~~I Prion catalyses I own formation
Altered protein (prion)
Fig. 4.18 The 'prion hypothesis' (redrawn from Wickner, 1997).
plexing field and the interested reader looking for a fuller explanation is directed to the excellent review of Wickner et al. (1999). From a brief perspective, work in the late 1960s and early '70s identified two infectious proteins (prions), which were able to convert the normal form of the protein into an abnormal form. [PSI] is the prion form of the SUP35 protein (involved in protein translation) and [URE3] is the prion form of the URE2 protein that switches off the utilisation of a poor nitrogen source in the presence of an alternative 'good' source. In her memorably titled review - 'Mad cows meet mad yeast: the prion hypothesis' - Lindquist (1996) argued that the work with yeast would provide 'new insights and approaches that will synergise with mammalian studies'. As if a premonition, Liu and Lindquist (1999) have reported that the conversion of the native protein (SUP35p) to [PSI] is triggered by increasing the number of characteristic short, oligopeptide repeats in the protein. The prion then propagates forming characteristic amyloid filaments that can be visualised using congo red staining and microscopy (Wickner et al., 1999). Conversely, deletion of the repeated sequences eliminates conversion to the prion format. As mammalian prion diseases are associated with an expansion in oligopeptide repeats, this work in yeast has identified a general mechanism in which mutation can lead to a protein misfolding disease. Further work (Ma & Lindquist, 1999) has focused on protein trafficking and the degradation of misfolded proteins. Rather than being degraded in situ within the secretary pathway, misfolded proteins are delivered by retrograde transport to the cytoplasm where they are deglycosylated and degraded. Occasionally, for reasons that have yet to be established, the abhorrent misfolded protein is not degraded and aggregates to become a prion-like protein. In conclusion, whatever the eventual outcome of the 'prion story', it seems likely that the yeast model will provide important insights into understanding and, ultimately, controlling and eradicating an important high profile human disease.
Unlike Candida and Cryptococcus yeast species (for a review see Hurley et al., 1987), S. cerevisiae has little clinical presence. Indeed, in a survey of pathogenic yeasts found in women, 80 distinct strains were found, consisting of six Candida species and a Saccharomyces. Of the 80 strains found, 59 isolates were C. albicans with S. cerevisiae accounting for only two isolates. This is in keeping with the view that S. cerevisiae is not normally recognised as being pathogenic to man although there are occasional clinical reports (notably in immunocompromised AIDS and transplant patients) of infections by S. cerevisiae. A recent review (Murphy & Kavanagh, 1999) has suggested that S. cerevisiae can be described as an 'opportunistic pathogen'. Significantly, when found, pathogenic isolates of S. cerevisiae can grow at the aty-pically high temperature (see Section 184.108.40.206) of 42°C (McCusker et al., 1994) and can (Murphy & Kavanagh, 1999) exhibit pseudohyphal growth which may facilitate the penetration of host tissue. Increasingly, reports of pathogenicity have focused on vaginitis and possible links to domestic exposure to yeast. However cases are rare, so much so that of 4943 cases of vaginitis, 19 (0.4%) were found to be caused by S. cerevisiae (McCullough et al., 1998). Intriguingly, these authors speculate that 'the proliferation of the use of S. cerevisiae as a health-food product, in home baking, and in home brewing may be a contributing factor in human colonisation and infection with this organism'. Similarly, Posteraro et al. (1999) reported a strong correlation between instances of yeast associated vaginitis and the 'frequent domestic use of yeast'. However, molecular fingerprinting of implicated commercial bakers' yeasts failed to show any epidemiological link with the clinical isolates.
Baking and brewing strains of S. cerevisiae have also been implicated in Crohn's disease (Walker, 1998), the chronic inflammation of the gut which is also associated with an increased risk of intestinal cancers. Sufferers exhibit an immune hypersensitivity to dietary yeast, with a specific response to specific cell wall mannan antigens (Young et al., 1998) (see Section 4.4.2). Persuasive evidence for the role of yeast in this disease comes from analysis of occupational mortality that shows bakers to have the highest odds ratio for Crohn's disease (Alic, 1999).
220.127.116.11 Yeast genome project and brewing yeast. It is telling - and a little disappointing - that such a major project as sequencing the yeast genome has provoked so little comment in the brewing press. Meaden (1996), in a brief review, succinctly captured the key facts. He noted that 'the overall genetic picture will be similar for brewers' yeast, whether an ale or lager strain'. However, inevitably, there will be differences between the sequenced yeast and brewing strains. The genome project involved a number of closely related haploid strains derived from the progenitor strain S288C (Philip Meaden, personal communication; Cherry et al., 1998). The most notable difference derives from the genealogy of S288C as reported by Mortimer and Johnston (1986). Although a complex and fascinating story, EM93 - the main progenitor strain of S288C which is estimated to contribute 88% of the genome of this strain - was originally isolated from a rotting fig in California in the late 1930s!
Fundamentally, the most obvious difference between the yeast whose genetic sequence we now have and brewing strains is one of ploidy. As is discussed in Section 18.104.22.168, brewing strains are polyploid and as such have three or four copies of each chromosome. The yeast used in the genome project is haploid and, by definition, has only one copy of each chromosome. Fundamentally, significant differences have been shown between specific chromosomes of laboratory haploid strains (such as those in the genome project) and the chromosomes in commercial strains such as distillers' 'M' strain (Wicksteed et al., 1994) and Carlsberg lager yeast (Casey, 1990). Consequently, it is no great surprise that the fine detail of the genetic 'snapshot' of a haploid strain of S. cerevisiae is anticipated to be different to brewing strains. As reported by Meaden (1996), the sequenced yeast has one copy of the genes responsible for maltose fer-
mentation whereas brewing strains typically contain ten or more sets of MAL genes. Similarly, some genes (such as the second alcohol acetyltransferase, Lg-ATF1) that are found in brewing yeast cannot be found in the genome sequence. Conversely, approaching from the other viewpoint, Goffeau el al. (1996) have noted that seemingly redundant genes may be 'more apparent than real'. Indeed, many apparently redundant genes may be required to deal with natural non-laboratory 'real world' environments such as rotting figs, grapes or more pertinently, the brewery (Goffeau el al., 1996; Oliver, 1996).
Clarification of just how significant these 'differences' are awaits the sequencing of a brewing strain of S. cerevisiae. As it seems likely that this will be a lager strain, there will be the dual challenge of sequencing the hybrid genome of S. monacenis and the more familiar S. cerevisiae (see Section 4.2.2). If and indeed when this might happen is not clear. Initial signs that a consortium of laboratories might tackle this challenge have come to nothing. A more likely scenario is that a single laboratory might undertake the task (Morten Kielland-Brandt, personal communication).
22.214.171.124 Chromosome number. Between 1960 and 1997, 12 genetic maps of S. cerevisiae have been published (Cherry el al., 1997). Although as recently as 1981 there were thought to be 17 chromosomes (Mortimer & Schild, 1981), by 'Edition 11A' (Mortimer el al., 1995) and the twelfth 'and probably the last' map (Cherry el al., 1997) the yeast genome consisted of 16 chromosomes. These range dramatically in size from the 230 kb of Chromosome 1 to 1532 kb of Chromosome IV (see Fig. 4.19). These provide a benchmark for chromosomal size, as chromosomal polymorphism is increasingly recognised (Section 126.96.36.199) and probed using karyotyping (Section 188.8.131.52).
It will come as no surprise that the genome sequencing project has played an
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