Genetic and environmental factors affecting the de novo appearance of the [PSI⁺] prion in Saccharomyces cerevisiae

It has previously been shown that yeast prion [PSI⁺] is cured by GuHCl, although reports on reversibility of curing were contradictory. Here we show that GuHCl treatment of both [PSI⁺] and [psi^-] yeast strains results in two classes of [psi^-] derivatives: Pin⁺, in which [PSI⁺] can be reinduced by Sup35p overproduction, and Pin^-, in which overexpression of the complete SUP35 gene does not lead to the [PSI⁺] appearance. However, in both Pin⁺ and Pin^- derivatives [PSI⁺] is reinduced by overproduction of a short Sup35p N-terminal fragment, thus, in principle, [PSI⁺] curing remains reversible in both cases. Neither suppression nor growth inhibition caused by SUP35 overexpression in Pin⁺ [psi^-] derivatives are observed in Pin^- [psi^-] derivatives. Genetic analyses show that the Pin⁺ phenotype is determined by a non-Mendelian factor, which, unlike the [PSI⁺] prion, is independent of the Sup35p N-terminal domain. A Pin^- [psi^-] derivative was also generated by transient inactivation of the heat shock protein, Hsp104, while [PSI⁺] curing by Hsp104 overproduction resulted exclusively in Pin⁺ [psi^-] derivatives. We hypothesize that in addition to the [PSI⁺] prion- determining domain in the Sup35p N-terminus, there is another self- propagating conformational determinant in the C-proximal part of Sup35p and that this second prion is responsible for the Pin⁺ phenotype.