L-Lysine production independent of the oxidative pentose phosphate pathway by Corynebacterium glutamicum with the Streptococcus mutans gapN gene.
Takeno, S., Hori, K., Ohtani, S., Mimura, A., Mitsuhashi, S. & Ikeda, M. (2016). Metabolic engineering, 37, 1-10.
We have recently developed a Corynebacterium glutamicum strain that generates NADPH via the glycolytic pathway by replacing endogenous NAD-dependent glyceraldehyde 3-phosphate dehydrogenase (GapA) with a nonphosphorylating NADP-dependent glyceraldehyde 3-phosphate dehydrogenase (GapN) from Streptococcus mutans. Strain RE2, a suppressor mutant spontaneously isolated for its improved growth on glucose from the engineered strain, was proven to be a high-potential host fof L-lysine production (Takeno et al., 2010). In this study, the suppressor mutation was identified to be a point mutation in rho encoding the transcription termination factor Rho. Strain RE2 still showed retarded growth despite the mutation rho696. Our strategy for reconciling improved growth with a high level of L-lysine production was to use GapA together with GapN only in the early growth phase, and subsequently shift this combination-type glycolysis to one that depends only on GapN in the rest of the growth phase. To achieve this, we expressed gapA under the myo-inositol-inducible promoter of iolT1 encoding a myo-inositol transporter in strain RE2. The resulting strain RE2Aiol was engineered into an L-lysine producer by introduction of a plasmid carrying the desensitized lysC, followed by examination for culture conditions with myo-inositol supplementation. We found that as a higher concentration of myo-inositol was added to the seed culture, the following fermentation period became shorter while maintaining a high level of L-lysine production. This finally reached a fermentation period comparable to that of the control GapA strain, and yielded a 1.5-fold higher production rate compared with strain RE2. The transcript level of gapA, as well as the GapA activity, in the early growth phase increased in proportion to the myo-inositol concentration and then fell to low levels in the subsequent growth phase, indicating that improved growth was a result of increased GapA activity, especially in the early growth phase. Moreover, blockade of the pentose phosphate pathway through a defect in glucose 6-phosphate dehydrogenase did not significantly affect L-lysine production in the engineered GapN strains, while a drastic decrease in L-lysine production was observed for the control GapA strain. Determination of the intracellular NADPH/NADP+ ratios revealed that the ratios in the engineered strains were significantly higher than the ratio of the control GapA strain irrespective of the pentose phosphate pathway. These results demonstrate that our strain engineering strategy allows efficient L-lysine production independent of the oxidative pentose phosphate pathway.
Regulation of myo-inositol biosynthesis by p53-ISYNA1 pathway.
Koguchi, T., Tanikawa, C., Mori, J., Kojima, Y. & Matsuda, K. (2016). International Journal of Oncology, 48(6), 2415-2424.
In response to various cellular stresses, p53 exerts its tumor suppressive effects such as apoptosis, cell cycle arrest, and senescence through the induction of its target genes. Recently, p53 was shown to control cellular homeostasis by regulating energy metabolism, glycolysis, antioxidant effect, and autophagy. However, its function in inositol synthesis was not reported. Through a microarray screening, we found that five genes related with myo-inositol metabolism were induced by p53. DNA damage enhanced intracellular myo-inositol content in HCT116 p53+/+ cells, but not in HCT116 p53-/- cells. We also indicated that inositol 3-phosphate synthase (ISYNA1) which encodes an enzyme essential for myo-inositol biosynthesis as a direct target of p53. Activated p53 regulated ISYNA1 expression through p53 response element in the seventh exon. Ectopic ISYNA1 expression increased myo-inositol levels in the cells and suppressed tumor cell growth. Knockdown of ISYNA1 caused resistance to adriamycin treatment, demonstrating the role of ISYNA1 in p53-mediated growth suppression. Furthermore, ISYNA1 expression was significantly associated with p53 mutation in bladder, breast cancer, head and neck squamous cell carcinoma, lung squamous cell carcinoma, and pancreatic adenocarcinoma. Our findings revealed a novel role of p53 in myo-inositol biosynthesis which could be a potential therapeutic target.
The myo-inositol/proton symporter IolT1 contributes to D-xylose uptake in Corynebacterium glutamicum.
Brüsseler, C., Radek, A., Tenhaef, N., Krumbach, K., Noack, S. & Marienhagen, J. (2017). Bioresource Technology, 249, 953-961.
Corynebacterium glutamicum has been engineered to utilize D-xylose as sole carbon and energy source. Recently, a C. glutamicum strain has been optimized for growth on defined medium containing D-xylose by laboratory evolution, but the mutation(s) attributing to the improved-growth phenotype could not be reliably identified. This study shows that loss of the transcriptional repressor IolR is responsible for the increased growth performance on defined D-xylose medium in one of the isolated mutants. Underlying reason is derepression of the gene for the glucose/myo-inositol permease IolT1 in the absence of IolR, which could be shown to also contribute to D-xylose uptake in C. glutamicum. IolR-regulation of iolT1 could be successfully repealed by rational engineering of an IolR-binding site in the iolT1-promoter. This minimally engineered C. glutamicum strain bearing only two nucleotide substitutions mimics the IolR loss-of-function phenotype and allows for a high growth rate on D-xylose-containing media (µmax).= 0.24 ± 0.01 h-1).
The myo‐inositol converting pathway via glucuronic acid does not contribute to ascorbic acid synthesis in Arabidopsis.
Ivanov Kavkova, E., Blöchl, C. & Tenhaken, R. (2018). Plant Biology, In Press.
Ascorbic acid (AsA) biosynthesis in plants predominantly occurs via a pathway with D‐mannose and L‐galactose as intermediates. One alternative pathway for AsA synthesis, which is similar to the biosynthesis route in mammals, is controversially discussed for plants. Here, myo‐inositol is cleaved to glucuronic acid and then converted via L‐gulonate to AsA. In contrast to animals, plants have an effective recycling pathway for glucuronic acid being a competitor for the metabolic rate. Recycling involves a phosphorylation at C1 by the enzyme glucuronokinase. Two previously described T‐DNA insertion lines in the gene coding for glucuronokinase1 show wildtype like expression levels of the mRNA in our experiments and do not accumulate glucuronic acid in labelling experiments disproving that these lines are true knockouts. As suitable T‐DNA insertion lines were not available, we generated frameshift mutations in the major expressed isoform glucuronokinase1 (At3g01640) to potentially redirect metabolites to AsA. However, radiotracer experiments with 3Hmyo‐inositol revealed that the mutants in glucuronokinase1 accumulate only glucuronic acid and incorporate less metabolite into cell wall polymers. AsA was not labeled, suggesting that Arabidopsis cannot efficiently use glucuronic acid for AsA biosynthesis. All four mutants in glucuronokinase as well as wildtype have the same level of AsA in leaves.