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.