GENETIC AND ENVIRONMENTAL-FACTORS AFFECTING THE DE-NOVO APPEARANCE OFTHE [PSI-CEREVISIAE(] PRION IN SACCHAROMYCES)

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 strain s results in two classes of [psi(-)] derivatives: Pin(+), in which [PS I+] can be reinduced by Sup35p overproduction, and Pin(-), in which ov erexpression of the complete SUP35 gene does not lead to the [PSI+] ap pearance. However, in both Pin(+) and Pin(-) derivatives [PSI+] is rei nduced 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 Pi n(+) [psi(-)] derivatives are observed in Pin(-) [psi(-)] derivatives. Genetic analyses show that the Pin(+) phenotype is determined by a no n-Mendelian factor, which, unlike the [PSI+] prion, is independent of the Sup35p N-terminal domain. A Pin(-) [psi(-)] derivative was also ge nerated by transient inactivation of the heat shock protein, Hsp104, w hile [PSI+] curing by Hsp104 overproduction resulted exclusively in Pi n(+) [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(+) pheno type.