Characterization of Saccharomyces cerevisiae strains from spontaneously fermented maize dough by profiles of assimilation, chromosome polymorphism, PCR and MAL genotyping

Several isolates of Saccharomyces cerevisiae from indigenous spontaneously fermented maize dough have been characterized with the purpose of selecting appropriate starter cultures and methods for their subspecies typing. The techniques applied included assimilation of carbon compounds by the API ID 32 C kit, determination of chromosome profiles by PFGE, PCR and MAL genotyping. For the 48 isolates investigated, use of the API ID 32 C kit resulted in eight different assimilation profiles. The most common assimilation profile was the ability of 50% of the isolates to assimilate galactose, saccharose, DL-lactate, raffinose, maltose and glucose. Both chromosome and PCR profiles could be used for subspecies typing of the isolates and on this basis, the isolates were grouped into clusters. The discriminative power of the two techniques was equal; a few isolates not separated by their chromosome profiles could be separated by their PCR profiles and vice versa. Four different MAL genotypes were observed with MAL11 and MAL31 predominating. MAL11 was seen for all isolates whereas no evidence of MAL21 and MAL41 was observed. Based on the results obtained, a high number of Saccharomyces cerevisiae isolates were found to be involved throughout the spontaneous fermentation of maize dough. All methods included appeared to be suitable for subspecies typing. However, the discriminative power was highest for the PFGE and PCR techniques.

Analysis of production brewing strains of yeast by DNA fingerprinting

Production brewing strains of the yeast Saccharomyces cerevisiae were analysed by DNA fingerprinting, using a Southern blotting and hybridization procedure and employing the Ty1-15 transposon as a probe. The ability to differentiate readily between strains was very dependent on the restriction enzyme used to digest the DNA prior to Southern blotting and hybridization; the enzymes EcoRI, PstI and SalI were found to be particularly useful in this respect. The method was applicable to the differentiation of both ale and lager yeasts, and was sufficiently sensitive to distinguish between very closely related strains. DNA fingerprinting by this approach confirmed, for example, that a flocculent strain isolated during a production-scale fermentation with a lager yeast was genotypically different from the parent.

Electrophoretic karyotyping of the yeast genus Torulaspora

Karyotypes of yeast strains in the genus Torulaspora were determined by pulse-field gel electrophoresis and compared with those of the related genus Zygosaccharomyces. The DNA bands ranged from 800 to 1600 kb in T. delbrueckii and 800 to 2000 kb in both T. globosa and T. pretoriensis and those numbers were about six in the three species. The chromosomes of Torulaspora strains comprised relatively smaller size of DNAs than Zygosaccharomyces strains.

Characterization of MEL genes in the genus Zygosaccharomyces

We cloned and sequenced a Zygosaccharomyces cidri MEL gene with a view to investigating the structure and regulation of yeast MEL genes. The amino acid sequence deduced from the nucleotide sequence showed 78.6% and 78.2% similarity to Saccharomyces cerevisiae and Saccharomyces pastorianus alpha-galactosidases, respectively. The expression of the MEL gene in several Zygosaccharomyces strains was induced by galactose. An electrophoretic karyotype of several Zygosaccharomyces species was obtained using contour-clamped electric field gel electrophoresis. The minimum number of chromosomes was five for Z. cidri, six for Z. fermentati, three for Z. florentinus, and four for Z. microellipsoides. The sizes of the chromosomes were generally larger than those of S. cerevisiae, the smallest containing approximately 0.4 megabase. The MEL gene was located, using the Z. cidri MEL gene as a probe, on the largest chromosome of the Z. cidri strains. In addition, a smaller chromosome (600 kb) in Z. cidri strain CBS4575 showed hybridization to the homologous MEL probe. This chromosome was absent in Z. cidri strain CBS5666. The probe hybridized to the largest chromosome of Mel+ Z. fermentati strains but failed to hybridize to any chromosome of Mel+ Z. mrakii or Z. florentinus strains. These results suggest the existence of a polymorphic MEL gene family in the yeast Zygosaccharomyces.