Saccharomyces pastorianus
From Wikipedia, the free encyclopedia
| iSaccharomyces pastorianus | ||||||||||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| Scientific classification | ||||||||||||||||
|
||||||||||||||||
|
|
||||||||||||||||
| Saccharomyces pastorianus Nguyen & Gaillardin ex. Beijerinck |
||||||||||||||||
|
|
||||||||||||||||
|
Saccharomyces carlsbergensis Saccharomyces uvarum |
Saccharomyces pastorianus is a yeast, used industrially for the production of lager beer.
Brewing yeasts are unicellular, eukaryotic ascomycete fungi of the Saccharomycetaceae family. They reproduce through asymmetric division, known as budding and are classified according to their physiology , biochemistry , DNA base sequence homology and enzyme patterns ).
Brewing yeast are polyploid and belong to the Saccharomyces genera. The brewing strains can be classified into two groups; the ale strains (Saccharomyces cerevisiae, generally used for ale and stout) and the lager strains Saccharomyces pastorianus also known as Saccharomyces cerevisiae, Saccharomyces carlsbergensis or Saccharomyces uvarum. Lager strains are a hybrid strain of Saccharomyces cerevisiae (ale strains) and Saccharomyces bayanus (wine strains) and are often referred to as bottom fermenting. In contrast, ale strains are referred to as top fermenting strains, reflecting their separation characteristics in open square fermenters. Although the two species differ in a number of ways including their response to temperature, sugar transport and utilisation, the Saccharomyces pastorianus and Saccharomyces cerevisiae species are closely related within the Saccharomyces genus
As Saccharomyces pastorianus is a hybrid of Saccharomyces bayanus and Saccharomyces cerevisiae it is not surprising that there is a degree of phenotypic and genomic similarity between the two species. The hybrid nature of Saccharomyces pastorianus also explains the genome size, which is up to 60% larger than that of Saccharomyces cerevisiae as it includes large parts of the two genomes (Montrocher et al, 1998)Montrocher R et al (1998). "Phylogenetic Analysis of the Saccharomyces cerevisiae Group Based on Polymorphisms of the rDNA Spacer Sequences". Int. J. Syst. Bacteriol. 48: 295–303.</ref>. However there is growing evidence that Saccharomyces pastorianus has inherited most of its genetic material from Saccharomyces bayanus (Montrocher et al, 1998). Indeed the mitochondrial DNA (Piskur et al, 1998) and ribosomal DNA (Montrocher et al, 1998) of Saccharomyces pastorianus appear to be derived from Saccharomyces bayanus rather than Saccharomyces cerevisiae.
The genomic difference between Saccharomyces pastorianus and Saccharomyces cerevisiae is responsible for a number of phenotypic traits which Saccharomyces pastorianus share with Saccharomyces bayanus, but not Saccharomyces cerevisiae . The ability of Saccharomyces pastorianus to breakdown melibose is dependent on up to ten MEL genes, which are exclusive to strains metabolising melibose such as Saccharomyces bayanus. Saccharomyces pastorianus never grows above 34oC, whereas Saccharomyces cerevisiae will grow at 37oC . Saccharomyces pastorianus exhibits a higher growth rate than Saccharomyces cerevisiae at 6 to 12oC.
It has been suggested that Saccharomyces pastorianus may be a hybrid of Saccharomyces cerevisiae and Saccharomyces monacensis, as the LEU2, MET2 and ACB1 genes of Saccharomyces pastorianus had been reported to have a high level of similarity or be identical to the Saccharomyces monacensis homologues (Casey and Pedersen, 1988; Hansen and Kielland-Brandt, 1994; Borsting et al, 1997). However, subtelomeric sequence hybridisation has suggested that Saccharomyces monacensis is likely to be a closely related hybrid to Saccharomyces pastorianus, rather than an ancestor (Casaregola et al, 2001).
Ale strains are genetically more diverse than lager strains, as lager strains are thought to derive from a hybrid gene pool (Casey, 1996). It is thought that the lager strains in use are derived from only one or two primary strains; Tuborg and Carlsberg (Casey, 1990; Casey, 1996).
