Isopropanol production with reutilization of glucose-derived CO2 by engineered Ralstonia eutropha

Heterotrophic and autotrophic production of L-isoleucine and L-valine by engineered Cupriavidus necator H16

Advancement in commodity chemical production from carbon dioxide (CO2) offers a promising path towards sustainable development goal. Cupriavidus necator is an ideal host to convert CO2 into high-value chemicals, thereby achieving this target. Here, C. necator was engineered for heterotrophic and autotrophic production of L-isoleucine and L-valine. Citramalate synthase was introduced to simplify isoleucine synthesis pathway. Blocking poly-hydroxybutyrate biosynthesis resulted in significant accumulation of isoleucine and valine. Besides, strategies like key enzymes screening and overexpressing, reducing power balancing and feedback inhibition removing were applied in strain modification. Finally, the maximum isoleucine and valine titers of the best isoleucine-producing and valine-producing strains reached 857 and 972 mg/L, respectively, in fed-batch fermentation using glucose as substrate, and 105 and 319 mg/L, respectively, in autotrophic fermentation using CO2 as substrate. This study provides a feasible solution for developing C. necator as a microbial factory to produce amino acids from CO2.

Microaerobic insights into production of polyhydroxyalkanoates containing 3-hydroxyhexanoate via native reverse β-oxidation from glucose in Ralstonia eutropha H16

BACKGROUND

Ralstonia eutropha H16, a facultative chemolitoautotroph, is an important workhorse for bioindustrial production of useful compounds such as polyhydroxyalkanoates (PHAs). Despite the extensive studies to date, some of its physiological properties remain not fully understood.

RESULTS

This study demonstrated that the knallgas bacterium exhibited altered PHA production behaviors under slow-shaking condition, as compared to its usual aerobic condition. One of them was a notable increase in PHA accumulation, ranging from 3.0 to 4.5-fold in the mutants lacking of at least two NADPH-acetoacetyl-CoA reductases (PhaB1, PhaB3 and/or phaB2) when compared to their respective aerobic counterpart, suggesting the probable existence of (R)-3HB-CoA-providing route(s) independent on PhaBs. Interestingly, PHA production was still considerably high even with an excess nitrogen source under this regime. The present study further uncovered the conditional activation of native reverse β-oxidation (rBOX) allowing formation of (R)-3HHx-CoA, a crucial precursor for poly(3-hydroxybutyrate-co-3-hydroxyhexanoate) [P(3HB-co-3HHx)], solely from glucose. This native rBOX led to the natural incorporation of 3.9 mol% 3HHx in a triple phaB-deleted mutant (∆phaB1∆phaB1∆phaB2-C2). Gene deletion experiments elucidated that the native rBOX was mediated by previously characterized (S)-3HB-CoA dehydrogenases (PaaH1/Had), β-ketothiolase (BktB), (R)-2-enoyl-CoA hydratase (PhaJ4a), and unknown crotonase(s) and reductase(s) for crotonyl-CoA to butyryl-CoA conversion prior to elongation. The introduction of heterologous enzymes, crotonyl-CoA carboxylase/reductase (Ccr) and ethylmalonyl-CoA decarboxylase (Emd) along with (R)-2-enoyl-CoA hydratase (PhaJ) aided the native rBOX, resulting in remarkably high 3HHx composition (up to 37.9 mol%) in the polyester chains under the low-aerated condition.

CONCLUSION

These findings shed new light on the robust characteristics of Ralstonia eutropha H16 and have the potential for the development of new strategies for practical P(3HB-co-3HHx) copolyesters production from sugars under low-aerated conditions.

Optimizing Hexose Utilization Pathways of Cupriavidus necator for Improving Growth and L-Alanine Production under Heterotrophic and Autotrophic Conditions

Cupriavidus necator is a versatile microbial chassis to produce high-value products. Blocking the poly-β-hydroxybutyrate synthesis pathway (encoded by the phaC1AB1 operon) can effectively enhance the production of C. necator, but usually decreases cell density in the stationary phase. To address this problem, we modified the hexose utilization pathways of C. necator in this study by implementing strategies such as blocking the Entner-Doudoroff pathway, completing the phosphopentose pathway by expressing the gnd gene (encoding 6-phosphogluconate dehydrogenase), and completing the Embden-Meyerhof-Parnas pathway by expressing the pfkA gene (encoding 6-phosphofructokinase). During heterotrophic fermentation, the OD600 of the phaC1AB1-knockout strain increased by 44.8% with pfkA gene expression alone, and by 93.1% with gnd and pfkA genes expressing simultaneously. During autotrophic fermentation, gnd and pfkA genes raised the OD600 of phaC1AB1-knockout strains by 19.4% and 12.0%, respectively. To explore the effect of the pfkA gene on the production of C. necator, an alanine-producing C. necator was constructed by expressing the NADPH-dependent L-alanine dehydrogenase, alanine exporter, and knocking out the phaC1AB1 operon. The alanine-producing strain had maximum alanine titer and yield of 784 mg/L and 11.0%, respectively. And these values were significantly improved to 998 mg/L and 13.4% by expressing the pfkA gene. The results indicate that completing the Embden-Meyerhof-Parnas pathway by expressing the pfkA gene is an effective method to improve the growth and production of C. necator.

Problems and corresponding strategies for converting CO2 into value-added products in Cupriavidus necator H16 cell factories

Elevated CO2 emissions have substantially altered the worldwide climate, while the excessive reliance on fossil fuels has exacerbated the energy crisis. Therefore, the conversion of CO2 into fuel, petroleum-based derivatives, drug precursors, and other value-added products is expected. Cupriavidus necator H16 is the model organism of the "Knallgas" bacterium and is considered to be a microbial cell factory as it can convert CO2 into various value-added products. However, the development and application of C. necator H16 cell factories has several limitations, including low efficiency, high cost, and safety concerns arising from the autotrophic metabolic characteristics of the strains. In this review, we first considered the autotrophic metabolic characteristics of C. necator H16, and then categorized and summarized the resulting problems. We also provided a detailed discussion of some corresponding strategies concerning metabolic engineering, trophic models, and cultivation mode. Finally, we provided several suggestions for improving and combining them. This review might help in the research and application of the conversion of CO2 into value-added products in C. necator H16 cell factories.

Effect of COD/ NO3−-N ratio on nitrite accumulation and microbial behavior in glucose-driven partial denitrification system

COD/NO₃ ^--N ratio was considered to be one of the key factors achieving effective nitrite accumulation during partial denitrification. In two parallel reactors incubated with glucose as carbon source at COD/NO₃ ^--N of 3 and 5, respectively, the microbial community structure shift and the nitrite accumulation performance during long-term operation were investigated. The maximum nitrite accumulation ratios at COD/NO₃ ^--N of 3 and 5 were 17.9% and 47.04%, respectively. Thauera was the dominant genus in both reactors on day 220 with the relative abundance of 18.67% and 64.01%, respectively. Batch experiments with different electron acceptors suggested that the distinction in nitrite accumulation at COD/NO₃ ^--N of 3 and 5 might be caused by the differences in the abundance of Thauera.