Abstract
Animal cytosolic aconitase and bacteria aconitase are able to switch to RNA binding proteins (IRPs), thereby playing a key role in the regulation of iron homeostasis. In the model plant Arabidopsis, we have identified three IRP1 homologues, named ACO1 to -3. To determine whether or not they may encode functional IRP proteins and regulate iron homeostasis in plants, we have isolated loss-of-function mutants in the three genes. The aco1-1 and aco3-1 mutants have a clear decrease in cytosolic aconitase activity. However, none of the mutants is affected in the accumulation of the ferritin transcript or protein in response to iron excess. Cis-acting elements potentially able to bind to the IRP have been searched in silico in the Arabidopsis genome. They appear to be very rare sequences, found in the 5´- or 3´-UTR of few genes unrelated to iron metabolism. They are therefore unlikely to play a functional role in the regulation of iron homeostasis. Taken together, our results demonstrate that, in plants, the cytosolic aconitase is not converted to IRP, and do not regulate iron homeostasis. In contrast to animals, the RNA binding activity of plant aconitase, if any, would be more likely related to a structural element, rather than to a canonical sequence.