Primary structure and regulation of a glucoamylase-encoding gene (STA2) in Saccharomyces diastaticus

Presence of Saccharomyces cerevisiae subsp. diastaticus in industry and nature and spoilage capacity of its vegetative cells and ascospores

Saccharomyces cerevisiae sub-species diastaticus (S. diastaticus) is the main fungal cause of spoilage of carbonated fermented beverages in the brewing industry. Here, prevalence of S. diastaticus in nature and breweries was assessed as well as the spoilage capacity of its vegetative cells and spores. S. diastaticus could only be enriched from 1 out of 136 bark and soil samples from the Netherlands, being the first described natural isolate of this yeast outside South America. On the other hand, it was identified by PCR and selective enrichment in 25 and 21 out of 54 biofilm samples from beer filling halls in Asia, Africa, Europe and North America. ITS sequencing revealed that S. cerevisiae (including S. diastaticus) represented <0.05% of fungal DNA in 17 out of 20 samples, while it represented 0.1, 2 and 32% in samples VH6, VH1 and VH3 respectively. Next, vegetative cells and ascospores of the natural S. diastaticus isolate MB523 were inoculated in a variety of beer products containing 0.0-5.0% alcohol (v/v). Ascospores spoiled all beer products, while vegetative cells did not grow in Radler lemon 0.0, Radler lime mint 0.0 and Radler lemon lime 0.0. Notably, vegetative cells could spoil these Radlers when they first had been grown in alcohol free beer either or not mixed with Radler lemon lime 0.0. Conversely, vegetative cells that had been grown in Radler lemon lime lost their spoilage potential of this beer product when they had grown in YPD medium for more than 24 h. In addition, it was shown that cells grown in alcohol free beer were more heat resistant than cells grown in YPD (D₅₂ 40 min and ≤ 10.3 min, respectively). Together, these data show that S. diastaticus is a less prevalent variant of S. cerevisiae in nature, while it accumulates in breweries in mixed biofilms. Data also show that both vegetative cells and spores can spoil all tested beer products, the latter cell type irrespective of its environmental history.

A re-evaluation of diastatic Saccharomyces cerevisiae strains and their role in brewing

Abstract

Diastatic strains of Saccharomyces cerevisiae possess the unique ability to hydrolyze and ferment long-chain oligosaccharides like dextrin and starch. They have long been regarded as important spoilage microbes in beer, but recent studies have inspired a re-evaluation of the significance of the group. Rather than being merely wild-yeast contaminants, they are highly specialized, domesticated yeasts belonging to a major brewing yeast lineage. In fact, many diastatic strains have unknowingly been used as production strains for decades. These yeasts are used in the production of traditional beer styles, like saison, but also show potential for creation of new beers with novel chemical and physical properties. Herein, we review results of the most recent studies and provide a detailed account of the structure, regulation, and functional role of the glucoamylase-encoding STA1 gene in relation to brewing and other fermentation industries. The state of the art in detecting diastatic yeast in the brewery is also summarized. In summary, these latest results highlight that having diastatic S. cerevisiae in your brewery is not necessarily a bad thing.

Key Points

•Diastatic S. cerevisiae strains are important spoilage microbes in brewery fermentations.

•These strains belong to the ‘Beer 2’ or ‘Mosaic beer’ brewing yeast lineage.

•Diastatic strains have unknowingly been used as production strains in breweries.

•The STA1-encoded glucoamylase enables efficient maltotriose use.

Electronic supplementary material

The online version of this article (10.1007/s00253-020-10531-0) contains supplementary material, which is available to authorized users.

