CHARACTERIZATION OF A FERRITIN MESSENGER-RNA FROM ARABIDOPSIS-THALIANA ACCUMULATED IN RESPONSE TO IRON THROUGH AN OXIDATIVE PATHWAY INDEPENDENT OF ABSCISIC-ACID

A ferritin cDNA, AtFer1, from seedlings of Arabidopsis thaliana has be en characterized. The deduced amino acid sequence of the AtFer1 protei n indicates that A. thaliana ferritin shares the same characteristics as the plant ferritin already characterized from the Leguminosae and G raminacea families: (i) it contains an additional sequence in its N-te rminal part composed of two domains: a transit peptide responsible for plastid targeting and an extension peptide; (ii) amino acids that for m the ferroxidase centre of H-type animal ferritin, as well as Glu res idues characteristic of L-type animal ferritin, are conserved in AtFer 1; (iii) the C-terminal part of the A. thaliana ferritin subunit defin ing the E-helix is divergent from its animal counterpart, and confirms that 4-fold-symmetry axis channels are hydrophilic in plant ferritin. Southern blot experiments indicate that AtFer1 is likely to be encode d by a unique gene in the A. thaliana genome, although a search in the NCBI dbEST database indicates that other ferritin genes, divergent fr om AtFer1, may exist. Iron loading of A. thaliana plantlets increased ferritin mRNA and protein abundance. In contrast to maize, the transcr ipt abundance of a gene responding to abscisic acid (RAB18) did not in crease in response to iron loading treatment, and A. thaliana ferritin mRNA abundance is not accumulated in response to a treatment with exo genous abscisic acid, at least in the culture system used in this stud y. In addition, iron-induced increases in ferritin mRNA abundance were the same as wild-type plants in abi1 and abi2 mutants of A. thaliana, both affected in the abscisic acid response in vegetative tissues. In creased AtFer1 transcript abundance in response to iron is inhibited b y the antioxidant N-acetylcysteine. These results indicate that an oxi dative pathway, independent of abscisic acid, could be responsible for the iron induction of ferritin synthesis in A. thaliana.