Jasper A. Diderich

Lager beer, made by fermentation of barley malt using the hybrid yeast Saccharomyces pastorianus, is extremely popular worldwide. The low genetic variation of S. pastorianus strains limits the scope of development for novel lager beer types. The wild ancestor Saccharomyces eubayanus has potential for use in lager brewing, but its application is constrained by formation of phenolic off-flavours (POF+ phenotype) and limited uptake of the wort α-oligoglucosides maltose and maltotriose. Here, we addressed both limitations, using screening and evolutionary engineering of the Himalayan S. eubayanus strain ABFM5L.1, one of the closest known relatives of S. eubayanus subgenome of modern hybrid lager yeasts. Prior to improving α-oligoglucoside utilisation, 4-vinylguaiacol formation was abolished by mutagenesis and screening for POF⁻ mutants. Subsequently, evolution for improved sugar utilisation and reverse engineering identified SeMALR1 (the S. eubayanus ortholog of S. cerevisiae MAL63) as a key regulatory control point. Single-nucleotide substitutions in this maltose regulon activator gene enabled rapid maltose uptake and partial maltotriose utilisation in a previously maltose-negative background. The evolved non-genetically modified POF⁻ strains showed near-complete maltose consumption (>90%–95%) and partial maltotriose consumption (∼25%–32%) in industrial wort. These results reveal substantial regulatory flexibility in Holarctic S. eubayanus and support its further development as a novel lager brewing yeast.

Saccharomyces pastorianus is an interspecies hybrid between S. cerevisiae and S. eubayanus. The identification of the parental species of S. pastorianus enabled the de novo reconstruction of hybrids that could potentially combine a wide array of phenotypic traits. Lager yeasts are characterized by their inability to decarboxylate ferulic acid present in wort, a phenotype also known as Pof^- (phenolic off-flavor). However, all known S. eubayanus strains characterized so far produce clove-like aroma specific of 4-vinyl guaiacol, a decarboxylated form of ferulic acid. This study explored a non-GMO approach to construct Pof^- S. eubayanus variants derived from the parental strain S. eubayanus CBS 12357. To rapidly screen a population of UV-mutagenized cells two complementary assays were developed. The first assay was based on the difference of light absorption spectra of ferulic acid and 4-vinyl guaiacol, while the second was based on the difference of sensitivity of Pof^- and Pof⁺ strains to cinnamic acid. The S. eubayanus variant HTSE042 was selected and was confirmed not to produce 4-vinyl guaiacol. Whole genome sequencing revealed that this variant lost the subtelomeric region of the CHRXIII right arm that carried the two clustered genes SePAD1- SeFDC1 whose deletion in a naïve S. eubayanus strain (CBS 12357/FM1318) resulted in an identical phenotype. Subsequently, the Pof^- variant was crossed with a Pof^- S. cerevisiae partner. The resulting hybrid was not able to convert ferulic acid demonstrating the undisputable value of the mutagenized variant HTSE042 to eventually construct S. cerevisiae × S. eubayanus hybrids with phenotypic characteristics of S. pastorianus.

We have recently reported about a Saccharomyces cerevisiae strain that, in addition to the Piromyces XylA xylose isomerase gene, overexpresses the native genes for the conversion of xylulose to glycolytic intermediates. This engineered strain (RWB 217) exhibited unprecedentedly high specific growth rates and ethanol production rates under anaerobic conditions with xylose as the sole carbon source. However, when RWB 217 was grown on glucose-xylose mixtures, a diauxic growth pattern was observed with a relatively slow consumption of xylose in the second growth phase. After prolonged cultivation in an anaerobic, xylose-limited chemostat, a culture with improved xylose uptake kinetics was obtained. This culture also exhibited improved xylose consumption in glucose-xylose mixtures. A further improvement in mixed-sugar utilization was obtained by prolonged anaerobic cultivation in automated sequencing-batch reactors on glucose-xylose mixtures. A final single-strain isolate (RWB 218) rapidly consumed glucose-xylose mixtures anaerobically, in synthetic medium, with a specific rate of xylose consumption exceeding 0.9 gg(-1)h(-1). When the kinetics of zero trans-influx of glucose and xylose of RWB 218 were compared to that of the initial strain, a twofold higher capacity (V(max)) as well as an improved K(m) for xylose was apparent in the selected strain. It is concluded that the kinetics of xylose fermentation are no longer a bottleneck in the industrial production of bioethanol with yeast.

