DETERMINATION OF BREWING YEAST PLOIDY BY DNA MEASUREMENT

Functional analysis and transcriptional regulation of two orthologs of ARO10, encoding broad-substrate-specificity 2-oxo-acid decarboxylases, in the brewing yeast Saccharomyces pastorianus CBS1483

The hybrid genomes of Saccharomyces pastorianus consist of subgenomes similar to those of S. cerevisiae and S. eubayanus, and impact of the genome structure on flavour production and its regulation is poorly understood. This study focuses on ARO10, a 2-oxo-acid decarboxylase involved in production of higher alcohols. In S. pastorianus CBS1483, four ARO10 copies were identified, three resembled S. cerevisiae ARO10 and one S. eubayanus ARO10. Substrate specificities of lager strain (Lg)ScAro10 and LgSeubAro10 were compared by individually expressing them in a pdc1Δ-pdc5Δ-pdc6Δ-aro10Δ-thi3Δ S. cerevisiae strain. Both isoenzymes catalysed decarboxylation of the 2-oxo-acids derived from branched-chain, sulphur-containing amino acids and preferably phenylpyruvate. Expression of both alleles was induced by phenylalanine, however in contrast to the S. cerevisiae strain, the two genes were not induced by leucine. Additionally, LgSeubARO10 showed higher basal expression levels during growth with ammonia. ARO80, which encodes ARO10 transcriptional activator, is located on CHRIV and counts three Sc-like and one Seub-like copies. Deletion of LgSeubARO80 did not affect LgSeubARO10 phenylalanine induction, revealing 'trans' regulation across the subgenomes. ARO10 transcript levels showed a poor correlation with decarboxylase activities. These results provide insights into flavour formation in S. pastorianus and illustrate the complexity of functional characterization in aneuploid strains.

Determination of the relative ploidy in different Saccharomyces cerevisiae strains used for fermentation and 'flor' film ageing of dry sherry-type wines

The full chromosomal karyotype of six enological Saccharomyces cerevisiae strains used for fermentation and biological ageing of sherry-type wines was studied. A genetic method based on the analysis of segregation frequencies of auxotrophic markers, among random spore progeny of hybrids, constructed between laboratory and industrial wine strains (Bakalinsky and Snow, 1990) was used. This method was combined with the analysis of strains by pulsed-field gel electrophoresis. The results obtained clearly indicate the presence of two, three or four copies of a chromosome in the industrial strains examined, and thus confirm that aneuploidy/polyploidy is not uncommon in these strains. In all strains examined, chromosome XIII polysomy is observed. This chromosome contains the ADH2 and ADH3 loci, that code for the ADHII and ADHIII isoenzymes of alcohol dehydrogenase, which are involved in ethanol oxidative utilization during biological ageing of wines. Tetrad analysis for the 'flor formation' character suggest two possibilities: this character is either regulated by at least a digenic system, or by only one gene present on a chromosome which is, at least, disomic.

Chemical and cytological changes during the autolysis of yeasts

Cell suspensions of Sacharomyces cerevisiae, Kloeckera apiculata and Candida stellata were autolyzed in phosphate buffer, pH 4.5, for up to 10 days. Cell dry weights decreased by 25-35% after 10 days. Based on initial cell dry weight, the soluble autolysate consisted of: carbohydrate (principally polysaccharide) 3-7%; organic acids 3-6%; protein 12-13%; free amino acids 8-12%; nucleic acid products 3-5%; and lipids 1-2%. The main organic acids in autolysates were propionic, succinic and acetic and the main amino acids were phenylalanine, glutamic acid, leucine, alanine and arginine. Approximately 85-90% of cellular RNA and 25-40% of cellular DNA were degraded during autolysis. Both neutral lipid and phospholipid components were degraded, with neural lipids but not phospholipids being found in autolysates. Scanning and transmission electron micrographs showed retention of cell wall structure and shape during autolysis, but there was extensive intracellular disorganization within S. cerevisiae and C. stellata. There were differences in the autolytic behavior of K. apiculata compared with S. cerevisiae and C. stellata.

