Redox responses in yeast to acetate as the carbon source

A hybrid inorganic-biological artificial photosynthesis system for energy-efficient food production

Artificial photosynthesis systems are proposed as an efficient alternative route to capture CO₂ to produce additional food for growing global demand. Here a two-step CO₂ electrolyser system was developed to produce a highly concentrated acetate stream with a 57% carbon selectivity (CO₂ to acetate), allowing its direct use for the heterotrophic cultivation of yeast, mushroom-producing fungus and a photosynthetic green alga, in the dark without inputs from biological photosynthesis. An evaluation of nine crop plants found that carbon from exogenously supplied acetate incorporates into biomass through major metabolic pathways. Coupling this approach to existing photovoltaic systems could increase solar-to-food energy conversion efficiency by about fourfold over biological photosynthesis, reducing the solar footprint required. This technology allows for a reimagination of how food can be produced in controlled environments.

Securing a furan-based biorefinery: disclosing the genetic basis of the degradation of hydroxymethylfurfural and its derivatives in the model fungus Aspergillus nidulans

Hydroxymethylfurfural (HMF) is a promising lignocellulosic-derived source for the generation of diverse chemical building blocks constituting an alternative to fossil fuels. However, it remains unanswered if ubiquitous fungi can ensure their efficient decay, similar to that observed in highly specialised fungi. To disclose the genetic basis of HMF degradation in aspergilli, we performed a comprehensive analysis of Aspergillus nidulans ability to tolerate and to degrade HMF and its derivatives (including an HMF-dimer). We identified the degradation pathway using a suite of metabolomics methods and showed that HMF was modified throughout sequential reactions, ultimately yielding derivatives subsequently channelled to the TCA cycle. Based on the previously revealed hmfFGH gene cluster of Cupriavidus basilensis, we combined gene expression of homologous genes in Aspergillus nidulans and functional analyses in single-deletion mutants. Results were complemented with orthology analyses across the genomes of twenty-five fungal species. Our results support high functional redundancy for the initial steps of the HMF degradation pathway in the majority of the analysed fungal genomes and the assignment of a single-copy furan-2,5-dicarboxylic acid decarboxylase gene in A. nidulans. Collectively our data made apparent the superior capacity of aspergilli to mineralise HMF, furthering the environmental sustainability of a furan-based chemistry.

Microbial upgrading of acetate into 2,3-butanediol and acetoin by E. coli W

BACKGROUND

Acetate is an abundant carbon source and its use as an alternative feedstock has great potential for the production of fuel and platform chemicals. Acetoin and 2,3-butanediol represent two of these potential platform chemicals.

RESULTS

The aim of this study was to produce 2,3-butanediol and acetoin from acetate in Escherichia coli W. The key strategies to achieve this goal were: strain engineering, in detail the deletion of mixed-acid fermentation pathways E. coli W ΔldhA ΔadhE Δpta ΔfrdA 445_Ediss and the development of a new defined medium containing five amino acids and seven vitamins. Stepwise reduction of the media additives further revealed that diol production from acetate is mediated by the availability of aspartate. Other amino acids or TCA cycle intermediates did not enable growth on acetate. Cultivation under controlled conditions in batch and pulsed fed-batch experiments showed that aspartate was consumed before acetate, indicating that co-utilization is not a prerequisite for diol production. The addition of aspartate gave cultures a start-kick and was not required for feeding. Pulsed fed-batches resulted in the production of 1.43 g l-1 from aspartate and acetate and 1.16 g l-1 diols (2,3-butanediol and acetoin) from acetate alone. The yield reached 0.09 g diols per g acetate, which accounts for 26% of the theoretical maximum.

CONCLUSION

This study for the first time showed acetoin and 2,3-butanediol production from acetate as well as the use of chemically defined medium for product formation from acetate in E. coli. Hereby, we provide a solid base for process intensification and the investigation of other potential products.

Overexpression of branched-chain amino acid aminotransferases rescues the growth defects of cells lacking the Barth syndrome-related gene TAZ1