A deletion in the STA1 promoter determines maltotriose and starch utilization in STA1+ Saccharomyces cerevisiae strains

Diastatic strains of Saccharomyces cerevisiae are common contaminants in beer fermentations and are capable of producing an extracellular STA1-encoded glucoamylase. Recent studies have revealed variable diastatic ability in strains tested positive for STA1, and here, we elucidate genetic determinants behind this variation. We show that poorly diastatic strains have a 1162-bp deletion in the promoter of STA1. With CRISPR/Cas9-aided reverse engineering, we show that this deletion greatly decreases the ability to grow in beer and consume dextrin, and the expression of STA1. New PCR primers were designed for differentiation of highly and poorly diastatic strains based on the presence of the deletion in the STA1 promoter. In addition, using publically available whole genome sequence data, we show that the STA1 gene is prevalent among the 'Beer 2'/'Mosaic Beer' brewing strains. These strains utilize maltotriose efficiently, but the mechanisms for this have been unknown. By deleting STA1 from a number of highly diastatic strains, we show here that extracellular hydrolysis of maltotriose through STA1 appears to be the dominant mechanism enabling maltotriose use during wort fermentation in STA1+ strains. The formation and retention of STA1 seems to be an alternative evolutionary strategy for efficient utilization of sugars present in brewer's wort. The results of this study allow for the improved reliability of molecular detection methods for diastatic contaminants in beer and can be exploited for strain development where maltotriose use is desired.

Relating growth dynamics and glucoamylase excretion of individual Saccharomyces cerevisiae cells

We have developed a novel flow cytometric procedure that allows determinations of properties of protein excretion in the growth medium on a cell-by-cell basis in Saccharomyces cerevisiae. The procedure is based on labelling of a periplasmically secreted protein with antibodies conjugated to a fluorescent marker such as fluorescein isothiocyanate (FITC). The staining conditions did not perturb cell growth after resuspension of stained cells in growth medium. Decrease in fluorescence was found to correlate with excretion of glucoamylase into the growth medium. The analysis of the staining pattern over time provides information on the behaviour of individual cells belonging to different cell-cycle phases and can be used to calculate the specific excretion rate of the overall population.

Nrg1 is a transcriptional repressor for glucose repression of STA1 gene expression in Saccharomyces cerevisiae

Expression of genes encoding starch-degrading enzymes is regulated by glucose repression in the yeast Saccharomyces cerevisiae. We have identified a transcriptional repressor, Nrg1, in a genetic screen designed to reveal negative factors involved in the expression of STA1, which encodes a glucoamylase. The NRG1 gene encodes a 25-kDa C2H2 zinc finger protein which specifically binds to two regions in the upstream activation sequence of the STA1 gene, as judged by gel retardation and DNase I footprinting analyses. Disruption of the NRG1 gene causes a fivefold increase in the level of the STA1 transcript in the presence of glucose. The expression of NRG1 itself is inhibited in the absence of glucose. DNA-bound LexA-Nrg1 represses transcription of a target gene 10.7-fold in a glucose-dependent manner, and this repression is abolished in both ssn6 and tup1 mutants. Two-hybrid and glutathione S-transferase pull-down experiments show an interaction of Nrg1 with Ssn6 both in vivo and in vitro. These findings indicate that Nrg1 acts as a DNA-binding repressor and mediates glucose repression of the STA1 gene expression by recruiting the Ssn6-Tup1 complex.

Coregulation of starch degradation and dimorphism in the yeast Saccharomyces cerevisiae

Saccharomyces cerevisiae, the exemplar unicellular eukaryote, can only survive and proliferate in its natural habitats through constant adaptation within the constraints of a dynamic ecosystem. In every cell cycle of S. cerevisiae, there is a short period in the G1 phase of the cell cycle where "sensing" transpires; if a sufficient amount of fermentable sugars is available, the cells will initiate another round of vegetative cell division. When fermentable sugars become limiting, the yeast can execute the diauxic shift, where it reprograms its metabolism to utilize nonfermentable carbon sources. S. cerevisiae can also initiate the developmental program of pseudohyphal formation and invasive growth response, when essential nutrients become limiting. S. cerevisiae shares this growth form-switching ability with important pathogens such as the human pathogen, Candida albicans, and the corn smut pathogen Ustilago maydis. The pseudohyphal growth response of S. cerevisiae has mainly been implicated as a means for the yeast to search for nutrients. An important observation made was that starch-degrading S. cerevisiae strains have the added ability to form pseudohyphae and grow invasively into a starch-containing medium. More significantly, it was also shown that the STA1-3 genes encoding three glucoamylase isozymes responsible for starch hydrolysis in S. cerevisiae are coregulated with a gene, MUC1, essential for pseudohyphal and invasive growth. At least two putative transcriptional activators, Mss10p and Mss11p, are involved in this regulation. The Muc1p is a putative integral membrane-bound protein similar to mammalian mucin-like proteins that have been implicated in the ability of cancer cells to invade other tissues. This provided us with an excellent example of integrative control between nutrient sensing, signaling, and differential development.