Determination of chromosome copy numbers in Saccharomyces cerevisiae strains via integrative probe and blot hybridization techniques

Methods have been devised for analyzing chromosome copy numbers in S. cerevisiae strains that may be polyploid or aneuploid, as is apparent in the case of many industrial strains. The initial step involved transformation of a strain with an integrative "ploidy probe" transplacement fragment that enabled the copy number of the targeted chromosomal locus to be determined via genomic Southern blotting and quantitative probe hybridization. Dual probe co-hybridization to Southern genomic DNA blots was used to extend such locus copy number determinations to other loci within the same chromosome, thereby screening for internal consistency along the length of the chromosome. This approach was also used to extend the analysis to other chromosomes in the genome. The method was established and verified with euploid series laboratory strains and then used to examine chromosome copy numbers in three industrial strains. One brewing strain apparently contained three copies of the chromosomes tested, whilst another brewing and a baking strain showed evidence of aneuploidy.

Endomitotic diploidization of Saccharomyces cerevisiae by heat treatment during spore germination

Diploid cells with ability to mate, hereafter referred to as diploid mater cells, were obtained at significant frequencies by the heat treatment of haploid spores at the early germination stage in Saccharomyces cerevisiae heterothallic strain CG5M (a/alpha diploid cells heterozygous for five auxotrophic markers). The highest frequency (ca. 11%) of diploidization was obtained from viable cells after heat treatment at 55 degrees C for 10 min when spores were precultivated for 30 min in liquid medium to initiate the germination. The diploid mater cells obtained were homozygous for mating type and for the auxotrophic markers. The diploidization of a spore is thus concluded to be due to endomitotic events in germinating heat-treated spores.

Direct induction of tetraploids or homozygous diploids in the industrial yeast Saccharomyces cerevisiae by hydrostatic pressure

Hydrostatic pressure and a dye plate method were used to investigate the direct induction of tetraploids or homozygous diploids from the industrial diploid or haploid yeast Saccharomyces cerevisiae. Above 200 MPa, hydrostatic pressure greatly inactivated the strains HF399s1 (alpha haploid), P-540 (a/alpha diploid), and P-544 (a/alpha diploid). At the same time, when pressure-treated cells of these strains were spread on a dye plate, some of the visible colonies were stained red/blue or dark blue (variant colonies); the rest stained violet, similar to colonies originating from diploid cells or haploid cells that were not pressure-treated. In addition, above 100 MPa, the formation of variant colonies increased with increasing pressure, and maximized (1 x 10(-1)) at 200 and 250 MPa, respectively. The size of almost all variant cells from P-544, P-540, and HF399s1 was visibly increased compared with that of untreated cells and the measured cellular DNA content of P-540 and HF399s1 was double that of untreated cells. Furthermore, based on random spore analysis and mass-matings, induced variants in the diploid strains were found to be tetraploid with an a/a/alpha/alpha genotype at the mating-type locus or, in the haploid strains, homozygous diploid with an alpha/alpha genotype. From these results we conclude that pressure treatment in combination with a dye plate is a useful method for strain improvement by direct induction of tetraploids or homozygous diploids from industrial strains whether diploid or haploid.

The value of electrophoretic fingerprinting and karyotyping in wine yeast breeding programmes

Electrophoretic banding patterns of total soluble cell proteins, DNA restriction fragments and chromosomal DNA were used to characterise ten strains of Saccharomyces cerevisiae used for commercial production of wine. These fingerprinting procedures provided unique profiles for all the different yeast strains and can therefore be used to identify and control industrial strains. Furthermore, the protein profiles, restriction fragments banding patterns and electrophoretic karyotyping by contour clamped homogeneous electric field electrophoresis (CHEF), were valuable to differentiate hybrid and parental strains in yeast breeding programmes. Hybrid strains, with desirable oenological properties, were obtained by mass spore-cell mating between a heterothallic killer yeast and two homothallic sensitive strains and all were shown to have unique DNA fingerprints and electrophoretic karyotypes.