Figure 2.

Novel effects of molecular chaperones on propagation of yeast prions. (a) Overexpression of SSA1-21 does not destabilize [PSI⁺] in the OT56 strain [73]. Shown are representative colonies of the OT56 strain bearing strong [PSI⁺] variant transformed with either pTEF-SSA1 [120] or pTEF-SSA1-21 (obtained using site-directed mutagenesis from pTEF-SSA1) plasmids for overproduction of wild-type Ssa1 or Ssa1-21, respectively, regulated by TEF1 promoter (b) Deletion of Sis1 dimerization domain may destabilize [PSI⁺]. Shown are representative 10-fold serial dilutions plated onto 1/4 YEPD medium [121] of the T-P^T-YAL2171 strain ([PSI⁺] derivative of YAL2171 [99]) expressing either full-length Sis1 or a variant with the deletion of dimerization domain (Sis1$\mathsf{\Delta }$DD) as the sole source of endogenous Sis1. In the top row, untagged SIS1 is expressed from its endogenous promoter. In the middle and bottom rows, the SIS1 gene is fused with EGFP on centromeric plasmid and expressed from under the constitutive ADH1 promoter. The red color of the colonies on the 1/4 YEPD medium indicates the loss of [PSI⁺]. On both (a,b), percentages of [psi⁻] and mosaic colonies are estimated from no less than three independent replicates with at least 30 colonies counted for each replicate. Mean and standard deviation are shown in each case.