Living cell reaction process forl-isoleucine andl-valine production

Production of 4-Hydroxybenzoic Acid by an Aerobic Growth-Arrested Bioprocess Using Metabolically Engineered Corynebacterium glutamicum

Corynebacterium glutamicum was metabolically engineered to produce 4-hydroxybenzoic acid (4-HBA), a valuable aromatic compound used as a raw material for the production of liquid crystal polymers and paraben. C. glutamicum was found to have a higher tolerance to 4-HBA toxicity than previously reported hosts used for the production of genetically engineered 4-HBA. To obtain higher titers of 4-HBA, we employed a stepwise overexpression of all seven target genes in the shikimate pathway in C. glutamicum Specifically, multiple chromosomal integrations of a mutated aroG gene from Escherichia coli, encoding a 3-deoxy-d-arabinoheptulosonic acid 7-phosphate (DAHP) synthase, and wild-type aroCKB from C. glutamicum, encoding chorismate synthase, shikimate kinase, and 3-dehydroquinate synthase, were effective in increasing product titers. The last step of the 4-HBA biosynthesis pathway was recreated in C. glutamicum by expressing a highly 4-HBA-resistant chorismate pyruvate-lyase (UbiC) from the intestinal bacterium Providencia rustigianii To enhance the yield of 4-HBA, we reduced the formation of by-products, such as 1,3-dihydroxyacetone and pyruvate, by deleting hdpA, a gene coding for a haloacid dehalogenase superfamily phosphatase, and pyk, a gene coding for a pyruvate kinase, from the bacterial chromosome. The maximum concentration of 4-HBA produced by the resultant strain was 36.6 g/liter, with a yield of 41% (mol/mol) glucose after incubation for 24 h in minimal medium in an aerobic growth-arrested bioprocess using a jar fermentor. To our knowledge, this is the highest concentration of 4-HBA produced by a metabolically engineered microorganism ever reported.IMPORTANCE Since aromatic compound 4-HBA has been chemically produced from petroleum-derived phenol for a long time, eco-friendly bioproduction of 4-HBA from biomass resources is desired in order to address environmental issues. In microbial chemical production, product toxicity often causes problems, but we confirmed that wild-type C. glutamicum has high tolerance to the target 4-HBA. A growth-arrested bioprocess using this microorganism has been successfully used for the production of various compounds, such as biofuels, organic acids, and amino acids. However, no production method has been applied for aromatic compounds to date. In this study, we screened for a novel final reaction enzyme possessing characteristics superior to those in previously employed microbial 4-HBA production. We demonstrated that the use of the highly 4-HBA-resistant UbiC from the intestinal bacterium P. rustigianii is very effective in increasing 4-HBA production.

Branched-Chain Amino Acids

Branched-chain amino acids (BCAAs), viz., L-isoleucine, L-leucine, and L-valine, are essential amino acids that cannot be synthesized in higher organisms and are important nutrition for humans as well as livestock. They are also valued as synthetic intermediates for pharmaceuticals. Therefore, the demand for BCAAs in the feed and pharmaceutical industries is increasing continuously. Traditional industrial fermentative production of BCAAs was performed using microorganisms isolated by random mutagenesis. A collection of these classical strains was also scientifically useful to clarify the details of the BCAA biosynthetic pathways, which are tightly regulated by feedback inhibition and transcriptional attenuation. Based on this understanding of the metabolism of BCAAs, it is now possible for us to pursue strains with higher BCAA productivity using rational design and advanced molecular biology techniques. Additionally, systems biology approaches using augmented omics information help us to optimize carbon flux toward BCAA production. Here, we describe the biosynthetic pathways of BCAAs and their regulation and then overview the microorganisms developed for BCAA production. Other chemicals, including isobutanol, i.e., a second-generation biofuel, can be synthesized by branching the BCAA biosynthetic pathways, which are also outlined.

