Fig. 1.

Most [PSI+] prion variants isolated in ssb1/2Δ, zuo1∆, and ssz1∆ strains are lost in the presence of the WT allele of SSB1, ZUO1, and SSZ1, respectively (see text). Such [PSI+] isolates in each deletion strain MS515, MS527, and MS510 were mated for 2 d on YPAD with either isogenic WT MS173 or each deletion strain MS520, MS528, or MS514 and replica-plated to minimal media with and without uracil. The presence of p1520 (pCEN LEU2 ura3-14) in all strains enables scoring [PSI+]. Diploids formed with WT are almost all Ura⁻ as a result of elimination of [PSI+] (Left). Diploids formed with deletion strains are Ura⁺ indicating stable maintenance of [PSI+] (Right).