[PSI+] Prion Induction within the Yeast Saccharomyces Genera
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Prions are self-perpetuating protein aggregates that cause neurodegenerative diseases in mammals and carry heritable traits in yeast. Yeast [PSI+] prion is the prion isoform of Sup35 protein, an essential translation termination factor. The [PSI+] prion state can be induced in [psi-] cells by the overexpression of the full-length Sup35 protein, or only a certain portion of the protein. However, this induction requires the presence of a second yeast prion known as [PIN+]. While [PSI+] is present in lab cultures of S. cerevisiae, it has not been reported in any other Saccharomyces species. In this study, we attempted to induce [PSI+] in S. paradoxus and S. bayanus, two close relatives of S. cerevisiae. As non-cerevisiae species of Saccharomyces lack other known prions, we employed a new induction approach based on the overproduction of a chimeric protein composed of portions of Sup35 and Human Progesterone Receptor 6.6 (HPR6.6). This construct can induce [PSI+] in the absence of other endogenous prions. We next engineered S. paradoxus and S. bayanus strains with markers that allowed for prion studies in these species. The novel Sup35-HPR6.6 fusion constructs led to [PSI+] induction in S. paradoxus, the sister species of S. cerevisiae, but not in the more distantly related S. bayanus. We also showed that the prion isoform of Sup35 from S. paradoxus, previously known to produce only unstable prions in the heterologous S. cerevisiae system, can generate mitotically stable prions in the homologous S. paradoxus system. Finally, we propose a model which explains the possible role of the Sup35 and HPR6.6 fusion constructs in [PIN+]-independent [PSI+] induction.