Improvement of the redox balance increases L-valine production by Corynebacterium glutamicum under oxygen deprivation conditions

Production of L-valine under oxygen deprivation conditions by Corynebacterium glutamicum lacking the lactate dehydrogenase gene ldhA and overexpressing the L-valine biosynthesis genes ilvBNCDE was repressed. This was attributed to imbalanced cofactor production and consumption in the overall L-valine synthesis pathway: two moles of NADH was generated and two moles of NADPH was consumed per mole of L-valine produced from one mole of glucose. In order to solve this cofactor imbalance, the coenzyme requirement for L-valine synthesis was converted from NADPH to NADH via modification of acetohydroxy acid isomeroreductase encoded by ilvC and introduction of Lysinibacillus sphaericus leucine dehydrogenase in place of endogenous transaminase B, encoded by ilvE. The intracellular NADH/NAD(+) ratio significantly decreased, and glucose consumption and L-valine production drastically improved. Moreover, L-valine yield increased and succinate formation decreased concomitantly with the decreased intracellular redox state. These observations suggest that the intracellular NADH/NAD(+) ratio, i.e., reoxidation of NADH, is the primary rate-limiting factor for L-valine production under oxygen deprivation conditions. The L-valine productivity and yield were even better and by-products derived from pyruvate further decreased as a result of a feedback resistance-inducing mutation in the acetohydroxy acid synthase encoded by ilvBN. The resultant strain produced 1,470 mM L-valine after 24 h with a yield of 0.63 mol mol of glucose(-1), and the L-valine productivity reached 1,940 mM after 48 h.

A new insertion sequence, IS14999, from Corynebacterium glutamicum

A new insertion sequence from Corynebacterium glutamicum ATCC 14999 was isolated and characterized. This IS element, designated IS14999, comprised a 1149 bp nucleotide sequence with 22 bp imperfect terminal inverted repeats. IS14999 carries a single open reading frame of 345 amino acids encoding a putative transposase that appears to have partial homology to IS642, an IS630/Tc1 superfamily element, at the C-terminal region in the amino acid sequence. This indicated that IS14999 belonged to the IS630/Tc1 superfamily, which was first identified in C. glutamicum. IS14999 has a unique distance of 38 amino acid residues between the second and third amino acids in the DDE motif, which is well known as the catalytic centre of transposase. This suggested that IS14999 constituted a new subfamily of the IS630/Tc1 superfamily. A phylogenetic tree constructed on the basis of amino acid sequences of transposases revealed that this new transposable element was more similar to eukaryotic Tc1/mariner family elements than to prokaryotic IS630 family elements. Added to the fact that IS14999 was present in only a few C. glutamicum strains, this implies that IS14999 was probably acquired by a recent lateral transfer event from eukaryotic cells. Analysis of the insertion site in C. glutamicum R revealed that IS14999 appeared to transpose at random and always caused a target duplication of a 5'-TA-3' dinucleotide upon insertion, like the other IS630/Tc1 family elements. These findings indicated that IS14999 could be a powerful tool for genetic manipulation of corynebacteria and related species.

Metabolic analysis of Corynebacterium glutamicum during lactate and succinate productions under oxygen deprivation conditions

Lactate and succinate were produced from glucose by Corynebacterium glutamicum under oxygen deprivation conditions without growth. Addition of bicarbonate to the reaction mixture led not only to a 3.6-fold increase in succinate production rate, but also to a 2.3- and 2.5-fold increase, respectively, of the rates of lactate production and glucose consumption, compared to the control. Furthermore, when small amounts of pyruvate were added to the reaction mixture, acid production rates and the glucose consumption rate were multiplied by a factor ranging from 2 to 3. These phenomena were paralleled by an increase in the NAD(+)/NADH ratio, thus corroborating the view that the efficient regeneration of NAD(+) could be triggered by the addition of either bicarbonate or pyruvate. To investigate the global metabolism of corynebacteria under oxygen deprivation conditions, we engineered several strains where the genes coding for key metabolic enzymes had been inactivated by gene disruption and replacement. A lactate dehydrogenase (LDH)-deficient mutant was not able to produce lactate, suggesting this enzyme has no other isozyme. Although a pyruvate carboxylase (pyc) mutant exhibited similar behavior to that of the wild type, phosphoenolpyruvate carboxylase (ppc) mutants were characterized by a dramatic decrease in succinate production, which was concomitant to decreased lactate production and glucose consumption rates. This set of observations corroborates the view that in coryneform bacteria under oxygen deprivation conditions the major anaplerotic reaction is driven by the ppc gene product rather than by the pyc gene product. Moreover, intracellular NADH concentrations in C. glutamicum were observed to correlate to oxygen-deprived metabolic flows.

