Translational repression and novel functions of Cth2 in the Saccharomyces cerevisiae response to iron deficiency

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Iron (Fe) is an essential micronutrient for all eukaryotes because of its redox properties. It participates as a cofactor in a wide range of biological processes, including protein translation, respiration (Krebs cycle and electron transport chain (ETC)) and DNA replication. The model organism Saccharomyces cerevisiae responds to iron deficiency (-Fe) by activating the Fe acquisition and recycling systems, and by remodeling cellular metabolism to promote Fe utilization in specific processes. The tandem zinc finger (TZF)-containing protein Cth2 plays an important role in prioritizing Fe by promoting the degradation of multiple mRNAs containing A/U-rich elements (AREs), including the CTH2 mRNA itself that is autoregulated. In this thesis, we identified and characterized new mechanisms involved in the global translational repression and novel functions of Cth2 in response to iron deficiency. Our results with polysome fractionation experiments demonstrate that the eIF2α/Gcn2 pathway is involved in the general repression of translational initiation during iron deficiency. The Gcn2 kinase specifically phosphorylates serine 51 of eIF2α in a Gcn1-dependent manner, causing a slight induction of GCN4 translation. The Gcn2 activation by uncharged tRNAs and TORC1 inactivation under iron deficiency is discussed. Besides, we show a role played by Cth2 in translational inhibition of several ARE-mRNAs in -Fe. Both the Cth2 TZF-domain as well as the AREs within SDH4 and CTH2 mRNAs are essential for translational repression, and we suggest a Cth2 general role on inhibition of translation of several ARE-mRNAs. Besides, while the amino-terminal domain (NTD) of Cth2 is important for both mRNA turnover and translational inhibition, its carboxy-terminal domain (CTD) is only involved in translational repression. Importantly, Cth2 carboxy-terminal domain is physiologically relevant in iron deficiency, and its mutation increases the protein levels of several Cth2 targets. Two novel Cth2 functions in -Fe include the regulation of mitochondrial respiration and the RNR3 catalytic subunit of the Fe-dependent ribonucleotide reductase (RNR) responsible of dNTP synthesis. The overexpression of CTH2 under Fe sufficiency decreases oxygen consumption as well as several Fe-enzymatic activities (Leu1, aconitase, complex II and III of the electron transport chain) while the complex IV activity is unaffected. Under iron deficient conditions, oxygen consumption decreases regardless Cth2, despite the decrease in complex II and III that is more pronounced when CTH2 is expressed. However the complex IV enzymatic activity is maintained by Cth2, probably by increasing Cox1 protein levels. Finally, RNR3 has been described to be highly expressed under genotoxic or replication stress by the Mec1–Rad53–Dun1 checkpoint pathway. However, RNR3 function is not clear as it is not the major isoform of the catalytic subunit of the enzyme ribonucleotide reductase. Our results suggest a higher RNR3 expression only under long-term iron deficiency comparable to that observed under those stresses. Besides, we demonstrate the participation of Cth2 (through its tandem zinc fingers) and Dun1 in the transcriptional induction of RNR3 under long-term iron deficient conditions. Cth2 partially promotes the RNR3 transcriptional derepression through Crt1 and Rox1 regulation under long-term iron deficiency, both transcriptional repressors of the RNR3 promoter. Importantly, unlike other stresses, RNR3 is physiologically relevant in iron deficiency
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