| Description |
Iron is an essential element required to almost all organisms for survival; however, iron can cause toxicity if it accumulates in excess. This dissertation is focused on delineating the mechanisms that yeast species utilize to control intracellular concentrations of iron. The transcription factor Aft1 induces the transcription of a set of genes required for high-affinity iron uptake and proper intracellular iron mobilization and storage. When yeast cells are grown in low-iron media, Aft1 accumulates in the nucleus where it activates transcription of its target genes. Conversely, when yeast cells are grown in sufficient iron media, Aft1 accumulates in the cytosol where it is inactive. Aft1 seems to be negatively regulated by a signal coming from the mitochondrial Fe-S cluster biogenesis pathway. This signal appears to be exported from the mitochondria to the cytosol through the inner mitochondrial membrane transporter Atm1. It is not clear what happens with this signal after it is exported to the cytosol or how Aft1 senses it. However, the cytosolic Fe-S assembly (CIA) machinery, which consists of the proteins Cfd1, Nar1 and Nbp35, is shown to be dispensable for this sensing mechanism. The CIA machinery matures a putative Fe-S intermediate coming from the mitochondria into [4Fe-4S] clusters. Thus, the inhibitory signal does not seem to contain a mature [4Fe-4S] cluster as it was previously thought. Two other elements that are necessary for the control of Aft1 activity in response to iron are the nuclear monothiol glutaredoxins Grx3 and Grx4. In the absence of both glutaredoxins, Aft1 is constitutively active regardless of the iron levels in the media. Furthermore, these glutaredoxins seem to be playing a direct role since their binding to Aft1 is necessary for its regulation. In addition, overexpression of Grx4 decreases the activity of Aft1 under low iron conditions. Grx3 and Grx4 contain a thioredoxin-like domain in addition to the glutaredoxin domain. The thioredoxin-like domain is dispensable in mediating iron-inhibition of Aft1 activity, whereas the glutaredoxin domain and its conserved cysteine are required for this function. Thus, nuclear monothiol glutaredoxins are novel components required for iron regulation of Aft1. |
