Tissue culture selection techniques were used to isolate a maize (Zea mays L.) variant D33, in which the aspartate family pathway was less sensitive to feedback inhibition by lysine. D33 was recovered by successively subculturing cultures originally derived from immature embryos on MS medium containing growth-inhibitory levels of lysine+threonine. The ability of D33 to grow vigorously on lysine+ threonine medium was retained after growth for 12 months on nonselection medium. New cultures initiated from shoot tissues of plants regenerated from D33 also were resistant to lysine+threonine inhibition. The Ki of aspartokinase for its feedback inhibitor, lysine, was about 9-fold higher in D33 than for the enzyme from unselected cultures. The free pools of lysine, threonine, isoleucine and methionine were increased 2-9-fold in D33 cultures. This was consistent with the observed change in feedback regulation of aspartokinase, the first enzyme common to the biosynthesis of these amino acids in the aspartate pathway. The accumulated evidence including the stability of resistance in the cultures, the resistance of cultures initiated from regenerated plants, the altered feedback regulation, and the increased free amino acids, indicates a mutational origin for these traits in line D33.
N-alkylated amino acids are intermediates of natural biological pathways and can be found incorporated in peptides or have physiological roles in their free form. The N-ethylated amino acid L-theanine shows taste-enhancing properties and health benefits. It naturally occurs in green tea as major free amino acid. Isolation of L-theanine from Camilla sinensis shows low efficiency, and chemical synthesis results in a racemic mixture. Therefore, biochemical approaches for the production of L-theanine gain increasing interest. Here, we describe metabolic engineering of Pseudomonas putida KT2440 for the fermentative production of Ltheanine from monoethylamine and carbon sources glucose, glycerol, or xylose using heterologous enzymes from Methylorubrum extorquens for L-theanine production and heterologous enzymes from Caulobacter crescentus for growth with xylose. L-Theanine (15.4 mM) accumulated in shake flasks with minimal medium containing monoethylamine and glucose, 15.2 mM with glycerol and 7 mM with xylose. Fed-batch bioreactor cultures yielded L-theanine titers of 10 g L −1 with glucose plus xylose, 17.2 g L −1 with glycerol, 4 g L −1 with xylose, and 21 g L −1 with xylose plus glycerol, respectively. To the best of our knowledge, this is the first L-theanine process using P. putida and the first compatible with the use of various alternative carbon sources.
Members of the genus Paenibacillus are widespread facultative anaerobic, endospore-forming bacteria. Some species such as Paenibacillus riograndensis or Paenibacillus polymyxa fix nitrogen and may play an important role in agriculture to reduce mineral nitrogen fertilization in particular for non-legume plants. The genetic manipulation of Paenibacillus is an imperative for the functional characterization, e.g., of its plant growth-promoting activities and metabolism. This study showed that P. riograndensis and P. polymyxa can be readily transformed using physical permeation by magnesium aminoclays. By means of the fluorescent reporter genes gfpUV, mcherry, and crimson, a two-plasmid system consisting of a theta-replicating plasmid and a rolling circle-replicating plasmid was shown to operate in both species. Xylose-inducible and mannitol-inducible fluorescent reporter gene expression was demonstrated in the compatible two-plasmid system by fluorescence-activated cell scanning. As a metabolic engineering application, the biotin requiring P. riograndensis was converted to a biotin-prototrophic strain based on mannitol-inducible expression of the biotin biosynthesis operon bioWAFDBI from Bacillus subtilis.
N-methylated amino acids are present in diverse biological molecules in bacteria, archaea and eukaryotes. There is an increasing interest in this molecular class of alkylated amino acids by the pharmaceutical and chemical industries. N-alkylated amino acids have desired functions such as higher proteolytic stability, enhanced membrane permeability and longer peptide half-lives, which are important for the peptide-based drugs, the so-called peptidomimetics. Chemical synthesis of N-methylated amino acids often is limited by incomplete stereoselectivity, over-alkylation or the use of hazardous chemicals. Here, we describe metabolic engineering of Pseudomonas putida KT2440 for the fermentative production of N-methylglutamate from simple carbon sources and monomethylamine. P. putida KT2440, which is generally recognized as safe and grows with glucose and the alternative feedstock glycerol as sole carbon and energy source, was engineered for the production of N-methylglutamate using heterologous enzymes from Methylobacterium extorquens. About 3.9 g L−1 N-methylglutamate accumulated within 48 h in shake flask cultures with minimal medium containing monomethylamine and glycerol. A fed-batch cultivation process yielded a N-methylglutamate titer of 17.9 g L−1.
The N-functionalized amino acid N-methylanthranilate is an important precursor for bioactive compounds such as anticancer acridone alkaloids, the antinociceptive alkaloid O-isopropyl N-methylanthranilate, the flavor compound O-methyl-N-methylanthranilate, and as a building block for peptide-based drugs. Current chemical and biocatalytic synthetic routes to N-alkylated amino acids are often unprofitable and restricted to low yields or high costs through cofactor regeneration systems. Amino acid fermentation processes using the Gram-positive bacterium Corynebacterium glutamicum are operated industrially at the million tons per annum scale. Fermentative processes using C. glutamicum for N-alkylated amino acids based on an imine reductase have been developed, while N-alkylation of the aromatic amino acid anthranilate with S-adenosyl methionine as methyl-donor has not been described for this bacterium. After metabolic engineering for enhanced supply of anthranilate by channeling carbon flux into the shikimate pathway, preventing by-product formation and enhancing sugar uptake, heterologous expression of the gene anmt encoding anthranilate N-methyltransferase from Ruta graveolens resulted in production of N-methylanthranilate (NMA), which accumulated in the culture medium. Increased SAM regeneration by coexpression of the homologous adenosylhomocysteinase gene sahH improved N-methylanthranilate production. In a test bioreactor culture, the metabolically engineered C. glutamicum C1* strain produced NMA to a final titer of 0.5 g·L−1 with a volumetric productivity of 0.01 g·L−1·h−1 and a yield of 4.8 mg·g−1 glucose.
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