[URE3] and [PSI] as prions ofSaccharomyces cerevisiae: genetic evidence and biochemical properties

We have proposed that the yeast non-Mendelian elements [URE3] and [PSI] are prions, altered forms of Ure2p and Sup35p, respectively. Both are curable, but can arise again in the cured strains. The relation of theure2andsup35mutations to [URE3] and [PSI] is that expected of prions, not that expected of nucleic acid replicons. Overproduction of Ure2p and Sup35p increases by 100-fold the frequency with which [URE3[ and [PSI] arise, respectively. The N-terminal 65 amino acid residues of Ure2p are necessary for the [URE3] change and Ure2p becomes partially protease-resistant in [URE3] strains. The chaperone, Hsp104, is necessary for the propagation of [PSI], but when overexpressed cures [PSI].