The yeast protein Taz1 is the orthologue of human Tafazzin, a phospholipid acyltransferase involved in cardiolipin (CL) remodeling via a monolyso CL (MLCL) intermediate. Mutations in Tafazzin lead to Barth syndrome (BTHS), a metabolic and neuromuscular disorder that primarily affects the heart, muscles, and immune system. Similar to observations in fibroblasts and platelets from patients with BTHS or from animal models, abolishing yeast Taz1 results in decreased total CL amounts, increased levels of MLCL, and mitochondrial dysfunction. However, the biochemical mechanisms underlying the mitochondrial dysfunction in BTHS remain unclear. To better understand the pathomechanism of BTHS, we searched for multi-copy suppressors of the taz1Δ growth defect in yeast cells. We identified the branched-chain amino acid transaminases (BCATs) Bat1 and Bat2 as such suppressors. Similarly, overexpression of the mitochondrial isoform BCAT2 in mammalian cells lacking TAZ improves their growth. Elevated levels of Bat1 or Bat2 did not restore the reduced membrane potential, altered stability of respiratory complexes, or the defective accumulation of MLCL species in yeast taz1Δ cells. Importantly, supplying yeast or mammalian cells lacking TAZ1 with certain amino acids restored their growth behavior. Hence, our findings suggest that the metabolism of amino acids has an important and disease-relevant role in cells lacking Taz1 function. KEY MESSAGES: Bat1 and Bat2 are multi-copy suppressors of retarded growth of taz1Δ yeast cells. Overexpression of Bat1/2 in taz1Δ cells does not rescue known mitochondrial defects. Supplementation of amino acids enhances growth of cells lacking Taz1 or Tafazzin. Altered metabolism of amino acids might be involved in the pathomechanism of BTSH.

Kinetic analysis of a Saccharomyces cerevisiae strain adapted for improved growth on glycerol: Implications for the development of yeast bioprocesses on glycerol

Glycerol is an agro-industrial residue generated in high amounts during the biodiesel production. The growing production of biodiesel is creating a worldwide glycerol surplus. Therefore, replacing sugar-based feedstock in bioprocesses by glycerol could be potentially attractive. Saccharomyces cerevisiae is one of the most commonly used microorganisms in the agri-food industry and therefore currently produced in large quantities from sugar-based feedstock. Unfortunately, growth of S. cerevisiae strains on glycerol is very low with reported μmax around 0.01 h(-1). This study demonstrates that successive growth of the S. cerevisiae CBS 8066, CEN.PK 113-7 D and Ethanol Red on glycerol as sole carbon source considerably improved the μmax from 0.01 up to 0.2 h(-1). The "adapted strain" CBS 8066-FL20 was kinetically characterized during aerobic and oxygen-limited cultivation in bioreactor and the results discussed in terms of their implication for developing glycerol-based S. cerevisiae bioprocesses.

Pnc1p supports increases in cellular NAD(H) levels in response to internal or external oxidative stress

Following transfer from medium with fermentable glucose to medium with nonfermentable acetate as the carbon source, cellular levels of NAD(H) were found to increase approximately 2-fold in a parental yeast strain. Similar transfer of a mutant strain subject to endogenous oxidative stress under these conditions produced more dramatic increases in cellular levels of NAD(H), and elevations above parental levels were shown to be due to the nicotinimidase Pnc1p. Similar transient increases in NAD(H) levels observed in the parental strain following addition of exogenous hydrogen peroxide were also attributable to Pnc1p.

Disulfide bond formation in yeast NAD+-specific isocitrate dehydrogenase

The tricarboxylic acid cycle NAD+-specific isocitrate dehydrogenase (IDH) of Saccharomyces cerevisiae is an octameric enzyme composed of four heterodimers of regulatory IDH1 and catalytic IDH2 subunits. Recent structural analyses revealed the close proximity of Cys-150 residues from IDH2 in adjacent heterodimers, and features of the structure for the ligand-free enzyme suggested that formation of a disulfide bond between these residues might stabilize an inactive form of the enzyme. We constructed two mutant forms of IDH, one containing a C150S substitution in IDH2 and the other containing C56S/C242S substitutions in IDH2 leaving Cys-150 as the sole cysteine residue. Treatment of the affinity-purified enzymes with diamide resulted in the formation of disulfide bonds and in decreased activities for the wild-type and C56S/C242S enzymes. Both effects were reversible by the addition of dithiothreitol. Diamide had no effect on the C150S mutant enzyme, suggesting that Cys-150 is essential for the formation of a disulfide bond that inhibits IDH activity. Diamide-induced formation of the Cys-150 disulfide bond was also observed in vivo for yeast transformants expressing the wild-type or C56S/C242S enzymes but not for a transformant expressing the C150S enzyme. Finally, natural formation of the Cys-150 disulfide bond with a concomitant decrease in cellular IDH activity was observed during the stationary phase for the parental strain and for transformants expressing wild-type or C56S/C242S enzymes but not for a transformant expressing the C150S enzyme. A reduction in viability for the latter strain suggests that a decrease in IDH activity is important for metabolic changes in stationary phase cells.