Segregation of yeast polymorphic STA genes in meiotic recombinants and analysis of glucoamylase production

Hybrid yeast strains were constructed using haploid Saccharomyces cerevisiae and Saccharomyces cerevisiae var. diastaticus strains to get haploid meiotic recombinants having more than one copy of STA1, STA2, and STA3 genes. STA genes were localized on the chromosomes by pulsed field gel electrophoresis. Working gene dosage effects were found among STA genes in liquid starch medium, indicating low levels of glucose repression. Growth of strains, however, was not influenced by their STA copy number.

FLO11, a yeast gene related to the STA genes, encodes a novel cell surface flocculin

We report the characterization of a gene encoding a novel flocculin related to the STA genes of yeast, which encode secreted glucoamylase. The STA genes comprise sequences that are homologous to the sporulation-specific glucoamylase SGA and to two other sequences, S2 and S1. We find that S2 and S1 are part of a single gene which we have named FLO11. The sequence of FLO11 reveals a 4,104-bp open reading frame on chromosome IX whose predicted product is similar in overall structure to the class of yeast serine/threonine-rich GPI-anchored cell wall proteins. An amino-terminal domain containing a signal sequence and a carboxy-terminal domain with homology to GPI (glycosyl-phosphatidyl-inositol) anchor-containing proteins are separated by a central domain containing a highly repeated threonine- and serine-rich sequence. Yeast cells that express FLO11 aggregate in the calcium-dependent process of flocculation. Flocculation is abolished when FLO11 is disrupted. The product of STA1 also is shown to have flocculating activity. When a green fluorescent protein fusion of FLO11 was expressed from the FLO11 promoter on a single-copy plasmid, fluorescence was observed in vivo at the periphery of cells. We propose that FLO11 encodes a flocculin because of its demonstrated role in flocculation, its structural similarity to other members of the FLO gene family, and the cell surface location of its product. FLO11 gene sequences are present in all yeast strains tested, including all standard laboratory strains, unlike the STA genes which are present only in the variant strain Saccharomyces cerevisiae var. diastaticus. FLO11 differs from all other yeast flocculins in that it is located near a centromere rather than a telomere, and its expression is regulated by mating type. Repression of FLO11-dependent flocculation in diploids is conferred by the mating-type repressor al/alpha2.

Muc1, a mucin-like protein that is regulated by Mss10, is critical for pseudohyphal differentiation in yeast

Pseudohyphal differentiation in Saccharomyces cerevisiae was first described as a response of diploid cells to nitrogen limitation. Here we report that haploid and diploid starch-degrading S. cerevisiae strains were able to switch from a yeast form to a filamentous pseudohyphal form in response to carbon limitation in the presence of an ample supply of nitrogen. Two genes, MSS10 and MUC1, were cloned and shown to be involved in pseudohyphal differentiation and invasive growth. The deletion of MSS10 resulted in extremely reduced amounts of pseudohyphal differentiation and invasive growth, whereas the deletion of MUC1 abolished pseudohyphal differentiation and invasive growth completely. Mss10 appears to be a transcriptional activator that responds to nutrient limitation and coregulates the expression of MUC1 and the STA1-3 glucoamylase genes, which are involved in starch degradation. MUC1 encodes a 1367-amino acid protein, containing several serine/threonine-rich repeats. Muc1 is a putative integral membrane-bound protein, similar to mammalian mucin-like membrane proteins that have been implicated to play a role in the ability of cancer cells to invade other tissues.

