Variants of the Sir4 Coiled-Coil Domain Improve Binding to Sir3 for Heterochromatin Formation in Saccharomyces cerevisiae

Increased volatile thiol release during beer fermentation using constructed interspecies yeast hybrids

Interspecies hybridization has been shown to be a powerful tool for developing and improving brewing yeast in a number of industry-relevant respects. Thanks to the popularity of heavily hopped ‘India Pale Ale’-style beers, there is an increased demand from brewers for strains that can boost hop aroma. Here, we explored whether hybridization could be used to construct strains with an enhanced ability to release hop-derived flavours through β-lyase activity, which releases desirable volatile thiols. Wild Saccharomyces strains were shown to possess high β-lyase activity compared to brewing strains, however, they also produced phenolic off-flavours (POF) and showed poor attenuation. To overcome these limitations, interspecies hybrids were constructed by crossing pairs of one of three brewing and one of three wild Saccharomyces strains (S. uvarum and S. eubayanus). Hybrids were screened for fermentation ability and β-lyase activity, and selected hybrids showed improved fermentation and formation of both volatile thiols (4MMP, 3MH and 3MH-acetate) and aroma-active esters compared to the parent strains. Undesirable traits (e.g. POF) could be removed from the hybrid by sporulation. To conclude, it was possible to boost the release of desirable hop-derived thiols in brewing yeast by hybridization with wild yeast. This allows production of beer with boosted hop aroma with less hops (thus improving sustainability issues).

Fruits of their labour: biotransformation reactions of yeasts during brewery fermentation

Abstract

There is a growing appreciation for the role that yeast play in biotransformation of flavour compounds during beverage fermentations. This is particularly the case for brewing due to the continued popularity of aromatic beers produced via the dry-hopping process. Here, we review the current literature pertaining to biotransformation reactions mediated by fermentative yeasts. These reactions are diverse and include the liberation of thiols from cysteine or glutathione-bound adducts, as well as the release of glycosidically bound terpene alcohols. These changes serve generally to increase the fruit and floral aromas in beverages. This is particularly the case for the thiol compounds released via yeast β-lyase activity due to their low flavour thresholds. The role of yeast β-glucosidases in increasing terpene alcohols is less clear, at least with respect to fermentation of brewer’s wort. Yeast acetyl transferase and acetate esterase also have an impact on the quality and perceptibility of flavour compounds. Isomerization and reduction reactions, e.g. the conversion of geraniol (rose) to β-citronellol (citrus), also have potential to alter significantly flavour profiles. A greater understanding of biotransformation reactions is expected to not only facilitate greater control of beverage flavour profiles, but also to allow for more efficient exploitation of raw materials and thereby greater process sustainability.

Key points

• Yeast can alter and boost grape- and hop-derived flavour compounds in wine and beer

• β-lyase activity can release fruit-flavoured thiols with low flavour thresholds

• Floral and citrus-flavoured terpene alcohols can be released or interconverted

Distinguishing between recruitment and spread of silent chromatin structures in Saccharomyces cerevisiae

The formation of heterochromatin at HML, HMR, and telomeres in Saccharomyces cerevisiae involves two main steps: Recruitment of Sir proteins to silencers and their spread throughout the silenced domain. We developed a method to study these two processes at single base-pair resolution. Using a fusion protein between the heterochromatin protein Sir3 and the non-site-specific bacterial adenine methyltransferase M.EcoGII, we mapped sites of Sir3-chromatin interactions genome-wide using long-read Nanopore sequencing to detect adenines methylated by the fusion protein and by ChIP-seq to map the distribution of Sir3-M.EcoGII. A silencing-deficient mutant of Sir3 lacking its Bromo-Adjacent Homology (BAH) domain, sir3-bah∆, was still recruited to HML, HMR, and telomeres. However, in the absence of the BAH domain, it was unable to spread away from those recruitment sites. Overexpression of Sir3 did not lead to further spreading at HML, HMR, and most telomeres. A few exceptional telomeres, like 6R, exhibited a small amount of Sir3 spreading, suggesting that boundaries at telomeres responded variably to Sir3 overexpression. Finally, by using a temperature-sensitive allele of SIR3 fused to M.ECOGII, we tracked the positions first methylated after induction and found that repression of genes at HML and HMR began before Sir3 occupied the entire locus.

The molecular biology of fruity and floral aromas in beer and other alcoholic beverages

Aroma compounds provide attractiveness and variety to alcoholic beverages. We discuss the molecular biology of a major subset of beer aroma volatiles, fruity and floral compounds, originating from raw materials (malt and hops), or formed by yeast during fermentation. We introduce aroma perception, describe the most aroma-active, fruity and floral compounds in fruits and their presence and origin in beer. They are classified into categories based on their functional groups and biosynthesis pathways: (1) higher alcohols and esters, (2) polyfunctional thiols, (3) lactones and furanones, and (4) terpenoids. Yeast and hops are the main sources of fruity and flowery aroma compounds in beer. For yeast, the focus is on higher alcohols and esters, and particularly the complex regulation of the alcohol acetyl transferase ATF1 gene. We discuss the release of polyfunctional thiols and monoterpenoids from cysteine- and glutathione-S-conjugated compounds and glucosides, respectively, the primary biological functions of the yeast enzymes involved, their mode of action and mechanisms of regulation that control aroma compound production. Furthermore, we discuss biochemistry and genetics of terpenoid production and formation of non-volatile precursors in Humulus lupulus (hops). Insight in these pathways provides a toolbox for creating innovative products with a diversity of pleasant aromas.