Genome Sequence of the Lager-Brewing Yeast Saccharomyces sp. Strain M14, Used in the High-Gravity Brewing Industry in China

Screening lager yeast with higher ethyl-acetate production by adaptive laboratory evolution in high concentration of acetic acid

Ethyl-acetate is important for the flavor and aroma of the alcoholic beverages, therefore, there have been extensive efforts toward increasing its production by engineering yeast strains. In this study, we reported a new approach to breed non-genetic modified producing yeast strain with higher ethyl-acetate production for beer brewing. First, we demonstrated the positive effect of higher acetic acid concentration on inducing the expression of acetyl-CoA synthetase (ACS). Then, we applied adaptive laboratory evolution method to evolve strain with higher expression level of ACS. As a result, we obtained several evolved strains with increased ACS expression level as well as ethyl-acetate production. In 3 L scale fermentation, the optimal strain EA60 synthesized more ethyl-acetate than M14 at the same time point. At the end of fermentation, the ethyl-acetate production in EA60 was 21.4% higher than M14, while the other flavor components except for acetic acid were changed in a moderate degree, indicating this strain had a bright prospect in industrial application. Moreover, this study also indicated that ACS1 played a more important role in increasing the acetic acid tolerance of yeast, while ACS2 contributed to the synthesis of cytosol acetyl-CoA, thereby facilitating the production of ethyl-acetate during fermentation.

Higher NADH Availability of Lager Yeast Increases the Flavor Stability of Beer

Flavor stability is a significant concern to brewers as the staling compounds impart unpleasant flavor to beer. Thus, yeasts with antistaling ability have been engineered to produce beer with improved flavor stability. Here, we proposed that increasing the NADH availability of yeast could improve the flavor stability of beer. By engineering endogenous pathways, we obtained an array of yeast strains with a higher reducing activity. Then, we carried out beer fermentation with these strains and found that the antistaling capacities of the beer samples were improved. For a better understanding of the underlying mechanism, we compared the flavor profiles of these strains. The production of staling components was significantly decreased, whereas the content of antistaling components, such as SO₂, was increased, in line with the increased antistaling ability. The other aroma components were marginally changed, indicating that this concept was useful for improving the antistaling stability without changing the flavor of beer.

Unraveling the Mechanisms for Low-Level Acetaldehyde Production during Alcoholic Fermentation in Saccharomyces pastorianus Lager Yeast

Acetaldehyde is produced by yeast during alcoholic fermentation, and its modification greatly affects beer flavor and quality. In the current study, we analyzed two yeast strains with a low level of acetaldehyde to reveal the potential mechanism underpinning the desirable low acetaldehyde production by these strains. We demonstrated that high alcohol dehydrogenase (ADH) activity and high NADH availability were the dominant factors for the low level of acetaldehyde in the fermentation liquor at the end of fermentation. High ADH activity resulted in reduced accumulation of acetaldehyde during the cell growth phase by increasing the flux to ethanol, whereas high NADH availability (in the cytosol or mitochondria) enhanced acetaldehyde reduction at the later phase of main fermentation. Furthermore, NADH availability is a more useful target trait than ADH activity for constructing yeast strains with a low level of acetaldehyde for industrial applications in terms of flavor contribution and unaltered fermentation period.

Horizontal transfer and proliferation of Tsu4 in Saccharomyces paradoxus

Background

Recent evidence suggests that horizontal transfer plays a significant role in the evolution of of transposable elements (TEs) in eukaryotes. Many cases of horizontal TE transfer (HTT) been reported in animals and plants, however surprisingly few examples of HTT have been reported in fungi.

Findings

Here I report evidence for a novel HTT event in fungi involving Tsu4 in Saccharomyces paradoxus based on (i) unexpectedly high similarity between Tsu4 elements in S. paradoxus and S. uvarum, (ii) a patchy distribution of Tsu4 in S. paradoxus and general absence from its sister species S. cerevisiae, and (iii) discordance between the phylogenetic history of Tsu4 sequences and species in the Saccharomyces sensu stricto group. Available data suggests the HTT event likely occurred somewhere in the Nearctic, Neotropic or Indo-Australian part of the S. paradoxus species range, and that a lineage related to S. uvarum or S. eubayanus was the likely donor species. The HTT event has led to massive proliferation of Tsu4 in the South American lineage of S. paradoxus, which exhibits partial reproductive isolation with other strains of this species because of multiple reciprocal translocations. Full-length Tsu4 elements are associated with both breakpoints of one of these reciprocal translocations.

Conclusions

This work shows that comprehensive analysis of TE sequences in essentially-complete genome assemblies derived from long-read sequencing provides new opportunities to detect HTT events in fungi and other organisms. This work also provides support for the hypothesis that HTT and subsequent TE proliferation can induce genome rearrangements that contribute to post-zygotic isolation in yeast.

Electronic supplementary material

The online version of this article (10.1186/s13100-018-0122-7) contains supplementary material, which is available to authorized users.