Cultural conditions for production of glucoamylase from Lactobacillus amylovorus ATCC 33621

Lactobacillus amylovorus ATCC 33621 is an actively amylolytic bacterial strain which produces a cell-bound glucoamylase (EC 3.2.1.3). Conditions of growth and glucoamylase production were investigated using dextrose-free de Man-Rogosa-Sharpe (MRS) medium in a 1.5 l fermenter, with varying dextrin concentration (0.1-1.5% (w/v)), pH (4.5-6.5) and temperature (25-55 degrees C). Cell extracts were prepared by subjecting cells to treatment with a French Pressure cell in order to release intracellular proteins. Glucoamylase activity was then assayed. The effects of pH (4.0-9.0), temperature (15-85 degrees C) and substrate (dextrin and starch, 0-2% w/v) concentration on crude enzyme activity were investigated. Optimal growth was obtained in MRS medium containing 1% (w/v) dextrin, at pH 5.5 and 37 degrees C. Glucoamylase production was maximal at the late logarithmic phase of growth, during 16-18 h. Crude enzyme had a pH optimum of 6.0 and temperature optimum of 60 degrees C. With starch as the substrate, maximal activity was obtained at a concentration of 1.5% (w/v). The effects of ions and inhibitors on glucoamylase activity were also investigated. Enzyme activity was not significantly influenced by Ca2+ and EDTA at 1 mmol l-1 concentration; however Pb2+ and Co2+ were found to inhibit the activity at concentrations of 1 mmol l-1. The crude enzyme was found to be thermolabile when glucoamylase activity decreased after about 10 min exposure at 60 degrees C. This property can be exploited in the brewing of low calorie beers where only mild pasteurization treatments are used to inactivate enzymes. The elimination of residual enzyme effect would prevent further maltodextrin degradation and sweetening during long-term storage, thus helping to stabilize the flavour of beer.

The S1, S2 and SGA1 ancestral genes for the STA glucoamylase genes all map to chromosome IX in Saccharomyces cerevisiae

The polymorphic extracellular glucoamylase-encoding genes STA1 (chr. IV), STA2 (chr. II) and STA3 (chr. XIV), from Saccharomyces cerevisiae var. diastaticus probably evolved by genomic rearrangement of DNA regions (S1, S2 and SGA1) present in S. cerevisiae, and subsequent translocation to unlinked regions of chromosomal regions. S1, encoding a homologue to the threonine/serine-rich domain of STA glucoamylases (GAI-III), mapped to the right arm of chromosome IX. S2, encoding the hydrophobic leader peptide of GAI-III), was also mapped on the right arm of chromosome IX, next to S1, close to DAL81. The SGA1 sporulation-specific, intracellular glucoamylase-encoding gene is located on the left arm of chromosome IX, 32 kb proximal of HIS5.

Inactivation of the UAS1 of STA1 by glucose and STA10 and identification of two loci, SNS1 and MSS1, involved in STA10-dependent repression in Saccharomyces cerevisiae

It has been reported that two upstream activation sites, UAS1 and UAS2, exist in the 5' non-coding region of the STA1 gene of Saccharomyces cerevisiae var. diastaticus. Based on studies using a UAS1STA1-CYC1-lacZ fusion, we divided UAS1 into two subsites, UAS1-1 and UAS1-2. The activation of the CYC1 promoter by UAS1STA1 was repressed by glucose in the culture medium and by the STA10 gene. The MATa/MAT alpha mating type configuration did not, however, affect UAS1STA1 activation. The UAS1STA1-CYC1-lacZ expression system was used to study STA10 repression further. A mutant insensitive to STA10-dependent repression was isolated. This sns1 mutation was not linked to STA10 and partially overcame the repressive effect of STA10 at the transcriptional level. From a genomic library constructed in the UAS1STA1-CYC1-lacZ expression vector, the MSS1 locus (multicopy suppressor of sns1) was isolated. This suppression of the sns1 mutation by multiple copies of the MSS1 locus occurred at the transcriptional level. When a gene disruption experiment was performed to examine the effect of a mss1 mutation, the sns1 mss1 double mutants produced 4 times higher levels of STA1 transcripts in the presence of STA10 than did the sns1 strain. Data presented in this paper suggest that both SNS1 and MSS1 loci are involved in STA10-dependent repression.

