Academic journal article Genetics

Effect of Domestication on the Spread of the [PIN+] Prion in Saccharomyces Cerevisiae

Academic journal article Genetics

Effect of Domestication on the Spread of the [PIN+] Prion in Saccharomyces Cerevisiae

Article excerpt

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PRIONS cause a variety of transmissible spongiform ence- phalopathies (TSEs) in mammals, including scrapie in sheep, bovine spongiform encephalopathy (BSE) in cattle, chronic wasting disease (CWD) in cervids, and Creutzfeldt- Jakob disease (CJD) in humans. With all TSEs, endogenous prion protein (PrP) misfolds into an altered, usually protease- resistant, isoform, termed "PrPres" ("res" for protease resistant).

Several prions have been described in fungi (Wickner 1994), with many prion variants of the [PSI+] and [URE3] prions showing toxic or even lethal effects in Saccharomyces cerevisiae (McGlinchey et al. 2011). The [PSI+] prion of S. cerevisiae is formed from the cytosolic protein Sup35p. There are three distinct regions of Sup35p: a glutamine/ asparagine-rich N-terminal domain (amino acids 1-123) that is necessary and sufficient for prion formation (Ter- Avanesyan et al. 1994) and that facilitates deadenylation and decay of messenger RNA (Hosoda et al. 2003); a middle M domain (amino acids 124 - 253) of unknown function; and an essential C-terminal domain that functions during trans- lation termination (reviewed by Wickner et al. 2004). The function of Rnq1p is not yet known, but the amyloid confor- mation of this protein, [PIN+], enhances the de novo forma- tion of other yeast prions, including [PSI+] (hence [PIN+], for [PSI+] inducibility) (Derkatch et al. 1997, 2001).

The natural abundance of prions should reflect their rate of spread via mating, relative fitness effects, and spontane- ous loss or gain of the prion state. Ecological factors could influence mating frequency and thus prion prevalence in the wild. Magwene et al. (2011) reported that strains from natural environments more readily undergo meiosis (under the tested conditions) compared to domesticated strains, whereas pseu- dohyphal growth was more readily induced in domesticated strains (Magwene et al. 2011). During meiosis wild yeast gen- erate four haploid spores that can subsequently mate with (1) cells from the same spore clone (homothallism), (2) cells from other spores within the ascus (intratetrad mating), or (3) spores from a different yeast clone (outcrossing) (Strathern et al. 1981). Studies conducted in Saccharomyces paradoxus have estimated the frequency of outcross mating at 1% of total matings and once per 105 mitotic divisions (Tsai et al. 2008). For S. cerevisiae, estimates of mating from genomic data reported two outcrosses per 105 mitotic doublings (Ruderfer et al. 2006), while another, more recent study reported one outcross per 100 mitotic divisions (Kelly et al. 2012).

Alternative to sporulation, nutrient deprivation can in- duce the formation of pseudohyphal filaments that allow yeast to invade solid media and forage for nutrients. This phenotype is highly variable, however, and even among genetically homogeneous strains only a subset of cells will undergo a pseudohyphal response to nutrient deprivation (Gimeno et al. 1992). Additionally, flocculation, character- ized by increased cell-cell adhesion resulting in aggregates of vegetative cells, is often observed when sugars are de- pleted from the media (Guo et al. 2000). Enhanced cellular aggregation provides protection in harsh environments (Bruckner and Mosch 2011), and flocculent yeast strains are often utilized in beer fermentation and other industrial- ized settings (Verstrepen and Klis 2006).

The yeast S. cerevisiae has been isolated globally from a va- riety of natural substrates (fruit, tree bark, soil) (Sniegowski et al. 2002; Wang et al. 2012) and from environments closely associated with human activity (breweries, bakeries, vine- yards) (Legras et al. 2007). We refer to strains isolated and adapted (or bred) for human use as domesticated strains. We include brewing, baking, vineyard, and clinical strains. We regard even "non-inoculated" wine strains isolated from vine- yards as domesticated since they have been isolated from agricultural crops after at least one human use. …

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