Identification and sequence determination of the acetohydroxy acid isomeroreductase gene from Brevibacterium flavum MJ233

The enzyme acetohydroxy acid isomeroreductase (AHAIR) is the second enzyme in the parallel isoleucine-valine biosynthetic pathway. We previously reported the cloning and sequencing of the acetohydroxy acid synthase (AHAS) genes from Brevibacterium flavum MJ233. Analysis of the sequence downstream of the AHAS genes identified another open reading frame highly homologous at the amino acid level to the AHAIR gene from Escherichia coli (ilvC). We subcloned the B. flavum AHAIR gene on a 2.1 kb Bg/II-EcoRI fragment by complementation of an E. coli ilvC mutant. The nucleotide sequence of the B. flavum AHAIR gene consists of 338 codons (molecular weight of 36158). Comparison of the deduced protein sequence revealed a high degree of identity with the sequences of ilvC genes from other organisms. Disruption of the B. flavum ilvC gene by a kanamycin resistance cassette resulted in L-isoleucine and L-valine auxotrophy.

Genomic organization of the biotin biosynthetic genes of coryneform bacteria: cloning and sequencing of the bioA-bioD genes from Brevibacterium flavum

Three coryneform bacteria, Brevibacterium flavum, Brevibacterium lactofermentum and Corynebacterium glutamicum have been shown to be able to convert 7-keto-8-aminopelargonic acid to biotin through a biotin synthetic pathway identical to that from Escherichia coli (Hatakeyama et al., DNA Sequence, in press, 1993). We report in this paper the cloning and sequencing of the biotin biosynthetic genes encoding the 7,8-diaminopelargonic acid aminotransferase (bioA) and the dethiobiotin synthetase (bioD) of B. flavum MJ233, by complementation of E. coli bioA and bioD mutants. Both bioA and bioD genes from B. flavum were located on a 4.0-kb SalI DNA fragment. Nucleotide sequence analysis of this fragment revealed that these genes consist of a 1272 bp and a 675 bp open reading frame, respectively. The deduced amino acid sequence of the 7,8-diaminopelargonic acid aminotransferase (BioA) is 51.3% and 31.9% identical to that of the E. coli and Bacillus spaericus bioA gene products, respectively. The deduced amino acid sequence of the dethiobiotin synthetase (BioD) is 25.9% and 32.7% identical to that of the E. coli and B. sphaericus bioD gene products, respectively. In addition, the genomic organization of the bioA, bioB and bioD genes in B. flavum has been shown to be different from that in E. coli and B. sphaericus.

Cloning and sequence determination of the acetohydroxy acid synthase genes from Brevibacterium flavum MJ233 by using the polymerase chain reaction

Taking advantage of highly conserved domains present in the three acetohydroxy acid synthase (AHAS) isozymes from E. coli K-12, we designed degenerate oligonucleotides corresponding to these regions. These synthetic DNA sequences were used as primers in polymerase chain reactions in order to amplify DNA sequences from Brevibacterium flavum MJ233 chromosomal DNA. The polymerase chain reaction product was used as a probe to recover genomic fragments from a lambda library of B. flavum MJ233. A 5.8-kb EcoRI fragment hybridizing to the probe was isolated and amplification of this fragment in a B. flavum strain resulted in increased AHAS-specific activity. Sequence analysis revealed two open reading frames (ilvL and ilvS) highly homologous at the amino acid level to the corresponding domains of the three AHAS isozymes of E. coli K-12. Moreover, disruption of the putative ilvL gene by a kanamycin resistance cassette resulted in L-isoleucine and L-valine auxotrophy. These observations demonstrate that the cloned fragment encodes the AHAS gene of B. flavum MJ233.