Multiple positive and negative cis-acting elements of the STA2 gene regulate glucoamylase synthesis in Saccharomyces cerevisiae

Expression of the glucoamylase-encoding gene (STA2) in Saccharomyces cerevisiae was previously shown to be regulated transcriptionally by both positive and negative factors. The objective of this work was to identify the cis-acting elements responsible for STA2 transcriptional activation as well as the transcriptional repressor effects of STA10 and MATa/MAT alpha. We identified two upstream activation regions (UAS). Three repressor regions responsive to STA10-mediated repression were identified, as well as two regions for down-regulation of STA2 expression. MATa/MAT alpha repression appears to effect STA2 expression either downstream from the translational start site or, indirectly, since no functional a1/alpha 2-responsive sequence was identified in the promoter region.

Properties and engineering of a mutant STA promoter of Saccharomyces diastaticus

A new allelic variant of the STA2 gene of S. diastaticus, designated as STA2K, was cloned and characterized (1; accompanying paper). An application-oriented analysis of the promoter region of STA2K is described, with an emphasis on its peculiar structural feature: A 1.1-kb natural deletion located 189 nucleotides upstream of the translation start codon. The strength of the STA2K promoter was found comparable to that of known strong constitutive yeast promoters (ADH1, GAPDH). Regulated glucoamylase expression was demonstrated by chimeric promoters, which were constructed by placing the STA2K promoter under the control of either the PHO5 or CYC1 upstream regulatory sequences. On high-copy-number vectors, induction of the UASPHO5-STA2K chimeric promoter by phosphate depletion resulted in a destructive overexpression of the secreted glucoamylase, which completely halted cell growth, and promoted cell decay. In contrast, UASCYC1 was shown to mediate a fine-tuned regulation both by glucose concentration and, indirectly, by starch, the substrate for the glucoamylase to produce glucose.

Cloning of a new allelic variant of a Saccharomyces diastaticus glucoamylase gene and its introduction into industrial yeasts

A new allelic variant of the STA2 gene, designated as STA2K, coding for a secreted glucoamylase, was cloned. Differences were revealed both in the structural gene and in the promoter region, as compared to other STA genes. The most peculiar structural features of STA2K are 1. a 1.1-kb natural deletion in its promoter located 189 nucleotides upstream of the translation start codon; and 2. an Asn-->Asp single amino acid change within the putative active site of the encoded glucoamylase. Neither the presence of glucose in the medium nor the host cell's mating type constellation affected the expression level of STA2K in S. cerevisiae. Self-replicating yeast plasmids containing STA2K were constructed and used to transform a laboratory yeast strain and various brewing strains. Pilot brewing tests with glucoamylase-secreting transformants of a brewing strain produced superattenuated beers at accelerated fermentation rates.

