Figure 7.

Increasing the gene dosage of SIR2 and ESA1 reciprocally suppresses the rDNA silencing defects associated with esa1-414 and sir2Δ mutants. (A) Increased gene dosage of SIR2 from a 2-micron plasmid suppressed the esa1-414 rDNA silencing defect. The esa1-414 strain transformed with ESA1 (LPY5141) or SIR2 (LPY5143) from URA3-marked 2-micron plasmids grew equivalently on plates assaying for rDNA silencing (right) compared with vector transformants (LPY5136) at 33°C. (B) Similarly, increasing the dosage of ESA1 suppressed the sir2Δ rDNA silencing defect. The sir2Δ strain transformed with SIR2 (LPY5166) or ESA1 (LPY5164) grew equivalently on plates assaying for rDNA silencing compared with vector transformants (LPY5159) at 28°C. The level of suppression was somewhat variable between isolates, perhaps reflecting a difference in expression levels of either ESA1 or SIR2 due to fluctuations in 2-micron plasmid copy number per isolate (Armstrong et al., 1989). The rDNA silencing assays were performed with 32 mg/ml canavanine in order to compare silencing effects in the sir2Δ and esa1 mutants directly.