Tight regulation of the expression of mRNAs encoding iron uptake proteins is essential to control iron homeostasis and avoid intracellular iron toxicity. We show that many mRNAs encoding iron uptake or iron mobilization proteins are expressed in iron-replete conditions in the absence of the S. cerevisiae RNase III ortholog Rnt1p or of the nuclear exosome component Rrp6p. Extended forms of these mRNAs accumulate in the absence of Rnt1p or of the 5'-->3' exonucleases Xrn1p and Rat1p, showing that multiple degradative pathways contribute to the surveillance of aberrant forms of these transcripts. RNase III-deficient cells are hypersensitive to high iron concentrations, suggesting that Rnt1p-mediated RNA surveillance is required to prevent iron toxicity. These results show that RNA surveillance through multiple ribonucleolytic pathways plays a role in iron homeostasis in yeast to avoid the potentially toxic effects of the expression of the iron starvation response in iron-replete conditions.
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