Yeast intragenic transcriptional control: activation and repression sites within the coding region of the Saccharomyces cerevisiae LPD1 gene

Though widely recognized in higher eukaryotes, the regulation of Saccharomyces cerevisiae genes transcribed by RNA polymerase II by proteins that bind within the coding sequence remains largely speculative. We have shown for the LPD1 gene, encoding lipoamide dehydrogenase, that the coding sequence between +13 and +469 activated gene expression of an LPD1::lacZ fusion by up to sixfold in the presence of the upstream promoter. This downstream region, inserted upstream of a promoterless CYC1::lacZ fusion, activated gene expression in a carbon source-dependent manner by a factor of 15 to 111, independent of orientation. Deletion and mutational analysis identified two downstream activation sites (DAS1 and DAS2) and two downstream repressor sites (DRS1 and DRS2) that influence the rate of LPD1 transcription rather than mRNA degradation or translation. Activation from the DAS1 region (positions +137 to +191), encompassing a CDEI-like element, is twofold under derepressive conditions. Activation from DAS2 (+291 to +296), a CRE-like motif, is 12-fold for both repressed and derepressed states. DRS1, a pair of adjacent and opposing ABF1 sites (+288 to +313), is responsible for a 1.3- to 2-fold repression of transcription, depending on the carbon source. DRS1 requires the concerted action of DRS2 (a RAP1 motif at position +406) for repression of transcription only when the gene is induced. Gel mobility shift analysis and in vitro footprinting have shown that proteins bind in vitro to these downstream elements.

Yeast intragenic transcriptional control: activation and repression sites within the coding region of the Saccharomyces cerevisiae LPD1 gene

Though widely recognized in higher eukaryotes, the regulation of Saccharomyces cerevisiae genes transcribed by RNA polymerase II by proteins that bind within the coding sequence remains largely speculative. We have shown for the LPD1 gene, encoding lipoamide dehydrogenase, that the coding sequence between +13 and +469 activated gene expression of an LPD1::lacZ fusion by up to sixfold in the presence of the upstream promoter. This downstream region, inserted upstream of a promoterless CYC1::lacZ fusion, activated gene expression in a carbon source-dependent manner by a factor of 15 to 111, independent of orientation. Deletion and mutational analysis identified two downstream activation sites (DAS1 and DAS2) and two downstream repressor sites (DRS1 and DRS2) that influence the rate of LPD1 transcription rather than mRNA degradation or translation. Activation from the DAS1 region (positions +137 to +191), encompassing a CDEI-like element, is twofold under derepressive conditions. Activation from DAS2 (+291 to +296), a CRE-like motif, is 12-fold for both repressed and derepressed states. DRS1, a pair of adjacent and opposing ABF1 sites (+288 to +313), is responsible for a 1.3- to 2-fold repression of transcription, depending on the carbon source. DRS1 requires the concerted action of DRS2 (a RAP1 motif at position +406) for repression of transcription only when the gene is induced. Gel mobility shift analysis and in vitro footprinting have shown that proteins bind in vitro to these downstream elements.

Genes required for derepression of an extracellular glucoamylase gene, STA2, in the yeast Saccharomyces

A diastatic strain of Saccharomyces cerevisiae producing the STA2-encoded extracellular glucoamylase (GA) in a pronounced glucose-repressible fashion was used as a parent for generating mutants with reduced GA activity under normal conditions of derepression. In addition to mutations in STA2, five other recessive mutations were identified which fell into four complementation groups designated haf1 through haf4. RNA blot analysis suggested that the haf mutations confer defects in STA2 transcription. The haf mutants were pleiotropically defective in utilization of alternative carbon sources and resembled the snf (sucrose non-fermenting) mutants identified previously as unable to derepress the expression of the SUC2 gene encoding invertase. We present evidence strongly suggesting that haf1 = snf2, haf3 = snf1 and haf4 = snf5. By phenotypic criteria, the postulated HAF2 gene (which is none of the SNF genes tested) appears to be similar to SNF2, SNF5 and SNF6, and is possibly a non-redundant extension of this group of functionally related SNF genes.

The STA2 and MEL1 genes of Saccharomyces cerevisiae are idiomorphic

The STA2 (glucoamylase) gene of Saccharomyces cerevisiae has been mapped close to the end of the left arm of chromosome II. Meiotic analysis of a cross between a haploid strain containing STA2, and another strain carrying the melibiase gene MEL1 (which is known to be at the end of the left arm of chromosome II) produced parental ditype tetrads only. Since there is no significant DNA sequence similarity between the STA2 and MEL1 genes, or their respective flanking regions, we conclude that these two genes are carried by separate non-hybridizing sequences of chromosomal DNA, either of which can reside at the end of the left arm of chromosome II. By analogy with the mating-type locus of Neurospora crassa, we suggest that the STA2 and MEL1 genes are idiomorphs with respect to one another.

