To test whether the level of rRNA, which reflects the amount of cellular ribosomes, may be somehow linked with cell survival under oxidative stress, we attempted to create a situation where the steady-state level of mature rRNAs will be increased or decreased. However, additional copies of rDNA present on a multicopy pNOY102 plasmid under control of the inducible GAL7 promoter (54) did not affect the amount of any mature rRNA species, possibly due to mechanisms that regulate ribosome abundance (data not shown). In contrast, the level of total genomic and plasmid-derived 18S and 25S rRNAs was, to our surprise, reduced to 70% in a strain transformed with the multicopy pJV12 plasmid expressing a tagged rDNA gene under the control of the constitutive PGK1 promoter (55) , when compared to a strain transformed with vector alone ( Figure 6A ). The basis of this effect is unclear, particularly that it was seen even though the tagged rRNA versions were expressed as confirmed by northern blots using probes specific for plasmid borne 25S rRNA ( Figure 6A ). Nevertheless, the strain carrying pJV12 showed a decreased viability already in the absence of oxidant (1.6-fold) and even more strikingly reduced following treatment with different concentrations of H 2 O 2 (4.6-fold for 0.6 mM and 6.24 for for 1 mM, respectively) relative to the strain with vector alone ( Figure 6B ). In another approach, we used the NOY504 strain, which carries a temperature sensitive (ts) RNA polymerase I (56) . At 378C, this strain ceases to grow but it sustains slow growth at 308C due to reduced levels of mature rRNA. Expression of additional copies of rDNA from pNOY102 or pJV12 plasmids improves growth at all temperatures and rescues the ts-lethal phenotype (55, 56) . Growth of NOY504 expressing additional rDNA under the control of GAL7 (pNOY102) or PGK1 (pJV12) promoters resulted in total rRNA levels lower by 15% in the latter case ( Figure 6C ). This relatively modest difference in rRNA abundance led to a 10% decrease in survival of cells exposed to oxidative stress ( Figure 6D ).