The chemotherapeutic drug 5-fluorouracil (5FU) disrupts DNA synthesis by inhibiting the enzymatic conversion of dUMP to dTMP. However, mounting evidence indicates that 5FU has important effects on RNA metabolism that contribute significantly to the toxicity of the drug. Strains with mutations in nuclear RNA-processing exosome components, including Rrp6p, exhibit strong 5FU hypersensitivity. Studies also suggest that 5FU-containing RNA can inhibit pseudouridylation, the most abundant post-transcriptional modification of noncoding RNA. We examined the effect of modulating the expression and activity of the essential yeast rRNA pseudouridylase Cbf5p on the 5FU hypersensitivity of an rrp6-delta mutant strain. Depletion of Cbf5p suppressed the 5FU hypersensitivity of an rrp6-delta strain, while high-copy expression enhanced sensitivity to the drug. A mutation in the catalytic site of Cbf5p also suppressed the 5FU hypersensitivity in the rrp6-Delta mutant, suggesting that RNA-based 5FU toxicity requires the pseudouridylation activity of Cbf5p. High-copy expression of box H/ACA snoRNAs also suppressed the 5FU hypersensitivity of an rrp6-delta strain, suggesting that sequestration of Cbf5p to a particular guide RNA reduces Cbf5p-dependent 5FU toxicity. On the basis of these results and previous reports that certain pseudouridylases form stable adducts with 5FU-containing RNA, we suggest that Cbf5p binds tightly to substrates containing 5FU, causing their degradation by the TRAMP/exosome-mediated RNA surveillance pathway.
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