Cloning and characterization of a Candida albicans maltase gene involved in sucrose utilization

In order to isolate the structural gene involved in sucrose utilization, we screened a sucrose-induced Candida albicans cDNA library for clones expressing alpha-glucosidase activity. The C. albicans maltase structural gene (CAMAL2) was isolated. No other clones expressing alpha-glucosidase activity. were detected. A genomic CAMAL2 clone was obtained by screening a size-selected genomic library with the cDNA clone. DNA sequence analysis reveals that CAMAL2 encodes a 570-amino-acid protein which shares 50% identity with the maltase structural gene (MAL62) of Saccharomyces carlsbergensis. The substrate specificity of the recombinant protein purified from Escherichia coli identifies the enzyme as a maltase. Northern (RNA) analysis reveals that transcription of CAMAL2 is induced by maltose and sucrose and repressed by glucose. These results suggest that assimilation of sucrose in C. albicans relies on an inducible maltase enzyme. The family of genes controlling sucrose utilization in C. albicans shares similarities with the MAL gene family of Saccharomyces cerevisiae and provides a model system for studying gene regulation in this pathogenic yeast.

The Saccharomyces cerevisiae GAM2/SIN3 protein plays a role in both activation and repression of transcription

We have cloned GAM2, which is required for transcription of STA1, a gene encoding an extracellular glucoamylase in Saccharomyces cerevisiae var. diastaticus. DNA sequence analysis revealed that GAM2 is the same gene as SIN3, known to be a general negative regulator of yeast genes. RNA blot analysis indicated that GAM2/SIN3 also acts as a positive regulator of GAM3/ADR6, which in turn is required for transcription of STA1 and ADH2. These results suggest that GAM2 regulates STA1 expression through transcriptional activation of GAM3 and indicate that GAM2/SIN3 protein is a transcriptional regulator that can play a role in both activation and repression of transcription.

Co-expression of a Saccharomyces diastaticus glucoamylase-encoding gene and a Bacillus amyloliquefaciens alpha-amylase-encoding gene in Saccharomyces cerevisiae

A glucoamylase-encoding gene (STA2) from Saccharomyces diastaticus and an alpha-amylase-encoding gene (AMY) from Bacillus amyloliquefaciens were cloned separately into a yeast-integrating shuttle vector (YIp5), generating recombinant plasmids pSP1 and pSP2, respectively. The STA2 and AMY genes were jointly cloned into YIp5, generating plasmid pSP3. Subsequently, the dominant selectable marker APH1, encoding resistance to Geneticin G418 (GtR), was cloned into pSP3, resulting in pSP4. For enhanced expression of GtR, the APH1 gene was fused to the GAL10 promoter and terminated by the URA3 terminator, resulting in pSP5. Plasmid pSP5 was converted to a circular minichromosome (pSP6) by the addition of the ARS1 and CEN4 sequences. Laboratory strains of Saccharomyces cerevisiae transformed with plasmids pSP1 through pSP6, stably produced and secreted glucoamylase and/or alpha-amylase. Brewers' and distillers' yeast transformed with pSP6 were also capable of secreting amylolytic enzymes. Yeast transformants containing pSP1, pSP2 and pSP3 assimilated soluble starch with an efficiency of 69%, 84% and 93%, respectively. The major starch hydrolysis products produced by crude amylolytic enzymes found in the culture broths of the pSP1-, pSP2- and pSP3-containing transformants, were glucose, glucose and maltose (1:1), and glucose and maltose (3:1), respectively. These results confirmed that co-expression of the STA2 and AMY genes synergistically enhanced starch degradation.