Post-transcriptional regulation of gene expression in response to iron deficiency in Saccharomyces cerevisiae
The ability of iron (Fe) to easily transition between two valence states makes it a preferred co-factor for innumerable biochemical reactions, ranging from cellular energy production, to oxygen transport, to DNA synthesis and chromatin modification. While Fe is highly abundant on the crust of the earth, its insolubility at neutral pH limits its bioavailability. As a consequence, organisms have evolved sophisticated mechanisms of adaptation to conditions of scarce Fe availability.
Studies in the baker's yeast Saccharomyces cerevisiae have shed light into the cellular mechanisms by which cells respond to limited Fe-availability. In response to Fe-deficiency, the transcription factors Aft1 and Aft2 activate a group of genes collectively known as the Fe-regulon. Genes in this group encode proteins involved in the high-affinity plasma membrane Fe-transport and siderophore uptake systems, as well as Fe-mobilization from intracellular stores and heme re-utilization. Concomitant with the up-regulation of the Fe-regulon, a large number of mRNAs encoding Fe-dependent proteins as well as proteins involved in many Fe-dependent processes are markedly down regulated. Thus, in response to low Fe-levels the cell activates the Fe-uptake and mobilization systems, while down-regulating mRNAs involved in highly Fe-demanding processes leading to a genome-wide remodeling of cellular metabolism that permits the funneling of the limiting Fe to essential Fe-dependent reactions.
The Fe-regulon member Cth2 belongs to a family of mRNA-binding proteins characterized by an RNA-binding motif consisting of two tandem zinc-fingers of the CX8CX5CX3H type. Members of this family recognize and bind specific AU-rich elements (AREs) located in the 3'untranslated region (3'UTRs) of select groups of mRNAs, thereby promoting their rapid degradation. In response to Fe-limitation, Cth2 binds ARE sequences within the 3'UTRs of many mRNAs encoding proteins involved in Fe-homeostasis and Fe-dependent processes, thereby accelerating their rate of decay.
Work described in this dissertation demonstrates that the Cth2 homolog, Cth1, is a bona fide member of the Fe-regulon, binds ARE-sequences within the 3'UTRs of select mRNAs and promotes their decay. Cth1 and Cth2 appear to be only partially redundant; Cth1 preferentially targets mRNAs encoding mitochondrial proteins, while Cth2 promotes the degradation of most of Cth1 targets in addition to other mitochondrial and non-mitochondrial Fe-requiring processes. The coordinated activity of Cth1 and Cth2 results in dramatic changes in glucose metabolism. In addition, experiments described in this dissertation indicate that the CTH1 and CTH2 transcripts are themselves subject to ARE-mediated regulation by the Cth1 and Cth2 proteins, creating an auto- and trans-regulatory circuit responsible for differences in their expression. Finally, work described here demonstrates that Cth2 is a nucleocytoplasmic shuttling protein and that shuttling is important for the early determination of cytosolic mRNA-fate.
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