific modifications into a cell's genome. Base Editing,Genome Editing,Editing, Base,Editing, Gene,Editing, Genome
D000076987 CRISPR-Associated Protein 9 An RNA-guided endodeoxyribonuclease that associates with CRISPR SEQUENCES in STREPTOCOCCUS PYOGENES and other bacteria where it participates in an adaptive immune function to cleave foreign DNA complimentary to small GUIDE RNA (sgRNAs). Structurally, Cas9 consists of an ALPHA-HELIX module and a nuclease module connected by a single helix. The nuclease module contains two enzymatic domains: RuvC, which cleaves non-target DNA strand, and an HNH nuclease domain, which cleaves the target strand. Specificity for the DNA target depends on the presence of a protospacer adjacent motif (PAM) sequence, a 2-6 nucleotide DNA sequence immediately following the sequence targeted by Cas9. Cas9 Endonuclease,Cas9 Enzyme,Cas9 Protein,CRISPR Associated Protein 9,Endonuclease, Cas9,Enzyme, Cas9
D001412 Bacillus subtilis A species of gram-positive bacteria that is a common soil and water saprophyte. Natto Bacteria,Bacillus subtilis (natto),Bacillus subtilis subsp. natto,Bacillus subtilis var. natto
D014633 Valine A branched-chain essential amino acid that has stimulant activity. It promotes muscle growth and tissue repair. It is a precursor in the penicillin biosynthetic pathway. L-Valine,L Valine
D015533 Transcriptional Activation Processes that stimulate the GENETIC TRANSCRIPTION of a gene or set of genes. Gene Activation,Genetic Induction,Transactivation,Induction, Genetic,Trans-Activation, Genetic,Transcription Activation,Activation, Gene,Activation, Transcription,Activation, Transcriptional,Genetic Trans-Activation,Trans Activation, Genetic
D015964 Gene Expression Regulation, Bacterial Any of the processes by which cytoplasmic or intercellular factors influence the differential control of gene action in bacteria. Bacterial Gene Expression Regulation,Regulation of Gene Expression, Bacterial,Regulation, Gene Expression, Bacterial
D053858 Metabolic Networks and Pathways Complex sets of enzymatic reactions connected to each other via their product and substrate metabolites. Metabolic Networks,Metabolic Pathways,Metabolic Network,Metabolic Pathway,Network, Metabolic,Networks, Metabolic,Pathway, Metabolic,Pathways, Metabolic
D055786 Gene Knockout Techniques Techniques to alter a gene sequence that result in an inactivated gene, or one in which the expression can be inactivated at a chosen time during development to study the loss of function of a gene. Gene Knock-Out Techniques,Gene Knock Out,Gene Knock Out Techniques,Gene Knockout,Gene Knock Outs,Gene Knock-Out Technique,Gene Knockout Technique,Gene Knockouts,Knock Out, Gene,Knock Outs, Gene,Knock-Out Technique, Gene,Knock-Out Techniques, Gene,Knockout Technique, Gene,Knockout Techniques, Gene,Knockout, Gene,Knockouts, Gene,Out, Gene Knock,Outs, Gene Knock,Technique, Gene Knock-Out,Technique, Gene Knockout,Techniques, Gene Knock-Out,Techniques, Gene Knockout

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