Structure and differential expression of the four members of the Arabidopsis thaliana ferritin gene family

Four ferritin genes are found within the complete sequence of the Arabidops is thaliana genome. All of them are expressed and their corresponding cDNA species have been cloned. The polypeptide sequences deduced from these four genes confirm all the properties of the ferritin subunits described so far , non-exhaustively, from various plant species. All are predicted to be tar geted to the plastids, which is consistent with the existence of a putative transit peptide at their N-terminal extremity. They also all possess a con served extension peptide in the mature subunit. Specific residues for ferro xidase activity and iron nucleation, which are found respectively in H-type or L-type ferritin subunits in animals, are both conserved within each of the four A. thaliana ferritin polypeptides. In addition, the hydrophilic na ture of the plant ferritin E-helix is conserved in the four A. thaliana fer ritin subunits. Besides this strong structural conservation, the four genes are differentially expressed in response to various environmental signals, and during the course of plant growth and development. AtFer1 and AtFer3 a re the two major genes expressed in response to treatment with an iron over load. Under our experimental conditions, AtFer4 is expressed with different kinetics and AtFer2 is not responsive to iron. HA activates the expression of AtFer1 and, to a smaller extent, AtFer3. Abscisic acid promotes the exp ression of only AtFer2, which is consistent with the observation that this is the only gene of the four to be expressed in seeds, whereas AtFer1, AtFe r4 and AtFer3 are expressed in various vegetative organs but not in seeds.