Ssd1-9A causes lysis phenotypes that are suppressed by cbk1 dosage suppressors. (A) Top panels: wild-type cells and cbk1-8 mutants at 34°C. The lysis phenotypes of cbk1-8 cells resemble those of Ssd1-9A–expressing cells at 22°C. Bottom panels: ssd1Δ cells carrying high copy plasmids of CCW12, SIM1, SRL1, UTH1, or ZRG8 were transformed with pRS316-Ssd1-9A. Cells were maintained at 22°C. Bars, 8 µm. (B) 10-fold dilution series of ssd1Δ cells (FLY2184) containing pRS316-Ssd1-9A and cbk1 dosage suppressor plasmids (pCCW12, pSIM1, pSRL1, pUTH1, and pZRG8). Negative control cells contain empty vectors pRS416 and pRS425. (C) Ssd1-9A toxicity is dependent on the mRNA-binding domain. Plasmids expressing Ssd1-RBDΔ (missing mRNA-binding domain, RBD 686–788) and Ssd1-9A-RBDΔ were introduced into ssd1Δ cells (FLE1210 and FLE1209). In contrast to Ssd1-9A expression, Ssd1-RBD and Ssd1-9A-RBDΔ expression is not toxic. Ssd1 expression was confirmed by immunoblots (not depicted).