Prolonged cultivation of Saccharomyces cerevisiae in aerobic, glucose-limited chemostat cultures (dilution rate, 0·10 h−1) resulted in a progressive decrease of the residual glucose concentration (from 20 to 8 mg l−1 after 200 generations). This increase in the affinity for glucose was accompanied by a fivefold decrease of fermentative capacity, and changes in cellular morphology. These phenotypic changes were retained when single-cell isolates from prolonged cultures were used to inoculate fresh chemostat cultures, indicating that genetic changes were involved. Kinetic analysis of glucose transport in an ‘evolved’ strain revealed a decreased K m, while V max was slightly increased relative to the parental strain. Apparently, fermentative capacity in the evolved strain was not controlled by glucose uptake. Instead, enzyme assays in cell extracts of the evolved strain revealed strongly decreased capacities of enzymes in the lower part of glycolysis. This decrease was corroborated by genome-wide transcriptome analysis using DNA microarrays. In aerobic batch cultures on 20 g glucose l−1, the specific growth rate of the evolved strain was lower than that of the parental strain (0·28 and 0·37 h−1, respectively). Instead of the characteristic instantaneous production of ethanol that is observed when aerobic, glucose-limited cultures of wild-type S. cerevisiae are exposed to excess glucose, the evolved strain exhibited a delay of ∼90 min before aerobic ethanol formation set in. This study demonstrates that the effects of selection in glucose-limited chemostat cultures extend beyond glucose-transport kinetics. Although extensive physiological analysis offered insight into the underlying cellular processes, the evolutionary ‘driving force’ for several of the observed changes remains to be elucidated.

A sugar-transporter-encoding gene, mstA, which is a member of the major facilitator superfamily, has been cloned from a genomic DNA library of the filamentous fungus Aspergillus niger. To enable the functional characterization of MSTA, a full-length cDNA was expressed in a Saccharomyces cerevisiae strain deficient in hexose uptake. Uptake experiments using ¹⁴C-labelled monosaccharides demonstrated that although able to transport d-fructose (K_m, 4.5±1.0 mM), d-xylose (K_m, 0.3±0.1 mM) and d-mannose (K_m, 60±20 µM), MSTA has a preference for d-glucose (K_m, 25±10 µM). pH changes associated with sugar transport indicate that MSTA catalyses monosaccharide/H⁺ symport. Expression of mstA in response to carbon starvation and upon transfer to poor carbon sources is consistent with a role for MSTA as a high-affinity transporter for d-glucose, d-mannose and d-xylose. Northern analysis has shown that mstA is subject to CreA-mediated carbon catabolite repression and pH regulation mediated by PacC. A. niger strains in which the mstA gene had been disrupted are phenotypically identical with isogenic reference strains when grown on 0.1–60 mM d-glucose, d-mannose, d-fructose or d-xylose. This indicates that A. niger possesses other transporters capable of compensating for the absence of MSTA.

Nutrient-limited Saccharomyces cerevisiae cells rapidly resume proliferative growth when transferred into glucose medium. This is preceded by a rapid increase in CLN3 , BCK2 , and CDC28 mRNAs encoding cell cycle regulatory proteins that promote progress through Start. We have tested the ability of mutations in known glucose signaling pathways to block glucose induction of CLN3 , BCK2 , and CDC28 . We find that loss of the Snf3 and Rgt2 glucose sensors does not block glucose induction, nor does deletion of HXK2 , encoding the hexokinase isoenzyme involved in glucose repression signaling. Rapamycin blockade of the Tor nutrient sensing pathway does not block the glucose response. Addition of 2-deoxy glucose to the medium will not substitute for glucose. These results indicate that glucose metabolism generates the signal required for induction of CLN3 , BCK2 , and CDC28 . In support of this conclusion, we find that addition of iodoacetate, an inhibitor of the glyceraldehyde-3-phosphate dehydrogenase step in yeast glycolysis, strongly downregulates the levels CLN3 , BCK2 , and CDC28 mRNAs. Furthermore, mutations in PFK1 and PFK2 , which encode phosphofructokinase isoforms, inhibit glucose induction of CLN3 , BCK2 , and CDC28 . These results indicate a link between the rate of glycolysis and the expression of genes that are critical for passage through G ₁ .

In glucose-limited aerobic chemostat cultures of a wild-type Saccharomyces cerevisiae and a derived hxk2 null strain, metabolic fluxes were identical. However, the concentrations of intracellular metabolites, especially fructose 1,6-bisphosphate, and hexose-phosphorylating activities differed. Interestingly, the hxk2 null strain showed a higher maximal growth rate and higher Crabtree threshold dilution rate, revealing a higher oxidative capacity for this strain. After a pulse of glucose, aerobic glucose-limited cultures of wild-type S. cerevisiae displayed an overshoot in the intracellular concentrations of glucose 6-phosphate, fructose 6-phosphate, and fructose 1,6-bisphosphate before a new steady state was established, in contrast to the hxk2 null strain which reached a new steady state without overshoot of these metabolites. At low dilution rates the overshoot of intracellular metabolites in the wild-type strain coincided with the immediate production of ethanol after the glucose pulse. In contrast, in the hxk2 null strain the production of ethanol started gradually. However, in spite of the initial differences in ethanol production and dynamic behaviour of the intracellular metabolites, the steady-state fluxes after transition from glucose limitation to glucose excess were not significantly different in the wild-type strain and the hxk2 null strain at any dilution rate.