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.